Armored scale insects are unusual in that a part of their bodies is genetically distinct from the rest. This extraordinary phenomenon challenges the notion of identity

I am a clone. That is, I am a colony of cells that developed from a single fertilized egg cell. Most animals are clones like me. It is a slight oversimplification to say that all of an animal's cells are genetically identical to each other. Some cells have mutations. In mammals, some cells (red blood cells) lack a nuclear genome entirely. Some cells have viruses—and when it's in a cell, a virus is basically a gene—that other cells lack. But a typical animal is a clone in the sense that all its cells arise from that single fertilized egg cell.Not all animals, however, are clones. Sometimes two tiny embryos developing inside their mother will fuse together into a single embryo and continue developing. The resulting animal is not a clone, but a chimera: a conglomeration of two different cell lineages into a single organism. Some species of monkeys (marmosets) typically have chimeric blood, from having shared a blood supply with a twin in utero ( Haig 1999 ), and rare cases of accidental chimerism are known from many animal species ( Tremblay and Caltagirone 1973 ;  van Dijk et al. 1996 ). In marine invertebrates, chimeric individuals often arise from the fusion of individuals later in development ( Buss 1987 ). Here I want to draw attention to a remarkable form of chimerism found in armored scale insects. These insects ( Figure 1 ) always develop not from a single fertilized egg but from two genetically different cells. One of these cells develops into a special organ (the bacteriome, which houses symbiotic bacteria) that has a nuclear genome different from that found in the rest of the body.Obligate chimerism—the presence of two genetically distinct cell lineages in every individual at each life stage—is found in a few families of scale insects, but nowhere else in nature. The avid naturalist wants to understand this sort of deep oddity for its own sake, but such understanding might have broader implications as well. For instance, although humans are not usually chimeras, we do have a quasi-chimeric phase in our life cycle: pregnancy. Some diseases of pregnancy are apparently due to conflicts between the genetically nonidentical tissues of mother and fetus ( Haig 2002 ). And the main things that humans eat are also quasi-chimeras: the seeds of flowering plants. In a grain of wheat, for instance, the germ, the endosperm, and the bran have three different nuclear genomes, and the conflicts between them may be similar in some ways to the conflicts seen in human pregnancy ( Alleman and Doctor 2000 ;  Santiago and Goodrich 2000 ). Ultimately, we might learn something about the general principles of conflict and cooperation between maternal and embryonic tissues that govern these cases if we can understand the uniquely stable and intimate chimerism of armored scale insects.Two Different Cell Lineages In all sexual animals and plants, production of an egg cell involves meiosis, the complex cellular process (involving DNA replication, recombination, and two nuclear divisions) whereby one diploid nucleus (with two copies of each chromosome) becomes four genetically different haploid nuclei (each with one copy of each chromosome). Only one of these four haploid nuclei becomes the egg cell (oocyte). In ordinary animals, the other three nuclei (the polar bodies) degenerate—they never divide again and are lost or destroyed—and the oocyte is the single maternal cell that (after fusion with a single paternal cell, the spermatocyte) develops into the embryo. But in armored scales, the polar bodies fuse together into a triploid cell (with three copies of each chromosome), and this triploid cell also winds up in the embryo ( Figure 2 ). The triploid cell derived from the polar bodies fuses with one cell from the embryo to become a pentaploid cell (with five copies of each chromosome). This pentaploid cell then proliferates to form the bacteriome of the embryo ( Brown 1965 ). Each cell in the bacteriome thus contains two copies of the mother's complete genome, in addition to the same haploid paternal genome as the rest of the embryo. In contrast, the rest of the embryo contains just one copy of half of the mother's genome. The apparent function of the bacteriome is to house intracellular bacteria. During embryonic development, bacteria move from the mother's bacteriome into the cells of the embryo's bacteriome. The precise role of the bacteria is not known, but it is thought that they synthesize essential nutrients ( Tremblay 1990 ), as they do in scale insects' close relatives, the aphids ( Shigenobu et al. 2000 ). Figure 2 Schematic Diagram of the Genetic System of a Diaspidid Scale Insect Zygote 1 is the fertilized ovum from which all organismal tissues except the bacteriome develop (black arrows). The bacteriome develops from zygote 2 (green arrows). Each haploid genome is represented by an N. A haploid genome may come from the oocyte (pink), sperm (blue), or polar bodies (black). The blue and pink arrows emphasize that the maternal versus paternal identity of a haploid genome is reset (imprinted) in each generation; thus, a male transmits only his maternal genome, but in his offspring the same chromosomes behave as a paternal genome (schematically, the pink N is converted to blue during spermatogenesis). Now the story gets even stranger. If the individual is a male, then the genetic difference between his bacteriome and the rest of his tissues becomes even greater as he develops. This is because most armored scale insects have an unusual genetic system called embryonic paternal genome elimination ( Herrick and Seger 1999 ). In males, the paternal genome is completely eliminated from most tissues very early in development—but it is never eliminated from the bacteriome. As a result, most of a male armored scale insect's tissues (including his sperm) have one copy of half of the mother's genome (the same genome as the oocyte from which he developed), but his bacteriome has two complete copies of the mother's genome and also has a paternal genome. Thus, 60% (three of five) of the gene copies in the male's bacteriome are absent from the rest of the male ( Figure 2 ).
Two Different Cell Lineages
In all sexual animals and plants, production of an egg cell involves meiosis, the complex cellular process (involving DNA replication, recombination, and two nuclear divisions) whereby one diploid nucleus (with two copies of each chromosome) becomes four genetically different haploid nuclei (each with one copy of each chromosome). Only one of these four haploid nuclei becomes the egg cell (oocyte). In ordinary animals, the other three nuclei (the polar bodies) degenerate—they never divide again and are lost or destroyed—and the oocyte is the single maternal cell that (after fusion with a single paternal cell, the spermatocyte) develops into the embryo. But in armored scales, the polar bodies fuse together into a triploid cell (with three copies of each chromosome), and this triploid cell also winds up in the embryo ( Figure 2 ). The triploid cell derived from the polar bodies fuses with one cell from the embryo to become a pentaploid cell (with five copies of each chromosome). This pentaploid cell then proliferates to form the bacteriome of the embryo ( Brown 1965 ). Each cell in the bacteriome thus contains two copies of the mother's complete genome, in addition to the same haploid paternal genome as the rest of the embryo. In contrast, the rest of the embryo contains just one copy of half of the mother's genome. The apparent function of the bacteriome is to house intracellular bacteria. During embryonic development, bacteria move from the mother's bacteriome into the cells of the embryo's bacteriome. The precise role of the bacteria is not known, but it is thought that they synthesize essential nutrients ( Tremblay 1990 ), as they do in scale insects' close relatives, the aphids ( Shigenobu et al. 2000 ).Now the story gets even stranger. If the individual is a male, then the genetic difference between his bacteriome and the rest of his tissues becomes even greater as he develops. This is because most armored scale insects have an unusual genetic system called embryonic paternal genome elimination ( Herrick and Seger 1999 ). In males, the paternal genome is completely eliminated from most tissues very early in development—but it is never eliminated from the bacteriome. As a result, most of a male armored scale insect's tissues (including his sperm) have one copy of half of the mother's genome (the same genome as the oocyte from which he developed), but his bacteriome has two complete copies of the mother's genome and also has a paternal genome. Thus, 60% (three of five) of the gene copies in the male's bacteriome are absent from the rest of the male ( Figure 2 ).Chimerism and Sibling Rivalry What could possibly be going on here? Why should scale insects, of all creatures, have obligate chimerism involving activated polar bodies? Essentially, we have no idea, largely because no one has even ventured a serious guess. When the phenomena were discovered, early in the 20th century, the theoretical tools for making sense of them were unavailable. One such tool is W. D.  Hamilton's (1964a)  theory of inclusive fitness, which holds that the degree of cooperation between two organisms (or tissues) must depend upon their degree of genetic relatedness. But the rise of Hamiltonian thinking coincided with the eclipse of classical cytogenetics in favor of the molecular biology of model organisms, and these remarkable little chimeras have languished in undeserved obscurity. Perhaps merely by looking at them with a modern eye, we can turn up some plausible hypotheses. Consider the special theoretical difficulty posed by chimerism between tissues derived from the oocyte and those derived from the polar bodies ejected by it during meiosis. Two siblings will typically exhibit some degree of sibling rivalry—their interests are not identical. If an individual were a chimera comprising full-sibling tissues (identical across approximately half of their genomes), there might be conflict between the two nonidentical cell lineages, as there is between the tissues of a mother mammal and her fetus (also identical across half of their genomes) during pregnancy ( Haig 2002 ). This may explain why obligate sibling chimerism never evolves (except perhaps in the very limited case of blood cells between sibling marmosets). But the problem of cooperation between tissues that derive from the oocyte and those that derive from the polar bodies is, if anything, even greater. The oocyte and the polar bodies are  less  closely related than two siblings would be, because the polar bodies are enriched for chromosome regions  not  present in the oocyte. If there were no crossing over between homologous chromosomes during meiosis, then the first meiotic division would consistently separate the chromosomes derived from the mother's mother from the chromosomes derived from the mother's father, producing two cells that are not related to each other  at all  (or, more precisely, exactly as closely related to each other as the mother's mother was to the mother's father). Crossing over prevents this, creating a mosaic of related and unrelated chromosome regions between the products of the first meiotic division and uncertain relationships between the final four meiotic products. Nonetheless, the consistently depressed relatedness between the oocyte and the polar bodies may help to explain why polar bodies are almost always eliminated—sibling rivalry might be even greater if some siblings were derived from each other's polar bodies.
