The picobirnaviruses  are currently thought to be animal viruses, as they are usually found in animal stool samples. However, no animal model or cell culture for their propagation has yet been found. In 2018, a hypothetical assumption about PBVs belonging to prokaryotic viruses was put forward and experimentally substantiated. This hypothesis is based on the presence of Shine–Dalgarno sequences in the genome of all PBVs before three reading frames (ORF) at the ribosomal binding site, with which the prokaryotic genome is saturated, while in the eukaryotic genome such regions occur with low frequency. The genome saturation with the Shine–Dalgarno sequences, as well as the preservation of this saturation in the progeny, according to scientists, allows us to attribute PBVs to prokaryotic viruses. On the other hand, there is a possibility that PBVs belong to viruses of eukaryotic hosts – fungi or invertebrates, since PBV-like sequences similar to the genome of fungal viruses from the families of mitoviruses and partitiviruses have been identified. In this regard, the idea arose that, in terms of reproduction mode, PBVs resemble fungal viruses. The divergence of views on the true PBV host(s) has sparked discussions among scientists and required further research to elucidate their nature. The review highlights the results of the search for a PBV host. The reasons for the occurrence of atypical sequences among the PBV genome sequences that use an alternative mitochondrial code of lower eukaryotes (fungi and invertebrates) for the translation of viral RNA-dependent RNA polymerase (RdRp) instead of the standard genetic code are analyzed. The purpose of the review was to collect arguments in support of the hypothesis about the phage nature of PBVs and to find the most realistic explanation of the reasons for identifying non-standard genomic sequences for PBVs. Based on the hypothesis about the genealogical relationship of PBVs with RNA viruses from other families with similar segmented genomes, such as Reoviridae, Cystoviridae, Totiviridae and Partitiviridae, virologists support the assumption of a decisive role in the origin of atypical PBV-like reassortment strains between PBVs and viruses of the listed families. The collected arguments given in this review indicate a high probability of a phage nature of PBVs. The data presented in the review show that the belonging of PBV-like progeny to prokaryotic or eukaryotic viruses is determined not only by its genome saturation level with a prokaryotic motif, standard or mitochondrial genetic code. The primary structure of the gene encoding the viral capsid protein responsible for the presence or absence of specific proteolytic properties of the virus that determine its ability for independent horizontal transmission into new cells may also be a decisive factor. Picobirnaviruses (PBVs) are small, nonenveloped bisegmenteddouble-stranded RNA viruses that have been detected ina wide variety of animal species including invertebrates and inenvironmental samples. Since PBVs are ubiquitous in faeces/gut contents of humans and other animals with or without diarrhea,they were considered opportunistic entericpathogens ofmammals and avian species. However, an animalcell cultureor a gnotobiotic animal for propagation of this virus has notyet been identified. This fact led some researchers to doubt thatpicobirnaviruses belong to eukaryotic viruses. Indian scientistsKrishnamurthy, Wang (2018) have analyzed a large number offull-size  genomic sequences of PBVs foundin faeces of humans and animals as well as environmentalsamples. This analysis revealed prokaryoticmotifs (regions)in the PBV genome located before the open reading framesat the ribosomal binding site. Based on the data obtained,a hypothesis was put forward and experimentally substantiatedthat PBVs belong to prokaryotic viruses – bacteriophages, which was later supported bya number of other studies .On the other hand, after the discovery of new PBV-likenucleotide sequences encoding RNA-dependent RNA polymerase,but using an alternative (non-standard) mitochondrialgenetic code (of molds or invertebrates) for translation, it wassuggested that PBVs could be fungal viruses with a reproductionmode resembling that of mitoviruses .These contradictions, which have caused a discussion inthe scientific community on the nature of the true host(s) ofPBVs, can be resolved by a hypothesis put forward in 2018by Wolf et al. (2018) and supported by other researchers. This hypothesis explains the originof abnormal strains of PBVs by the tendency of these virusesto abrupt genetic modification following the reassortment ofgenome segments described earlier  and the acquisition of the ability of the bacteriophageto reproduce in the cells of the organism of another taxonomicgroup – the lower eukaryote.This review analyzes the available publications on modernideas about the origin and evolution of PBVs, as well as adiscussion on the prokaryotic or eukaryotic nature of their truehosts. The purpose of the review was to collect arguments tosupport the hypothesis about the phage nature of PBVs andto search for the most realistic explanation of the reasons forthe identification of genomic sequences that are nonstandardfor viruses.Picobirnaviruses (PBVs) are small, noneveloped particles33–37 nm in diameter with icosahedral type of symmetry(T = 2) belonging to the only genus Picobirnavirus withinthe family Picobirnaviridae of the order Diplohnavirales.Double-stranded (ds) RNA-genome of PBVs consists of twosegments 2525 and 1745 bp in length . Informationabout the structure of the virion and genome of PBV, the areaof prevalence, connection with diarrhea, the level of excretion,the opportunistic  and zoonotic natureof the virus, the wide tissue tropism and genetic variabilityis given in previously published reviews . The methodsof amplification, PCR diagnostics and genome sequencing ofthese viruses are also described thereUsing phylogenetic analysis based on the nucleotide sequenceof the RNA-dependent RNA polymerase (RdRp) genelocated in the segment 2 of the genome, researchers dividePBVs into five genogroups: GI, GII , GIII, GIV and GV , amongwhich there are genetically variable clusters. GenogroupsGI and GII are more common in the PBV cluster detectedin vertebrates and humans. Genogroup GIII has been identifiedin invertebrates  while genogroups GIVand GV have been identified in fungal and prokaryotic hostcells . All five PBV genogroups identifiedin one host (marmot) are shown in the phylogram  inthe study . The main PBV genogroups aregenogroups GI and GII, of which PBVs of genogroup GIare the most common .It has been established that viruses belonging to the sameRdRp-genogroup can be detected in suspected hosts belongingto different species. It has also been shown that PBVsof different genogroups are detected in the host of the samespecies . At the same time,the true host of viruses has not yet been identified. Among thehigher eukaryotes, they did not succeed in identifying eithera cell culture or gnotobiotic animals for the virus propagation.In the future, as the PBV studies continued, researchersbegan to doubt the fact that the cells of higher eukaryotescould be the hosts of these viruses . Recently ithas been discovered that PBVs differ from dsRNA viruses ofhigher eukaryotes (Reoviridae) not only in the architecture ofthe capsid, but also in presumably being able to infect prokaryoticcells .Prior to the hypothesis of Krishnamurthy and Wang (2018),PBVs were thought to be eukaryotic viruses because theywere identified in a wide variety of animal species, includinginvertebrates. Since PBVs are ubiquitous in the gut contentsof humans and other animals with or without diarrhea, theywere considered opportunistic enteric pathogens. But intestinalvirom in animals contains not only eukaryotic, but alsoprokaryotic viruses, which usually make up the largest shareof it . It was logical to assume that PBVsare not present in the gut cells, but in the gut contents and canbe prokaryotic viruses of the gut microbiome .In this case, the level of virus identification should correspondto the number of bacteria in which they multiply. Inparticular, the study of Kleymann et al. (2020) reported highrates of identification of GI PBVs  in Indianmongoose stool samples on the island of St. Kitts (one of the Lesser Antilles). The percentage of PBV identification onthe island of St. Kitts could mean the concentration of hostbacteria in this area. Moreover, there was a difference in thefrequency of PBV identification in mongooses from the urbanand wild habitats, 33.33 % (19/57) and 40.00 % (10/25)respectively, which could indicate a difference in bacterialload in these placesThe suggestion that the PBVs may be prokaryotic virusesappeared when researchers began to analyze the diversity offull-sized  sequences of their genome. Withthe help of next-generation sequencing technologies (NGS)and polymerase chain reaction (PCR) using specific primers,they succeeded in identifying some features of the PBV genomethat indicate that these viruses may actually be prokaryoticor fungal viruses In 2018, while analyzing different reference genomes ofRNA-containing viruses individually and at the family level,Indian scientists Krishnamurthy and Wang have discoveredconservative regions in the PBV genome called Shine–Dalgarnosequences or SD-sequences .Such regions are present in the genomes of all prokaryotic