Chimerism and Sibling Rivalry
What could possibly be going on here? Why should scale insects, of all creatures, have obligate chimerism involving activated polar bodies? Essentially, we have no idea, largely because no one has even ventured a serious guess. When the phenomena were discovered, early in the 20th century, the theoretical tools for making sense of them were unavailable. One such tool is W. D.  Hamilton's (1964a)  theory of inclusive fitness, which holds that the degree of cooperation between two organisms (or tissues) must depend upon their degree of genetic relatedness. But the rise of Hamiltonian thinking coincided with the eclipse of classical cytogenetics in favor of the molecular biology of model organisms, and these remarkable little chimeras have languished in undeserved obscurity. Perhaps merely by looking at them with a modern eye, we can turn up some plausible hypotheses.Consider the special theoretical difficulty posed by chimerism between tissues derived from the oocyte and those derived from the polar bodies ejected by it during meiosis. Two siblings will typically exhibit some degree of sibling rivalry—their interests are not identical. If an individual were a chimera comprising full-sibling tissues (identical across approximately half of their genomes), there might be conflict between the two nonidentical cell lineages, as there is between the tissues of a mother mammal and her fetus (also identical across half of their genomes) during pregnancy ( Haig 2002 ). This may explain why obligate sibling chimerism never evolves (except perhaps in the very limited case of blood cells between sibling marmosets). But the problem of cooperation between tissues that derive from the oocyte and those that derive from the polar bodies is, if anything, even greater. The oocyte and the polar bodies are  less  closely related than two siblings would be, because the polar bodies are enriched for chromosome regions  not  present in the oocyte.lessnotIf there were no crossing over between homologous chromosomes during meiosis, then the first meiotic division would consistently separate the chromosomes derived from the mother's mother from the chromosomes derived from the mother's father, producing two cells that are not related to each other  at all  (or, more precisely, exactly as closely related to each other as the mother's mother was to the mother's father). Crossing over prevents this, creating a mosaic of related and unrelated chromosome regions between the products of the first meiotic division and uncertain relationships between the final four meiotic products. Nonetheless, the consistently depressed relatedness between the oocyte and the polar bodies may help to explain why polar bodies are almost always eliminated—sibling rivalry might be even greater if some siblings were derived from each other's polar bodies.at allTowards an Explanation So how and why did two families of scale insects tame and domesticate their potentially fractious polar bodies, rather than killing them like normal animals do? There are at least three lines of thinking that seem promising for unraveling this mystery. Histological eusociality and relatedness. There are interesting parallels between, on the one hand, the chimerism seen in armored scale insects and, on the other hand, the eusociality (true sociality) seen in ants and honeybees. In ants and honeybees, sterile individuals (workers) provide nutrition to their potentially fertile siblings. In armored scale insects, a genetically distinct but ultimately sterile cell lineage (the bacteriome) provides nutrition to its potentially fertile “sibling” cell lineage (the rest of the scale insect)—though, of course, polar body-derived cells are “sibling” in a strange special sense. Like ants and bees, armored scale insects are effectively haplodiploid: males transmit only the chromosomes they inherited from their mother, and all of a male's sperm are identical to each other. This “clonality” of sperm boosts the relatedness between sisters, and  Hamilton (1964b)  pointed out that this high relatedness can explain the high level of cooperation between sisters seen in eusocial ants and honeybees. High levels of cooperation between sisters have since been found in haplodiploid thrips as well ( Crespi 1992 ). It is tempting to speculate that similar explanations can be applied to “histological eusociality” seen in the cooperation between related tissues in scale insects. This temptation increases when we consider another case from the old cytogenetics literature of apparent histological eusociality, though not of true permanent chimerism. This occurs in a few families of parasitoid wasps ( Tremblay and Caltagirone 1973 ;  Strand and Grbic 1997 ), in which cells derived from polar bodies form a membrane around the yolkless egg that is deposited in the wasp's insect host (which is often a scale insect!). Similar to the worker ant and the scale insect bacteriome, this membrane is thought to mediate nutrition of the developing embryo, and, similar to ants, honeybees, and scale insects, these wasps are haplodiploid. But although it is tempting to conclude that haplodiploidy plays the same role in promoting histological eusociality as it does in promoting organismal eusociality, the temptation should probably be resisted. In the case of the parasitoid wasps, the polar body-derived tissue has no paternal genome, so the clonality of sperm cannot boost its relatedness to anything. In the case of armored scale insect chimerism, the bacteriome does have a copy of the paternal genome, and that copy is identical to the paternal genome in the rest of the embryo, so in that sense the two tissues do have a high relatedness similar to the high relatedness of full siblings under haplodiploidy. But the scale insect bacteriome gets its copy of the paternal genome directly from the embryo, so the clonality of sperm (the source of elevated relatedness between haplodiploid sisters) apparently has nothing to do with it. Some other explanation is probably needed. Maternal interests. Note that, whatever the relationship between the polar body-derived tissues and the rest of the insect, the polar bodies contain the mother's complete genome. And while your polar bodies may be  less  related to you, on average, than your siblings are, they are  more  related to your siblings (and, of course, to your mother) than they are to you! Perhaps the polar bodies function to somehow enforce some maternal or family interest, nipping in the bud some sibling rivalry that would otherwise suppress the family's fitness. In haplodiploid groups, females are more closely related to their full sisters (with whom they share three-quarters of their gene copies) than to their brothers (with whom they share only one-quarter of their gene copies). So if there is competition between siblings for resources, females are expected to behave more antagonistically towards brothers and more cooperatively towards sisters. In contrast, a female is equally related to a son as she is to a daughter (each carries half of her gene copies). These asymmetries in relatedness lead to struggles in haplodiploid social groups, with a mother seeking to direct more resources towards sons and with sisters seeking to direct more resources away from their brothers and towards each other ( Seger and Stubblefield 2002 ). It is difficult to see how such a struggle might play itself out in scale insects, which are hardly social insects. The only motile stage in a female's life is the first instar (“crawler”), after which she settles in one spot permanently. The male is slightly more mobile, having a motile (usually winged) adult form. Nonetheless, (1) the low motility of females, and the fact that they live mostly on long-lived woody plants, means that maternal kin may interact over long timescales, as in social insects; (2) some scale insects appear to make relatively sophisticated social decisions about where to settle, settling nearer to (and thus possibly competing more closely with) non-kin than kin ( Kasuya 2000 ); (3) although most scale insects use phloem sap, an almost inexhaustible resource, armored scales use parenchyma tissues ( Rosen 1990 ), which may be locally exhausted, and therefore may compete against neighbors for food. Thus, it is conceivable that females may compete against brothers and sisters for resources, that they may make decisions that affect the intensity of that competition, and that such decisions may have different optima from the perspectives of maternal versus paternal genes. Possibly, the presence of a massive contingent of maternal genes (a double dose of the complete maternal genome) in a nutritionally significant tissue like the bacteriome might somehow affect such decisions in ways favorable to maternal interests. Proximal mechanisms might include effects on signals or perceptions related to relatedness, gender, site quality, or satiety. Similar manipulation of intersibling interaction might be going on in the case of the parasitoid wasps that have polar body-derived membranes around their eggs. Sometimes these wasps lay a single unfertilized (male) egg and a single fertilized (female) egg into the same host. Both eggs divide to form embryos, which divide into a clone of many embryos. Some of the embryos become sterile “precocious” larvae that can apparently attack other larvae trying to use the same host—including, potentially, their own siblings ( Ode and Hunter 2002 ). Here is a situation in which the polar body genes (in the membranes surrounding the