andeukaryotic viruses and usually consist of six nucleotides –AGGAGG.They are located in the 5′-untranslated regionbefore the open reading frames (ORFs) at a distance of 1 to18 nucleotides (spacer region) to the start codon (AUG) initiatingthe translation of the viral genome products . FunctionalSD-sequences are ribosome binding sites and promote thetranslation of viral proteins.However, genome enrichment with SD-sequences was observedonly in families of viruses that infect prokaryotes, butnot in families infecting eukaryotes. This observation made itpossible for Krishnamurthy and Wang to suggest that the highfrequency of appearance of SD-sequences in the viral genomesmay be a defining feature of the prokaryotic type of virus, andany viral families the genomes of which are enriched with suchSD-regions are prokaryotic viral families. Among the virusesinfecting prokaryots, for example, some bacteriophages of thefamily Cystoviridae have a high content of SD-sequences,the genome of which consists of several fragments of dsRNA.In the PBV genome, SD-sequences were present before allORFs in segments 1 and 2. The level of enrichment with SDregionsin PBVs is higher than in any known prokaryotic virusfamily and this level was constant (in 100 % of the studiedgenes), while not in all prokaryotic viruses it is maintained inthe virus replication process. Such a high level of preservationof prokaryotic regions in the PBV genome should correlatewith the level of their preservation in the genome of bacteria ofa certain type from the spectrum of hosts that PBVs can infect.This level varies in different viral families . Preservation of the level of enrichment withprokaryotic regions in the genomes of prokaryotic virusesdepends on whether the host bacterium itself retains them inits genes. It is known that not all bacterial species preservethe prokaryotic Shine–Dalgarno sequence to the same extent.For example, in the genome of bacteria belonging to the typeFirmicutes, the prokaryotic motif is preserved in more than80 % of genes, while in Bacteroides, less than 10 % . It is known that different families of bacterialRNA-viruses can consist of evolutionarily related virusescapable of infecting one type of bacteria. From this fact itfollows that PBVs can infect bacteria within the type Firmicutesthat most corresponds to the level of preservation ofthe prokaryotic motif in genes (more than 80 %) to PBVs andis most common in the fecal microbiota .The hypothesis about the phage nature of PBVs put forwardby Krishnamurthy and Wang (2018) is confirmed by the resultsobtained by other researchers . In particular, the study ofBoros et al. (2018) revealed in the genome of chicken PBVsSD-regions that were located in segment 1 before the threeORFs and in segment 2 before ORFs above the initiation codons.Using 6xHis tagging and western blotting of genomicsegment 1 of PBVs containing the SD-motif, these researchershave succeeded in showing in vivo the possibility of itsexpression and functionalization in Escherichia coli . The results obtained, according to the authors,serve as proof of the existence of a bacterial culture for thereproduction of PBVs.The assumption that PBVs represent a new family of RNAbacteriophages with a high level of genomic diversity wasalso confirmed in the work of Adriaenssens et al. (2018). Theauthors of this work found a hexameric prokaryotic AGGAGGmotif in 100 % of the genomic sequences of PBVs, whileeukaryoticviruses from different families had SD-regionswith low frequency and mainly consisted of tetramers  . The review ofGhosh, Malik (2021) presents a number of conservative prokaryoticsequences (motives) found before all ORFs in segments1 and 2 of PBV and PBV-like genomes, indicatingthe location and with access numbers in GenBank .However, despite the practical results obtained, indicatingthe possible PBV affiliation to prokaryotic viruses, manyauthorsbelieve that it is premature to talk about the finalproof of the phage nature of PBVs . Thehost in which PBVs would successfully reproduce has notyet been identified. Given the fact that the gut microbiomeconsists of several hundreds of mostly uncultivable bacteria,the identification of true bacterial or archeal host(s) of PBVs(if any) will be challenging .In addition, ORFs encoding the RdRp gene were found inthe genome of some PBV strains, in which, during translation,instead of the expected standard genetic code, an alternativecode of fungi and invertebrates was used . So an assumptionwas made that the PBV hosts may be cells of lowereukaryotes.As is known, the gene sequences in the viral genomes havetheir characteristic conserved regions – motifs by which virusesare identified. Motives characteristic of PBV and PBVlikegenomes include the prokaryotic region Shine– Dalgar- no AGGAGG , terminal sequences 5′-GUAAAand 3′-ACUGC , and three regionsrepresented in amino acid sequences DFXKFD, SGSGGT andGDD When translating the RdRp gene of most picobirnaviruses,the standard genetic code is used. However, recently, newPBV-like RdRp gene sequences with an alternative (nonstandard)mitochondrial genetic code have been identified inhuman , mongoose ,bat  and invertebrate  fecalsamples. The mitochondrial code is characteristic of virusesof mold fungi and invertebrates. In particular, five PBV-likegenomic sequences consisting of dsRNA with mitochondrialcode were isolated from the gut contents of bats . In four of them  they failed to identify the presence of the capsid gene.These PBV-like genomes contained only RdRp gene sequencewith the mitochondrial genetic code of the mold. The absenceof the capsid protein gene in them resembled the genome ofmitoviruses from the family Mitoviridae, which are knownto infect the mitochondria of unicellular mold fungi .The mitovirus genome as well as four abnormal PBVlikegenomes consists of dsRNA and encodes only RdRp.Mitoviruses lack a capsid, replicate in mitochondria and usethe genetic mitochondrial code of mold fungi for the RdRptranslation. So an assumption was made that mold fungi canalso be PBV hosts .However, unlike mitoviruses, the noncapsid PBV-likestrains identified by Yinda et al. (2018) in a single segment ofthe RdRp genome contained conserved regions characteristicof PBVs. In the genome of the fifth PBV-like strain (P16-366),in addition to the conservative features of the RdRp genecharacteristic of PBVs, a sequence encoding the capsid proteinwas found. This strain was clustered on the phylogenetic treealong with GII PBVs. However, it used an alternative geneticcode and was very similar in terms of genome organizationto fungal viruses of the family Partitiviridae . These families have a minimalistic genomeconsisting of two dsRNA segments encoding RdRp and thecapsid protein, respectively, which in these families is clearlyhomologous .Similarly, researchers Kleymann et al. (2020) isolated theM17A strain among typical PBV-like strains from mongoose,the RdRp gene of which retained all conservative for PBVsmotives, but had an alternative mitochondrial mold code, andthe capsid sequence in the genome of this strain was absent.Assumption that PBVs can be fungal or invertebrate viruses,as Ghosh, Malik (2021) rightly noted, has further complicatedthe discussion about the true PBV hostDuring the discussion about the true PBV hosts, researchers attemptedto interpret the causes of the appearance of PBV-likestrains . In particular,Yinda et al. (2018) suggested that PBV-like strains found inthe gut contents of different eukaryotic hosts, with a genomeresembling the mitovirus genome, may have a reproductionmode similar to mitoviruses. Following the assumption ofYinda et al. (2018), the capsid protein gene is not neededwith this reproduction mode, since mitoviruses do not use thepathway of the independent horizontal transmission from cellto cell, but are transmitted vertically from mother to daughtercells (during division) or horizontally (by merging hyphae).Similarly, when assembling new PBV-like particles, simplifiedstructures can be formed with the adaptation to the existencein a fungal cell characteristic of mitoviruses and lost capsidprotein gene. Such an interpretation of the appearance ofnoncapsid RNA viruses is consistent with one of the trendsin the evolution of RNA viruses associated with the loss oftheir structural module by their genome, noticed by Wolf etal. (2018).The trend in the evolution of RNA viruses associatedwith the loss of the genome segment with the capsid gene isdemonstrated by the pedigree diagram of eukaryotic viruseswith the RNA genome, shown in Figure 4. This scheme alsoexplains the origin of capsidated RNA viruses of unicellulareukaryotesThe evolution of capsid-free viruses of lower eukaryotes(fungi and invertebrates) follows the path of genome reductiondue to the loss of the capsid protein gene of the probable ancestor– levivirus infecting prokaryotes . In accordance with thescheme, the ancestor of mitoviruses from the family Mitoviridaedescended from the family Leviviridae, previouslycontaining the capsid protein gene. An illustrative example ofgenome reduction is hypoviruses – capsid-free viruses withdsRNA, the structure and phylogenetic analysis of the genomeof which showed that they originated from potiviruses that lostthe capsid protein and the capsid protein gene, entering the cells of unicellulareukaryotes. Replication of mitoviruses occurs only in mitochondria,where their capsid-free (‘naked’) RNA genomes(replicons) are replicated. With the loss of the structural gene,mitoviruses have lost the ability for independent horizontaltransmission .Similarly, the origin of the group of capsid-free PBV-likestrains isolated by Yinda et al. (2018) and Kleymann et al.