proliferating embryos) might have very different selective optima for levels of between-sibling aggressiveness—and even for rates of development—than the genes in the embryos they surround, and (because they apparently mediate the nutrition of the embryos) they might be able to influence how the embryos develop. Gender crypsis. The endosymbiotic bacteria that dwell in the scale insects' bacteriomes are maternally inherited. Thus, from the perspective of the bacteria, male insects are deadends. Many maternally inherited bacteria have evolved to manipulate the hosts' genetic system for their own advantage. Some bacterial lineages induce parthenogenesis or feminize males. Bacteria may even evolve to suicidally kill male embryos that they find themselves in, if the death of the male frees up resources that his sisters can use ( Majerus 2003 ). In order to do this, bacteria must respond to some cue that indicates the gender of the individual they are in. Potentially, a host could evolve resistance to such manipulation by maternally inherited bacteria by depriving those bacteria of cues indicating gender. In armored scales, the bacteriome has exactly the same genome (two complete copies of the mother's diploid genome and one complete copy of the father's genome) in all full siblings, whether they are male or female, and the same is usually true in mealybugs. This might explain why the bacteriome is the only tissue in which the father's genome remains present and active in both males and females.
Towards an Explanation
So how and why did two families of scale insects tame and domesticate their potentially fractious polar bodies, rather than killing them like normal animals do? There are at least three lines of thinking that seem promising for unraveling this mystery.Histological eusociality and relatedness. There are interesting parallels between, on the one hand, the chimerism seen in armored scale insects and, on the other hand, the eusociality (true sociality) seen in ants and honeybees. In ants and honeybees, sterile individuals (workers) provide nutrition to their potentially fertile siblings. In armored scale insects, a genetically distinct but ultimately sterile cell lineage (the bacteriome) provides nutrition to its potentially fertile “sibling” cell lineage (the rest of the scale insect)—though, of course, polar body-derived cells are “sibling” in a strange special sense. Like ants and bees, armored scale insects are effectively haplodiploid: males transmit only the chromosomes they inherited from their mother, and all of a male's sperm are identical to each other. This “clonality” of sperm boosts the relatedness between sisters, and  Hamilton (1964b)  pointed out that this high relatedness can explain the high level of cooperation between sisters seen in eusocial ants and honeybees. High levels of cooperation between sisters have since been found in haplodiploid thrips as well ( Crespi 1992 ). It is tempting to speculate that similar explanations can be applied to “histological eusociality” seen in the cooperation between related tissues in scale insects. This temptation increases when we consider another case from the old cytogenetics literature of apparent histological eusociality, though not of true permanent chimerism. This occurs in a few families of parasitoid wasps ( Tremblay and Caltagirone 1973 ;  Strand and Grbic 1997 ), in which cells derived from polar bodies form a membrane around the yolkless egg that is deposited in the wasp's insect host (which is often a scale insect!). Similar to the worker ant and the scale insect bacteriome, this membrane is thought to mediate nutrition of the developing embryo, and, similar to ants, honeybees, and scale insects, these wasps are haplodiploid. But although it is tempting to conclude that haplodiploidy plays the same role in promoting histological eusociality as it does in promoting organismal eusociality, the temptation should probably be resisted. In the case of the parasitoid wasps, the polar body-derived tissue has no paternal genome, so the clonality of sperm cannot boost its relatedness to anything. In the case of armored scale insect chimerism, the bacteriome does have a copy of the paternal genome, and that copy is identical to the paternal genome in the rest of the embryo, so in that sense the two tissues do have a high relatedness similar to the high relatedness of full siblings under haplodiploidy. But the scale insect bacteriome gets its copy of the paternal genome directly from the embryo, so the clonality of sperm (the source of elevated relatedness between haplodiploid sisters) apparently has nothing to do with it. Some other explanation is probably needed.
Histological eusociality and relatedness.
There are interesting parallels between, on the one hand, the chimerism seen in armored scale insects and, on the other hand, the eusociality (true sociality) seen in ants and honeybees. In ants and honeybees, sterile individuals (workers) provide nutrition to their potentially fertile siblings. In armored scale insects, a genetically distinct but ultimately sterile cell lineage (the bacteriome) provides nutrition to its potentially fertile “sibling” cell lineage (the rest of the scale insect)—though, of course, polar body-derived cells are “sibling” in a strange special sense. Like ants and bees, armored scale insects are effectively haplodiploid: males transmit only the chromosomes they inherited from their mother, and all of a male's sperm are identical to each other. This “clonality” of sperm boosts the relatedness between sisters, and  Hamilton (1964b)  pointed out that this high relatedness can explain the high level of cooperation between sisters seen in eusocial ants and honeybees. High levels of cooperation between sisters have since been found in haplodiploid thrips as well ( Crespi 1992 ). It is tempting to speculate that similar explanations can be applied to “histological eusociality” seen in the cooperation between related tissues in scale insects.This temptation increases when we consider another case from the old cytogenetics literature of apparent histological eusociality, though not of true permanent chimerism. This occurs in a few families of parasitoid wasps ( Tremblay and Caltagirone 1973 ;  Strand and Grbic 1997 ), in which cells derived from polar bodies form a membrane around the yolkless egg that is deposited in the wasp's insect host (which is often a scale insect!). Similar to the worker ant and the scale insect bacteriome, this membrane is thought to mediate nutrition of the developing embryo, and, similar to ants, honeybees, and scale insects, these wasps are haplodiploid.But although it is tempting to conclude that haplodiploidy plays the same role in promoting histological eusociality as it does in promoting organismal eusociality, the temptation should probably be resisted. In the case of the parasitoid wasps, the polar body-derived tissue has no paternal genome, so the clonality of sperm cannot boost its relatedness to anything. In the case of armored scale insect chimerism, the bacteriome does have a copy of the paternal genome, and that copy is identical to the paternal genome in the rest of the embryo, so in that sense the two tissues do have a high relatedness similar to the high relatedness of full siblings under haplodiploidy. But the scale insect bacteriome gets its copy of the paternal genome directly from the embryo, so the clonality of sperm (the source of elevated relatedness between haplodiploid sisters) apparently has nothing to do with it. Some other explanation is probably needed.Maternal interests. Note that, whatever the relationship between the polar body-derived tissues and the rest of the insect, the polar bodies contain the mother's complete genome. And while your polar bodies may be  less  related to you, on average, than your siblings are, they are  more  related to your siblings (and, of course, to your mother) than they are to you! Perhaps the polar bodies function to somehow enforce some maternal or family interest, nipping in the bud some sibling rivalry that would otherwise suppress the family's fitness. In haplodiploid groups, females are more closely related to their full sisters (with whom they share three-quarters of their gene copies) than to their brothers (with whom they share only one-quarter of their gene copies). So if there is competition between siblings for resources, females are expected to behave more antagonistically towards brothers and more cooperatively towards sisters. In contrast, a female is equally related to a son as she is to a daughter (each carries half of her gene copies). These asymmetries in relatedness lead to struggles in haplodiploid social groups, with a mother seeking to direct more resources towards sons and with sisters seeking to direct more resources away from their brothers and towards each other ( Seger and Stubblefield 2002 ). It is difficult to see how such a struggle might play itself out in scale insects, which are hardly social insects. The only motile stage in a female's life is the first instar (“crawler”), after which she settles in one spot permanently. The male is slightly more mobile, having a motile (usually winged) adult form. Nonetheless, (1) the low motility of females, and the fact that they live mostly on long-lived woody plants, means that maternal kin may interact over long timescales, as in social insects; (2) some scale insects appear to make relatively sophisticated social decisions about where to settle, settling nearer to (and thus possibly competing more closely