(2020) from Cameroonian bats  and from a mongoose (strain M17A) can beexplained by the loss of a genome segment encoding the capsidprotein by the parent strainOn the other hand, a capsid-free RNA virus (mitovirus) inthe process of evolution could gain a capsid . Encapsulated strains of RNA viruses,and probably PBV-like analogues, could have occurred in thisevolutionary way.Researchers explain shuffling or loss of genome segmentsin RNA viruses by another trend in their evolution – theability to reassort – to redistribute gene modules (RdRp andcapsid protein) between closely related virus families withsimilar genes. Reassortment modification of the genome hasalso been observed in PBVs .The creative role of reassortment between families of RNAviruses with similar genes explains the origin of the PBVlikestrain P16-366 found by Yinda et al. (2018) in bats. Thisstrain contained, together with the gene sequence RdRp , the sequence encodingthe capsid protein. The proposed genetic similarity betweenfamilies of viruses with a bisegmented dsRNA genome isbased on the hypothesis of a reassortment mechanism for theevolution of these virusesThe phylogenetic relationship between the families of RNAviruses with a segmented genome is based on the informationabout the primary structure of the RdRp gene. This geneis universal and, despite a much greater distribution amongeukaryotic viruses, is present in almost all RNA viruses (includingcapsid-less RNA replicons) with the exception of somesatellite viruses .The universality of the RdRp gene indicates the possibilityof its common origin in RNA viruses. In 2018, Wolf et al. putforward a hypothesis suggesting the presence of phylogeneticrelationships between families of RNA-containing viruses,and a phylogenetic tree was constructed , the topology of which demonstrates a possible relationship betweenthe familiesThe phylogenetic tree constructed on the basis of a setof phylogenetic data gives an idea of the possible origin ofPBVs. This tree confirms the evolution scenario in whichthe last common ancestors of virus lineages with a dsRNAgenome were simple viruses the segmented genome of whichcontained two genes (RdRp and capsid protein). Accordingto the authorsof the hypothesis, all viruses with a dsRNAgenome in branches 2 and 4 of the phylogenetic tree havesimilar capsid proteins that can be combined with the genomeRdRp from different viruses with +RNA genomeViruses with the dsRNA genome – Partitiviridae, Picobirnaviridae,Cystoviridae, Reoviridae and Totiviridae formseparate lineages in two branches of the phylogenetic tree.Viruses of the family Picobirnaviridae are located on thesame line with the families Hypoviridae, Amalgaviridaeand Partitiviridae. The most phylogenetically close familiesforming one cluster are Picobirnaviridae and Partitiviridae.The close location of these families demonstrates a high degreeof relationship between them. These families are unitedby a similar organization of virions and homologous capsidproteins . The differencebetween these viruses lies in the fact that the surfacePBV capsid proteins, unlike those of partiviruses, have perforationactivity, which determines the ability of the virus topenetrate into the cell. In addition, due to the ability of PBVsfor horizontal transmission, two segments of its genome arecombined into one capsid during assembly, and in partiviruses,the RdRp gene and the capsid protein gene use separate capsids.The PBV capsid protein gene is distantly related to thecapsid protein genes of other viruses with the dsRNA genome, which make upthree parallel evolutionary lineages in branch 4 of the phylogenetictree . Significant homology of thegenes encoding the capsid protein of viruses of the familiesReoviridae, Totiviridae, Cystoviridae, Picobirnaviridae andPartitiviridae was noted earlier .Three evolutionary lineages of branch 4 of the phylogenetictree are formed by families of viruses infecting both prokaryotes(Cystoviridae) and eukaryotes .The location of these viral families on one branch,according to the authors of the phylogenetic tree, does notexclude the possibility of the origin of eukaryotic +RNAviruses from their prokaryotic analogues. They admit thatmitoviruses (replicating in the mitochondria of mold cells)originated from an ancestor common with leviviruses – a prokaryoticRNA virus parasitizing in enterobacteria. The proofof this is the evolutionary relationship between cystoviruses and reoviruses .According to supporters of the creative role of reassortment,the origin of the PBV-like strain P16-366 could resultfrom a reassortment between an as yet undiscovered PBVrelative from branch 2, which has passed to reproduction inthe mitochondria of fungal cells, and one of the viruses withthe dsRNA genome of branch 4 . Moreover, it wasnoted that genetic restrictions on the ability to create reassortantsduring coinfection with viruses of families formingbranch 4 of the phylogenetic tree may be less strict for theprokaryotic virus family Cystoviridae. The appearance of theencapsulated PBV-like strain P16-366 occurred due to theunification of the segment with the gene RdRp of the +RNAvirus of branch 2 (possibly a naked RNA replicon) using themitochondrial code, with a fragment of the capsid gene of thedsRNA virus of branch 4.Another direction in the evolution of viruses with a bisegmenteddsRNA genome is the acquisition of partition – thepackaging of genome segments into one (monopartite) orseparate (bi-multipartite) particles. This trend is interestingbecause it gives us some insight into the nature of the hostsof PBV-like strains (prokaryotic or eukaryotic). For example,bi-partition is observed only in fungal viruses with a dsRNAgenome and in plant viruses with a ssDNA genome (Begomoviruses). In bacterial viruses in general andwith a segmented dsRNA genome, in particular (Cystoviridae),the packaging of genome segments into individual particlesis not observed. Partition is related to the virus transmissionmode (independent or non-independent). For example, partitivirusesparasitizing in fungal cells do not have the abilityfor independent  transmission and they are bipartite,while PBVs transmitting independently are monopartite.The horizontal (independent) transmission pathway allowsthem to transfer both segments of the genome into a new cellwith a high probability. The ability for independent transmission into new cells can be considered as oneof the main criteria for determining the true host of the detectedvirus.During familiarization with the studies solving the questionof the nature of PBV and PBV-like strains, we came to understandthat the belonging of these viruses to the bacterialviruses, higher or lower eukaryotes can be determined notonly by the characteristic saturation level of their genome witha prokaryotic motif, standard or mitochondrial genetic code.This affiliation should no less be determined by the presenceor absence of specific (in relation to the host cell) proteolyticactivity of the capsid protein, which determines the possibilityof independent penetration of the virus into the cell. The abilityfor independent horizontal transmission is characteristicof animal viruses and phages, while it is absent in viruses oflower eukaryotes . PBV capsidproteinshave perforating activity .This makes PBVs capable of independent penetration intocells and, therefore, can characterize them as bacterial viruses.Thus, we can conclude that in solving the question of thetrue PBV host in which they can reproduce, in addition toconservative motifs characteristic of the PBV genome, thedetermining factor is the presence of a mechanism of specifichorizontal penetration into the cell – capsid proteins with specificperforating activity. If these proteins are specific to thereceptors of a prokaryotic cell, for example, Firmicutes cells, then we are dealing with a phage,if they are specific to the receptors of a eukaryotic cell – witha virus of higher eukaryotes, and if there are no capsid proteinsat all, then such a virus can be attributed to viruses of lowereukaryotes (not capable of independent transmission).The existence of atypical PBV-like strains cannot constitutea refutation of supposed phage nature of PBVs for a numberof reasons. According to the hypothesis of Wolf et al. (2018),there is a related relationship between the families of viruseswith the dsRNA genome in the structure of the phylogenetictree, as evidenced by the presence of homologous genes anda common ancestor related to prokaryotic RNA viruses. Moreover,the families of eukaryotic and prokaryotic RNA virusesmay be related, as evidenced by the evolutionary relationshipbetween cystoviruses and reoviruses . This supports the assumptionof the possibility of the exchange of homologous segments ofthe genome between prokaryotic viruses and related virusesof lower eukaryotes, when both are in the mold fungus cell.It has recently become known that bacterial viruses can notonly infect bacterial cells, but also pass through the epithelialcells of eukaryotes using the mechanism of phage transcytosis.From epithelial cells through the blood or lymph, phages canenter various organs and tissues of animals. However, phagescan penetrate inside eukaryotic cell only with a