with) non-kin than kin ( Kasuya 2000 ); (3) although most scale insects use phloem sap, an almost inexhaustible resource, armored scales use parenchyma tissues ( Rosen 1990 ), which may be locally exhausted, and therefore may compete against neighbors for food. Thus, it is conceivable that females may compete against brothers and sisters for resources, that they may make decisions that affect the intensity of that competition, and that such decisions may have different optima from the perspectives of maternal versus paternal genes. Possibly, the presence of a massive contingent of maternal genes (a double dose of the complete maternal genome) in a nutritionally significant tissue like the bacteriome might somehow affect such decisions in ways favorable to maternal interests. Proximal mechanisms might include effects on signals or perceptions related to relatedness, gender, site quality, or satiety. Similar manipulation of intersibling interaction might be going on in the case of the parasitoid wasps that have polar body-derived membranes around their eggs. Sometimes these wasps lay a single unfertilized (male) egg and a single fertilized (female) egg into the same host. Both eggs divide to form embryos, which divide into a clone of many embryos. Some of the embryos become sterile “precocious” larvae that can apparently attack other larvae trying to use the same host—including, potentially, their own siblings ( Ode and Hunter 2002 ). Here is a situation in which the polar body genes (in the membranes surrounding the proliferating embryos) might have very different selective optima for levels of between-sibling aggressiveness—and even for rates of development—than the genes in the embryos they surround, and (because they apparently mediate the nutrition of the embryos) they might be able to influence how the embryos develop.
Maternal interests.
Note that, whatever the relationship between the polar body-derived tissues and the rest of the insect, the polar bodies contain the mother's complete genome. And while your polar bodies may be  less  related to you, on average, than your siblings are, they are  more  related to your siblings (and, of course, to your mother) than they are to you! Perhaps the polar bodies function to somehow enforce some maternal or family interest, nipping in the bud some sibling rivalry that would otherwise suppress the family's fitness. In haplodiploid groups, females are more closely related to their full sisters (with whom they share three-quarters of their gene copies) than to their brothers (with whom they share only one-quarter of their gene copies). So if there is competition between siblings for resources, females are expected to behave more antagonistically towards brothers and more cooperatively towards sisters. In contrast, a female is equally related to a son as she is to a daughter (each carries half of her gene copies). These asymmetries in relatedness lead to struggles in haplodiploid social groups, with a mother seeking to direct more resources towards sons and with sisters seeking to direct more resources away from their brothers and towards each other ( Seger and Stubblefield 2002 ).lessmoreIt is difficult to see how such a struggle might play itself out in scale insects, which are hardly social insects. The only motile stage in a female's life is the first instar (“crawler”), after which she settles in one spot permanently. The male is slightly more mobile, having a motile (usually winged) adult form. Nonetheless, (1) the low motility of females, and the fact that they live mostly on long-lived woody plants, means that maternal kin may interact over long timescales, as in social insects; (2) some scale insects appear to make relatively sophisticated social decisions about where to settle, settling nearer to (and thus possibly competing more closely with) non-kin than kin ( Kasuya 2000 ); (3) although most scale insects use phloem sap, an almost inexhaustible resource, armored scales use parenchyma tissues ( Rosen 1990 ), which may be locally exhausted, and therefore may compete against neighbors for food. Thus, it is conceivable that females may compete against brothers and sisters for resources, that they may make decisions that affect the intensity of that competition, and that such decisions may have different optima from the perspectives of maternal versus paternal genes. Possibly, the presence of a massive contingent of maternal genes (a double dose of the complete maternal genome) in a nutritionally significant tissue like the bacteriome might somehow affect such decisions in ways favorable to maternal interests. Proximal mechanisms might include effects on signals or perceptions related to relatedness, gender, site quality, or satiety.Similar manipulation of intersibling interaction might be going on in the case of the parasitoid wasps that have polar body-derived membranes around their eggs. Sometimes these wasps lay a single unfertilized (male) egg and a single fertilized (female) egg into the same host. Both eggs divide to form embryos, which divide into a clone of many embryos. Some of the embryos become sterile “precocious” larvae that can apparently attack other larvae trying to use the same host—including, potentially, their own siblings ( Ode and Hunter 2002 ). Here is a situation in which the polar body genes (in the membranes surrounding the proliferating embryos) might have very different selective optima for levels of between-sibling aggressiveness—and even for rates of development—than the genes in the embryos they surround, and (because they apparently mediate the nutrition of the embryos) they might be able to influence how the embryos develop.Gender crypsis. The endosymbiotic bacteria that dwell in the scale insects' bacteriomes are maternally inherited. Thus, from the perspective of the bacteria, male insects are deadends. Many maternally inherited bacteria have evolved to manipulate the hosts' genetic system for their own advantage. Some bacterial lineages induce parthenogenesis or feminize males. Bacteria may even evolve to suicidally kill male embryos that they find themselves in, if the death of the male frees up resources that his sisters can use ( Majerus 2003 ). In order to do this, bacteria must respond to some cue that indicates the gender of the individual they are in. Potentially, a host could evolve resistance to such manipulation by maternally inherited bacteria by depriving those bacteria of cues indicating gender. In armored scales, the bacteriome has exactly the same genome (two complete copies of the mother's diploid genome and one complete copy of the father's genome) in all full siblings, whether they are male or female, and the same is usually true in mealybugs. This might explain why the bacteriome is the only tissue in which the father's genome remains present and active in both males and females.
Gender crypsis.
The endosymbiotic bacteria that dwell in the scale insects' bacteriomes are maternally inherited. Thus, from the perspective of the bacteria, male insects are deadends. Many maternally inherited bacteria have evolved to manipulate the hosts' genetic system for their own advantage. Some bacterial lineages induce parthenogenesis or feminize males. Bacteria may even evolve to suicidally kill male embryos that they find themselves in, if the death of the male frees up resources that his sisters can use ( Majerus 2003 ). In order to do this, bacteria must respond to some cue that indicates the gender of the individual they are in. Potentially, a host could evolve resistance to such manipulation by maternally inherited bacteria by depriving those bacteria of cues indicating gender. In armored scales, the bacteriome has exactly the same genome (two complete copies of the mother's diploid genome and one complete copy of the father's genome) in all full siblings, whether they are male or female, and the same is usually true in mealybugs. This might explain why the bacteriome is the only tissue in which the father's genome remains present and active in both males and females.Prospects Scale insects and their bacteriomes challenge our notion of what an individual is. Is a scale insect's bacteriome a kind of sibling? Is it half sibling, half self? Is it a sterile slave, under control? Is it an extension of the mother, exerting control? In all other organisms, chimeras are temporary and unstable. How have scale insects suppressed the conflicts that normally tear chimeras apart? To approach such questions, we'll have to revive the empirical study of scale insect bacteriomes, combining approaches from recent studies of aphid bacteriomes ( Braendle et al. 2003 ) and of human pregnancy ( Haig 2002 ). We can better understand the nature of genetic conflicts in scale insects by studies of the genetic structure of scale insect populations, together with studies of sex ratio variation and the proximate mechanisms of sex determination. For simplicity, I have described only the most common of the huge variety of very different scale insect genetic systems and modes of bacteriome development ( Tremblay 1977 ,  1990 ;  Nur 1980 ). This diversity (greater than for the comparable cases of mammalian placentas and flowering-plant endosperms) means there is a huge scope for comparative ecological and genetic studies that could help elucidate general principles. The study of truly strange creatures can tell us what kinds of things are possible. That's why we will be so interested in any life found on another planet and why, in the meantime, we should take a close look at scale insects.
Prospects
Scale insects and their bacteriomes challenge our notion of what an individual is. Is a scale insect's bacteriome a kind of sibling? Is it half sibling, half self? Is it a sterile slave, under control? Is it an extension of the mother, exerting control? In all other organisms, chimeras are temporary and unstable. How have scale insects suppressed the conflicts that normally tear chimeras apart? To approach such questions, we'll have to revive the empirical study of scale insect bacteriomes, combining approaches from recent studies of aphid bacteriomes ( Braendle et al. 2003 ) and of human pregnancy ( Haig 2002 ). We can better understand the nature of genetic conflicts in scale insects by studies of the genetic structure of scale insect populations, together with studies of sex ratio variation and the proximate mechanisms of sex determination. For simplicity, I have described only the most common of the huge variety of very different scale insect genetic systems and modes of bacteriome development ( Tremblay 1977 ,  1990 ;  Nur 1980 ). This diversity (greater than for the comparable cases of mammalian placentas and flowering-plant endosperms) means there is a huge scope for comparative ecological and genetic studies that could help elucidate general principles. The study of truly strange creatures can tell us what kinds of things are possible. That's why we will be so interested in any life found on another planet and why, in the meantime, we should take a close look at scale insects.


Background Laparoscopic cholecystectomy is one of the commonest surgical procedures carried out in the world today. Occasionally patients present with undiagnosed situs inversus and acute cholecystitis. We discuss one such case and outline how the diagnosis was made and the pitfalls encountered during surgery and how they were overcome.
Background
Laparoscopic cholecystectomy is one of the commonest surgical procedures carried out in the world today. Occasionally patients present with undiagnosed situs inversus and acute cholecystitis. We discuss one such case and outline how the diagnosis was made and the pitfalls encountered during surgery and how they were overcome.Case presentation A 32 year old female presented to our department with epigastric pain radiating through to the back. A diagnosis of acute cholecystitis in a patient with situs inversus totalis was made following clinical examination and radiological investigation. Laparoscopic cholecystectomy was subsequently performed and the patient made an uneventful recovery.
Case presentation
A 32 year old female presented to our department with epigastric pain radiating through to the back. A diagnosis of acute cholecystitis in a patient with situs inversus totalis was made following clinical examination and radiological investigation. Laparoscopic cholecystectomy was subsequently performed and the patient made an uneventful recovery.Conclusion Situs inversus presenting with acute cholecystitis is very rare. The surgeon must appreciate that care should be taken to set up the operating theatre in the mirror image of the normal set-up for cholecystectomy, and that right handed surgeons must modify their technique to adapt to the mirror image anatomy.
Conclusion
Situs inversus presenting with acute cholecystitis is very rare. The surgeon must appreciate that care should be taken to set up the operating theatre in the mirror image of the normal set-up for cholecystectomy, and that right handed surgeons must modify their technique to adapt to the mirror image anatomy.

Background In 1600 the first known case of situs inversus in humans was reported by Fabricius [ 1 ]. The incidence is thought to be in the region of 1:5000 to 1:20000 [ 2 ]. The condition may affect the thoracic organs, abdominal organs or both. It is associated with a number of other conditions such as Kartagener's (bronchiectasis, sinusitis, situs inversus) and cardiac anomalies. There is no current evidence that situs inversus predisposes to cholelithiasis [ 3 ]. Since Mouret first performed it in 1987, laparoscopic cholecystectomy has become the standard operative procedure for gallbladder disease. It is associated with reduced hospital stay, fewer respiratory complications, less pain and a faster return to work.
Background
In 1600 the first known case of situs inversus in humans was reported by Fabricius [ 1 ]. The incidence is thought to be in the region of 1:5000 to 1:20000 [ 2 ]. The condition may affect the thoracic organs, abdominal organs or both. It is associated with a number of other conditions such as Kartagener's (bronchiectasis, sinusitis, situs inversus) and cardiac anomalies. There is no current evidence that situs inversus predisposes to cholelithiasis [ 3 ].Since Mouret first performed it in 1987, laparoscopic cholecystectomy has become the standard operative procedure for gallbladder disease. It is associated with reduced hospital stay, fewer respiratory complications, less pain and a faster return to work.Case presentation A thirty two year old female was admitted with a three hour history of epigastric pain radiating into her back in keeping with biliary colic. She had vomited a number of times. In the previous week she had two episodes of a similar nature. On examination there was no jaundice or pyrexia. The apex beat was in the right fifth intercostal space, midclavicular line. She had epigastric tenderness but was not tender in the right or left upper quadrants. Her white cell count and amylase level was normal but her C-reactive protein level (CRP) was elevated at 290 mg/L. An electrocardiograph showed right axis deviation and right ventricular hypertrophy, in keeping with dextrocardia. An ultrasound scan of the upper abdomen identified the gallbladder, which contained stones, in the left upper quadrant. The spleen was visualised in the right upper quadrant. There was no evidence of common bile duct or intrahepatic duct dilatation. Chest X-Ray confirmed the clinical and electrocardiograph diagnosis of dextrocardia. The diagnosis of acute cholecystitis and situs inversus was made. The patient settled clinically over two to three days and was discharged home to be admitted electively for laparoscopic cholecystectomy. In order to conduct the laparoscopic cholecystectomy all theatre equipment including diathermy, monitors and CO 2  insufflator were positioned in the mirror image of their normal position. The surgical team also changed sides with the primary surgeon and first assistant on the patients right and the second assistant on the left. The ports were inserted in the usual way but on the left side. At laparoscopy the entirety of the abdominal contents were indeed reversed. The main difficulty encountered was that the primary surgeon, who was right handed, would have had to cross hands to retract on Hartmann's pouch while dissecting Calot's triangle. We overcame this difficulty by allowing the first assistant to retract on Hartmann's pouch, while the primary surgeon dissected Calot's triangle using his right hand via the epigastric port without hindrance. The common bile duct and cystic duct were identified, as was the cystic artery, which lay anterior to the cystic duct. The surgery proceeded without incident and the patient recovered and was discharged the next day.