bacterial cellinfected by them . The ubiquitous isolationof PBVs from environments where bacteria occur meansthat these viruses may not be intracellular eukaryotic viruses,but prokaryotic viruses of the gut microbiome . The assumption that PBVscan infect prokaryotic cells is confirmed by the presenceofa conservative prokaryotic Shine–Dalgarno sequence in theirgenome .The identification of atypical PBV-like sequences witha non-standard mitochondrial genetic code of fungi and invertebratescan serve as evidence that PBVs are capable of reproducing their own kind in the cells of lower eukaryotes,undergoing genetic changes when changing hosts .Therefore, it can be assumed that PBVs can get into eukaryoticcell , where they will meet eukaryotic or prokaryoticvirus with similar genome. Viruses like mitoviruses, partitiviruses,or cystoviruses may be among the PBV reassortmentpartners. After rearranging the genome segments of the PBVswith partner viruses, PBV-like reassortants described by Yindaet al. (2018) and Kleymann et al. (2020) can apppear. Then,according to the assumptions of the authors of the reassortationinteraction hypothesis, depending on the presence or absenceof prokaryotic motives and motives characteristic of PBVs,as well as depending on the genetic code used by the genesof a new virus  will determinenot only the degree of its relationship with PBV, but also itsprokaryotic or eukaryotic nature.The hypothesis explaining the formation of atypical PBVstrains through the exchange of homologous genome segmentsbetween related viral families does not exclude the possibilityof their appearance using satellite relationships between PBVsand helper viruses from 2nd or 4th branches of the phylogenetictree. Viruses from the families Partitiviridaе, Totiviridae,Reoviridae are known as helper viruses, which require somesatellite dsRNAs for their reproduction .Assuming that PBV is a satellite that uses an RdRp enzymefor reproduction in the fungal cells similar to mitovirus (frombranch 2), it is possible to explain the appearance of non-capsidPBV-like strains with mitochondrial genetic code P11-300,P11-378, P14-90 and P15-218, isolated by Yinda et al. (2018)or M17A isolated by Kleymann et al. (2020).By using as a helper a virus from branches 2 or 4  theappearance of the encapsulated PBV-like strain P16-366 canbe explained. In other words, in this case, the ability of theprogeny resulting from the interaction of the satellite viruswith the helper virus to reproduce in a bacterial cell or in themitochondria of mold will depend on the nature of the helpervirus. And this does not contradict the supposed prokaryoticnature of PBVs, but only means the possibility of its changein the reassortment process.PBVs are found wherever bacteria are found – in environmentalsamples, in lower eukaryotes (fungi and invertebrates), inthe gut contents of higher eukaryotes (including reptiles). Thismeans that PBVs may not be intracellular eukaryotic virusesbut prokaryotic viruses of the gut microbiome .Like prokaryotic cystoviruses, PBV genomes contain andpreserve in a saturated state (in both genome segments ineach reading frame) functional binding sites for bacterial-typeribosomes , while in eukaryoticviruses the genome is not saturated with them .The identification of related PBV strains in different animalspecies may mean that the PBV hosts are a specific type ofbacteria, possibly Firmicutes, found in the gut of differentvertebrates and invertebrates. The level of preservation ofprokaryotic sites in the genome of these bacteria (more than80 % of the genes) corresponds to that of PBVs .PBV identification in the gut, respiratory organs of animals, in blood and respiratorysamples  doesnot refute the assumption that bacterial cells can be their hosts.Phages cannot directly infect cells of different organs of highereukaryotes, but they can get into these organs by non-specifictranslocation through gut epithelium with blood flow  or with the help of bacteria in which they multiply. This is how phages penetrate intothe blood, lymph, organs and even the brain.The presence of a capsid protein with perforating activity(the ability to penetrate through the cell membrane), as inviruses capable of infecting animal cells , does not contradict the fact that PBVs can be prokaryoticRNA viruses. It is known that representatives of thefamily of bacterial RNA viruses also have a molecular apparatusfor penetrating into bacterial cells using transcytosis.The acquisition of immunity to PBVs by infected animalsalso does not mean that PBVs can be considered eukaryoticviruses, since it has been established that host immune responsescan also occur against bacterial viruses . Possibly, PBVs cause animmune response to