Case presentation
A thirty two year old female was admitted with a three hour history of epigastric pain radiating into her back in keeping with biliary colic. She had vomited a number of times. In the previous week she had two episodes of a similar nature.On examination there was no jaundice or pyrexia. The apex beat was in the right fifth intercostal space, midclavicular line. She had epigastric tenderness but was not tender in the right or left upper quadrants. Her white cell count and amylase level was normal but her C-reactive protein level (CRP) was elevated at 290 mg/L. An electrocardiograph showed right axis deviation and right ventricular hypertrophy, in keeping with dextrocardia.An ultrasound scan of the upper abdomen identified the gallbladder, which contained stones, in the left upper quadrant. The spleen was visualised in the right upper quadrant. There was no evidence of common bile duct or intrahepatic duct dilatation. Chest X-Ray confirmed the clinical and electrocardiograph diagnosis of dextrocardia.The diagnosis of acute cholecystitis and situs inversus was made. The patient settled clinically over two to three days and was discharged home to be admitted electively for laparoscopic cholecystectomy.In order to conduct the laparoscopic cholecystectomy all theatre equipment including diathermy, monitors and CO 2  insufflator were positioned in the mirror image of their normal position. The surgical team also changed sides with the primary surgeon and first assistant on the patients right and the second assistant on the left. The ports were inserted in the usual way but on the left side. At laparoscopy the entirety of the abdominal contents were indeed reversed.The main difficulty encountered was that the primary surgeon, who was right handed, would have had to cross hands to retract on Hartmann's pouch while dissecting Calot's triangle. We overcame this difficulty by allowing the first assistant to retract on Hartmann's pouch, while the primary surgeon dissected Calot's triangle using his right hand via the epigastric port without hindrance. The common bile duct and cystic duct were identified, as was the cystic artery, which lay anterior to the cystic duct. The surgery proceeded without incident and the patient recovered and was discharged the next day.Conclusion In this case the patient presented with epigastric pain only and had no definite left upper quadrant pain. It has been noted in 30% of previous reported cases of acute cholecystitis in patients with situs inversus that the pain was felt in the epigastrium alone and in 10% the pain was localised to the right upper quadrant [ 4 ]. The proposed explanation for this is that the central nervous system may not share in the general transposition. Previous reports have confirmed that situs inversus is not a contraindication for laparoscopic cholecystectomy [ 1 - 3 , 5 , 6 ]. The procedure is, however, more difficult and care and time must be taken to re-arrange the equipment set-up in theatre, and to recognise the mirror-image anatomy which can cause difficulties with orientation. At least two thirds of surgeons are right handed. It is necessary for these surgeons, and their assistants, to modify their usual surgical technique to comfortably and safely carry out the procedure. Rather than the clumsy crossing of hands to retract on Hartmann's pouch for dissection of Calot's triangle, we suggest that retraction on Hartmann's pouch may be carried out by the assistant, thus allowing the surgeon to operate in a more ergodynamic fashion. Learning points 1. Detailed clinical examination is important in diagnosing previously unknown situs inversus. 2. Patients with gallabladder disease and situs inversus may have pain in the right upper quadrant, epigastrium or left upper quadrant. 3. Theatre equipment must be moved to the mirror image of their normal positions before surgery. 4. The surgeon must recognise the mirror image anatomy and modify his or her technique appropriately.
Conclusion
In this case the patient presented with epigastric pain only and had no definite left upper quadrant pain. It has been noted in 30% of previous reported cases of acute cholecystitis in patients with situs inversus that the pain was felt in the epigastrium alone and in 10% the pain was localised to the right upper quadrant [ 4 ]. The proposed explanation for this is that the central nervous system may not share in the general transposition.Previous reports have confirmed that situs inversus is not a contraindication for laparoscopic cholecystectomy [ 1 - 3 , 5 , 6 ]. The procedure is, however, more difficult and care and time must be taken to re-arrange the equipment set-up in theatre, and to recognise the mirror-image anatomy which can cause difficulties with orientation. At least two thirds of surgeons are right handed. It is necessary for these surgeons, and their assistants, to modify their usual surgical technique to comfortably and safely carry out the procedure. Rather than the clumsy crossing of hands to retract on Hartmann's pouch for dissection of Calot's triangle, we suggest that retraction on Hartmann's pouch may be carried out by the assistant, thus allowing the surgeon to operate in a more ergodynamic fashion.Learning points 1. Detailed clinical examination is important in diagnosing previously unknown situs inversus. 2. Patients with gallabladder disease and situs inversus may have pain in the right upper quadrant, epigastrium or left upper quadrant. 3. Theatre equipment must be moved to the mirror image of their normal positions before surgery. 4. The surgeon must recognise the mirror image anatomy and modify his or her technique appropriately.
Learning points
1. Detailed clinical examination is important in diagnosing previously unknown situs inversus.2. Patients with gallabladder disease and situs inversus may have pain in the right upper quadrant, epigastrium or left upper quadrant.3. Theatre equipment must be moved to the mirror image of their normal positions before surgery.4. The surgeon must recognise the mirror image anatomy and modify his or her technique appropriately.Abbreviations CRP- C reactive protein. CO 2 - Carbon dioxide.
Abbreviations
CRP- C reactive protein.CO 2 - Carbon dioxide.Competing interests The author(s) declare that they have no competing interests.
Competing interests
The author(s) declare that they have no competing interests.Pre-publication history The pre-publication history for this paper can be accessed here:
Pre-publication history
The pre-publication history for this paper can be accessed here:


Background To investigate whether low birth weight increases the risk of myocardial infarction later in life in women.
Background
To investigate whether low birth weight increases the risk of myocardial infarction later in life in women.Methods Nationwide population-based case-control study. Patients and controls: 152 patients with a first myocardial infarction before the age of 50 years in the Netherlands. 568 control women who had not had a myocardial infarction stratified for age, calendar year of the index event, and area of residence.
Methods
Nationwide population-based case-control study. Patients and controls: 152 patients with a first myocardial infarction before the age of 50 years in the Netherlands. 568 control women who had not had a myocardial infarction stratified for age, calendar year of the index event, and area of residence.Results Birth weight in the patient group was significantly lower than in control women (3214 vs. 3370 gram, mean difference -156.3 gram (95%CI -9.5 to -303.1). The odds ratio for myocardial infarction, associated with a birth weight lower than 3000 gram (20 th  percentile in controls) compared to higher than 3000 gram was 1.7 (95%CI 1.1–2.7), while the odds ratio for myocardial infarction for children with a low birth weight (< 2000 g) compared to a birth weight ≥ 2000 g was 2.4 (95%CI 1.0 – 5.8). Both figures did not change after adjustment for putative confounders (age, education level, body mass index, waist-hip ratio, hypertension, diabetes, hypercholesterolemia, smoking, and family history of cardiovascular disease).
Results
Birth weight in the patient group was significantly lower than in control women (3214 vs. 3370 gram, mean difference -156.3 gram (95%CI -9.5 to -303.1). The odds ratio for myocardial infarction, associated with a birth weight lower than 3000 gram (20 th  percentile in controls) compared to higher than 3000 gram was 1.7 (95%CI 1.1–2.7), while the odds ratio for myocardial infarction for children with a low birth weight (< 2000 g) compared to a birth weight ≥ 2000 g was 2.4 (95%CI 1.0 – 5.8). Both figures did not change after adjustment for putative confounders (age, education level, body mass index, waist-hip ratio, hypertension, diabetes, hypercholesterolemia, smoking, and family history of cardiovascular disease).Conclusions Low birth weight is associated with an increased risk of myocardial infarction before age of 50 in Dutch women.
Conclusions
Low birth weight is associated with an increased risk of myocardial infarction before age of 50 in Dutch women.

Background Intrauterine malnutrition, as reflected by birth weight and abnormal thinness at birth, has been associated with an increased incidence of risk factors for arterial disease, i.e. hypertension, impaired glucose tolerance, diabetes and to a lesser extent hyperlipidemia and body fat distribution in adulthood [ 1 - 10 ]. This observation has become known as the 'fetal origins of adult disease' or 'Barker hypothesis', which suggests that several of the major diseases of later life, including coronary heart disease, stroke and cardiovascular death, originate in impaired intrauterine growth and development [ 11 , 12 ]. In cohort studies, Barker [ 13 - 15 ] in England and Finland, Rich-Edwards  et al . [ 16 ] as part of the Nurses' Health Study in the USA and Leon  et al.  [ 17 ] from Uppsala in Sweden showed an inverse relationship between birth weight and the clinical endpoint ischemic heart disease. Leon  et al.  [ 17 ] found a significant relationship only among male singletons and adjusted their results for gestational age and socioeconomic confounding. The association was not found in a cohort study from Gothenburg [ 18 ]. Our aim was to investigate the association in a case control study among Dutch women.
Background
Intrauterine malnutrition, as reflected by birth weight and abnormal thinness at birth, has been associated with an increased incidence of risk factors for arterial disease, i.e. hypertension, impaired glucose tolerance, diabetes and to a lesser extent hyperlipidemia and body fat distribution in adulthood [ 1 - 10 ]. This observation has become known as the 'fetal origins of adult disease' or 'Barker hypothesis', which suggests that several of the major diseases of later life, including coronary heart disease, stroke and cardiovascular death, originate in impaired intrauterine growth and development [ 11 , 12 ]. In cohort studies, Barker [ 13 - 15 ] in England and Finland, Rich-Edwards  et al . [ 16 ] as part of the Nurses' Health Study in the USA and Leon  et al.  [ 17 ] from Uppsala in Sweden showed an inverse relationship between birth weight and the clinical endpoint ischemic heart disease. Leon  et al.  [ 17 ] found a significant relationship only among male singletons and adjusted their results for gestational age and socioeconomic confounding. The association was not found in a cohort study from Gothenburg [ 18 ].et alet al.et al.Our aim was to investigate the association in a case control study among Dutch women.Methods The RATIO (Risk of Arterial Thrombosis In relation to Oral contraceptives) study is a population-based case-control study on myocardial infarction in relation to oral contraceptive use among women aged 18 to 49 years in the Netherlands [ 19 ]. An additional standardized questionnaire was sent to all 218 patients and 769 controls from whom also blood samples had been taken for determination of metabolic risk factors (diabetes and hypercholesterolemia). Questions elicited information on birth weight, waist and hip circumference and data on the menstrual cycle. For 13 women no current address could be found (12 patients, 1 control). Four women had died since the index date (2 patients, 2 controls), which was the date of the first myocardial infarction for the patients and the midyear for the controls. One hundred and fifty two patients (71%) and 568 controls (75%) responded to the questionnaire and women were asked to measure their waist and hip circumference. Body mass index (BMI) was calculated as body weight (kg) divided by height squared (m 2 ). Waist-hip-ratio was calculated as waist circumference divided by hip circumference. Multiple linear and unconditional logistic regression were used to analyze the data. Odds ratios for the relationship between birth weight and myocardial infarction were calculated and 95% confidence intervals (95%CI) were derived from the models. Birth weights were categorized according to quintiles in control women in order to investigate an association between birth weight and the risk of myocardial infarction later in life. These were <3000 g, 3000 to 3199 g, 3200 to 3499 g, 3500 to 3883 g, and >3884 g, respectively. To determine whether women with a lower birth weight had a higher risk for a myocardial infarction, patients were divided in a group with a birth weight equally or higher than 2000 g and a group with a birth weight lower 2000 g [ 20 ]. Odds ratios were adjusted for age, education level, body mass index, waist-hip ratio, hypertension, diabetes, hypercholesterolemia, smoking, and family history of cardiovascular disease, when appropriate. Interaction between low birth weight and low education level was investigated by computing a dummy variable.
Methods
The RATIO (Risk of Arterial Thrombosis In relation to Oral contraceptives) study is a population-based case-control study on myocardial infarction in relation to oral contraceptive use among women aged 18 to 49 years in the Netherlands [ 19 ]. An additional standardized questionnaire was sent to all 218 patients and 769 controls from whom also blood samples had been taken for determination of metabolic risk factors (diabetes and hypercholesterolemia). Questions elicited information on birth weight, waist and hip circumference and data on the menstrual cycle. For 13 women no current address could be found (12 patients, 1 control). Four women had died since the index date (2 patients, 2 controls), which was the date of the first myocardial infarction for the patients and the midyear for the controls. One hundred and fifty two patients (71%) and 568 controls (75%) responded to the questionnaire and women were asked to measure their waist and hip circumference. Body mass index (BMI) was calculated as body weight (kg) divided by height squared (m 2 ). Waist-hip-ratio was calculated as waist circumference divided by hip circumference.Multiple linear and unconditional logistic regression were used to analyze the data. Odds ratios for the relationship between birth weight and myocardial infarction were calculated and 95% confidence intervals (95%CI) were derived from the models. Birth weights were categorized according to quintiles in control women in order to investigate an association between birth weight and the risk of myocardial infarction later in life. These were <3000 g, 3000 to 3199 g, 3200 to 3499 g, 3500 to 3883 g, and >3884 g, respectively. To determine whether women with a lower birth weight had a higher risk for a myocardial infarction, patients were divided in a group with a birth weight equally or higher than 2000 g and a group with a birth weight lower 2000 g [ 20 ]. Odds ratios were adjusted for age, education level, body mass index, waist-hip ratio, hypertension, diabetes, hypercholesterolemia, smoking, and family history of cardiovascular disease, when appropriate. Interaction between low birth weight and low education level was investigated by computing a dummy variable.Results The characteristics of 152 women with myocardial infarction and 568 control women at the index date are shown in Table  1 . At the moment of completing the questionnaire, patients were aged 32–59 years (mean 50), and control women 25–60 years (mean 47). The mean body mass index was 25.1 kg/m 2  for the patients and 23.4 kg/m 2  for control women, mean difference 1.76 kg/m 2  (95%CI 1.05–2.47), p < 0.001. Ninety-seven patients (64%) and 415 (73%) controls could give their birth weight. Compared with control women, patients had a significantly lower mean birth weight (3214 vs. 3370 g, mean difference -156.3 g (95%CI -9.5 to -303.1). The odds ratio for myocardial infarction for children with a low birth weight (< 2000 g) compared to a birth weight ≥ 2000 g was 2.4 (95%CI 1.0 to 5.8). After adjustment for putative confounders (age, education level, body mass index, waist-hip ratio, hypertension, diabetes, hypercholesterolemia, smoking, and family history of cardiovascular disease) the odds ratio did not change. Odds ratios for myocardial infarction in different categories of birth weight as compared to the reference category (birth weight higher than 3884 g) were 1.3 (95%CI 0.5–3.3) for a birth weight 3500 to 3883 g, 1.4 (95%CI 0.6–3.4) for a birth weight 3200 to 3499 g, 1.7 (95%CI 0.6–5.1) for a birth weight 3000 to 3199 g, and 2.3 (95%CI 1.0–5.4) for a birth weight lower than 3000 g (Table  2 ). The risk of myocardial infarction was 6.2 fold increased (95%CI 2.7–13.9) among women with low birth weight and a low educational level compared to women with a high birth weight and a high educational level (reference category). Table 1 Characteristics of patients with a first myocardial infarction and control women Characteristic Patients (N = 152) Control women (N = 568) Age – yr (SD) 42.1 (0.5) 38.6 (0.3) Caucasian ethnicity (%) 142 (93) 538 (95) Educational level - Primary school or less (%) 83 (55) 160 (28) - Secondary school (%) 52 (34) 257 (45) - Higher education or university (%) 17 (11) 149 (26) Current smokers (%) 128 (84) 218 (39) History of hypertension (%) 35 (23) 35 (6) History of hypercholesterolemia (%) 16 (11) 14 (3) History of diabetes (%) 8 (5) 7 (1) Family history of cardiovascular disease (%) 98 (66) 194 (36) Birth weight-gram - Mean (SD) 3214 (676) 3370 (659) - Median (range) 3150 (1500–5010) 3500 (1500–5800) Body Mass Index – kg/m 2  – Mean (SD) 25.1 (0.4) 23.4 (0.2) Waist circumference – cm (SD) 89.5 (13.1) 83.0 (10.2) Waist/hip ratio (SD) 0.85 (0.006) 0.81 (0.008) Premenopausal (%) 132 (87) 480 (85) Table 2 Odds ratios (95%CI) for myocardial infarction in quintiles of birth weight as compared to the reference category Birth weight (g) Odds Ratio (95% CI)  > 3884 1*  3500–3883 1.3 (0.5–3.3)  3200–3499 1.4 (0.6–3.4)  3000–3199 1.7 (0.6–3.4)  < 3000 2.3 (1.0–5.4) * Reference category
Results