infection not of the host cells, but of thebacterial cells that make up its microbiome, which does notexclude the possibility that PBVs are prokaryotic virusesLike the phages, which are virulent and moderate, in aninfected organism, PBVs can be active (excreted) and inactive(temporarily not excreted), while infected animals willbe asymptomatic carriers .The identification of PBV-like strains with the RdRp geneusing an alternative mitochondrial genetic code of eukaryotes for translation is also not a refutation ofthe assumption that PBVs belong to prokaryotic viruses. Accordingto the hypothesis of Wolf et al. (2018), they are theresult of reassortment between the PBVs and the families ofRNA viruses with a similar genome. The appearance of atypicalPBV-like strains only indicates the possibility of an evolutionarytransition from the prokaryotic nature of the virus tothe eukaryotic nature and back as a result of rearrangement ofgenomic segments, and is explained by a change in the degreeof saturation of the genome of the new virus with prokaryoticregions, which may change its nature. The possibility of satelliterelationships between PBVs and RNA viruses of lowereukaryotes such as Partitiviridaе, Totiviridae and Reoviridae,which are known as helper viruses, is not excluded. There areknown cases of satellite relationships between prokaryoticand lower eukaryotic viruses infecting one single-celled host,allowing phages to reproduce new progeny in a eukaryoticcell .The expression and functioning of PBV segment 1 in vivoin Escherichia coli  was carried out, whichconfirms the existence of a bacterial culture for the propagationof PBVs.The arguments given convincingly show that PBVs can indeedbe prokaryotic viruses, since they are found wherever bacteriaare found – in environmental samples, in the gut contents ofvertebrates, in the cells of fungi and invertebrates.Similar to prokaryotic cystoviruses, PBV genomes containand retain functional binding sites for bacterial-type ribosomesin a saturated state, while in eukaryotic viruses thegenome is not saturated with these motifs. The high level ofpreservation of prokaryotic sites in the PBV genome suggeststhat they belong to a new family of RNA viruses that infecta certain type of bacteria with a high content of SD-sequencesin the genome. Such host bacteria of PBVs can be Firmicutesfound in the gut of vertebrates and invertebrates, which havea similar level of prokaryotic motifs characteristic of bacteria.The high frequency of the presence of SD-sequences in thePBV genome can be considered one of the main criteria foridentifying new virus families for affiliation to a prokaryoticor eukaryotic host.But in order for a virus to be finally classified as a phage,its genome must contain a structural gene encoding a proteinwith specific proteolytic properties that determine the abilityof the virus to independently penetrate into a bacterial cell.A protein with proteolytic properties is present in the PBVcapsid, which indicates the ability of this virus for horizontaltransmission (independently) from cell to cell and from onehost to another. At the same time, the identification of PBVsin the gut of various hosts may suggest that their horizontaltransmission can be carried out by bacteria and, therefore,these viruses themselves are phages. The identification ofPBVs in various tissues of the body can also be explained bythe fact that, being phages, they are capable of nonspecifictranslocation through the walls of gut epithelial cells.The selection of a culture for the virus propagation is necessaryfor the final determination of its nature. To date, thebelonging of PBVs to viruses of higher eukaryotes has notbeen proven, since it was not possible to isolate them fromeukaryotic cell cultures. To establish definitively whetherPBVs are prokaryotic viruses, it is necessary to direct effortsto select a host for their reproduction among prokaryotic cellsobtained from the gut microbiome of mutants and conditionsfor the cultivation of these cells.And, finally, the presence of probable relatives among thefamilies of RNA viruses with similar genes with which PBVscan exchange genome segments, replicating atypical geneticvariants, does not contradict the correctness of the hypothesisabout the phage nature of PBVs, but indicates their potentialability to adapt to new conditions of existence, allowing themto infect eukaryotic or prokaryotic host cells.The authors declare no conflict of interest.Adriaenssens E.M., Farkas K., Harrison C., Jones D.L., Allison H.E.,McCarthy A.J. Viromic analysis of wastewater input to a river catchmentreveals a diverse assemblage of RNA viruses. mSystems. 2018;3(3):1-18. 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