The characteristics of 152 women with myocardial infarction and 568 control women at the index date are shown in Table  1 . At the moment of completing the questionnaire, patients were aged 32–59 years (mean 50), and control women 25–60 years (mean 47). The mean body mass index was 25.1 kg/m 2  for the patients and 23.4 kg/m 2  for control women, mean difference 1.76 kg/m 2  (95%CI 1.05–2.47), p < 0.001. Ninety-seven patients (64%) and 415 (73%) controls could give their birth weight. Compared with control women, patients had a significantly lower mean birth weight (3214 vs. 3370 g, mean difference -156.3 g (95%CI -9.5 to -303.1). The odds ratio for myocardial infarction for children with a low birth weight (< 2000 g) compared to a birth weight ≥ 2000 g was 2.4 (95%CI 1.0 to 5.8). After adjustment for putative confounders (age, education level, body mass index, waist-hip ratio, hypertension, diabetes, hypercholesterolemia, smoking, and family history of cardiovascular disease) the odds ratio did not change. Odds ratios for myocardial infarction in different categories of birth weight as compared to the reference category (birth weight higher than 3884 g) were 1.3 (95%CI 0.5–3.3) for a birth weight 3500 to 3883 g, 1.4 (95%CI 0.6–3.4) for a birth weight 3200 to 3499 g, 1.7 (95%CI 0.6–5.1) for a birth weight 3000 to 3199 g, and 2.3 (95%CI 1.0–5.4) for a birth weight lower than 3000 g (Table  2 ). The risk of myocardial infarction was 6.2 fold increased (95%CI 2.7–13.9) among women with low birth weight and a low educational level compared to women with a high birth weight and a high educational level (reference category).Characteristics of patients with a first myocardial infarction and control womenCharacteristicPatients (N = 152)Control women (N = 568)Odds ratios (95%CI) for myocardial infarction in quintiles of birth weight as compared to the reference categoryBirth weight (g)Odds Ratio (95% CI)* Reference categoryDiscussion In this case control study we have found that women with a low birth weight had a higher risk of myocardial infarction than women with a higher birth weight. The data confirm the association found in cohort studies [ 13 - 17 ] and as such support the 'fetal origins of adult disease' or 'Barker hypothesis'. A limitation of the study is that we have no information on gestational age. However, recently it has been demonstrated that both children who had been born prematurely and children who are small for gestational age had a reduction in insulin resistance [ 21 , 22 ]. The self-report of birth weights as well as the rather high percentage of missing values for birth weight may also limit the study, but the random events among cases and controls cannot explain our results. Socio-economic factors associated with low birth weight are also associated to risk factors for arterial disease later in life. As pointed out by several others, it will be nearly impossible to disentangle these effects [ 16 , 17 ]. In the present study we confirmed an interactive effect between low birth weight and a low educational level on the risk of myocardial infarction in women. However, when we adjusted for age, education level, body mass index, waist-hip ratio, hypertension, diabetes, hypercholesterolemia, smoking, and family history of cardiovascular disease, factors of which some may partly been seen as (the result of) socio-economic/environmental and genetic factors, we still observed an association between low birth weight and a higher risk of myocardial infarction. This is in agreement with others who also found that genetic and socio-economic circumstances at birth and in adult life can not completely explain the association between low birth weight and disease late in life [ 10 , 16 , 17 , 23 - 25 ]. Among the proposed underlying biological mechanisms to explain the association is impaired endothelial development. Already at very young age, individuals with low birth weight exhibit endothelial dysfunction that persists into childhood and adult life, suggesting that endothelial dysfunction precedes the development of vascular related diseases later in life and represents the link between low birth weight and these diseases [ 26 - 32 ]. Furthermore, Smith  et al . [ 33 ] found that mothers, who once gave birth to thin babies, have a higher risk of developing ischemic heart disease later in life. Therefore, these mothers seem to have, just as their children, an impaired endothelial function. If the impaired endothelial function is already manifest during the process of implantation, it might lead to inadequate development of the vasculature in the maternal part of the placenta, which enfeebles the function of the placenta resulting in low birth weight.
Discussion
In this case control study we have found that women with a low birth weight had a higher risk of myocardial infarction than women with a higher birth weight. The data confirm the association found in cohort studies [ 13 - 17 ] and as such support the 'fetal origins of adult disease' or 'Barker hypothesis'.A limitation of the study is that we have no information on gestational age. However, recently it has been demonstrated that both children who had been born prematurely and children who are small for gestational age had a reduction in insulin resistance [ 21 , 22 ]. The self-report of birth weights as well as the rather high percentage of missing values for birth weight may also limit the study, but the random events among cases and controls cannot explain our results. Socio-economic factors associated with low birth weight are also associated to risk factors for arterial disease later in life. As pointed out by several others, it will be nearly impossible to disentangle these effects [ 16 , 17 ].In the present study we confirmed an interactive effect between low birth weight and a low educational level on the risk of myocardial infarction in women. However, when we adjusted for age, education level, body mass index, waist-hip ratio, hypertension, diabetes, hypercholesterolemia, smoking, and family history of cardiovascular disease, factors of which some may partly been seen as (the result of) socio-economic/environmental and genetic factors, we still observed an association between low birth weight and a higher risk of myocardial infarction. This is in agreement with others who also found that genetic and socio-economic circumstances at birth and in adult life can not completely explain the association between low birth weight and disease late in life [ 10 , 16 , 17 , 23 - 25 ].Among the proposed underlying biological mechanisms to explain the association is impaired endothelial development. Already at very young age, individuals with low birth weight exhibit endothelial dysfunction that persists into childhood and adult life, suggesting that endothelial dysfunction precedes the development of vascular related diseases later in life and represents the link between low birth weight and these diseases [ 26 - 32 ]. Furthermore, Smith  et al . [ 33 ] found that mothers, who once gave birth to thin babies, have a higher risk of developing ischemic heart disease later in life. Therefore, these mothers seem to have, just as their children, an impaired endothelial function. If the impaired endothelial function is already manifest during the process of implantation, it might lead to inadequate development of the vasculature in the maternal part of the placenta, which enfeebles the function of the placenta resulting in low birth weight.et alConclusions In conclusion, our study shows that a low birth weight (<2000 g) is associated with a 2.4 fold higher risk of myocardial infarction before the age of 50 as compared with a birth weight ≥ 2000 g. Because the risk of cardiovascular disease is known to increase with an increasing number of risk factors, women with a low birth weight should try to avoid acquired risk factors, like smoking and obesity. In addition, extra attention should be given in detecting diabetes or hypertension at a later stage in life and in detecting exaggerated growth during childhood, as these individuals seem most prone to develop disease later on in life [[ 34 - 36 ]].
Conclusions
In conclusion, our study shows that a low birth weight (<2000 g) is associated with a 2.4 fold higher risk of myocardial infarction before the age of 50 as compared with a birth weight ≥ 2000 g. Because the risk of cardiovascular disease is known to increase with an increasing number of risk factors, women with a low birth weight should try to avoid acquired risk factors, like smoking and obesity. In addition, extra attention should be given in detecting diabetes or hypertension at a later stage in life and in detecting exaggerated growth during childhood, as these individuals seem most prone to develop disease later on in life [[ 34 - 36 ]].Competing interests The author(s) declare that they have no competing interests.
Competing interests
The author(s) declare that they have no competing interests.Authors' contributions BT participated in the design, execution and analysis of the RATIO- study and drafted the manuscript. KK analyzed the data and drafted the manuscript. RH collected the data and performed statistical analyses. FR initiated the study and helped to draft the manuscript. FH participated in the design of the study and the writing of the paper.
Authors' contributions
BT participated in the design, execution and analysis of the RATIO- study and drafted the manuscript.KK analyzed the data and drafted the manuscript.RH collected the data and performed statistical analyses.FR initiated the study and helped to draft the manuscript.FH participated in the design of the study and the writing of the paper.