Variation and evolution of herkogamy in Exochaenium (Gentianaceae): implications for the evolution of distyly.
BACKGROUNDS AND AIMS
The spatial separation of stigmas and anthers (herkogamy) in flowering plants functions to reduce self-pollination and avoid interference between pollen dispersal and receipt. Little is known about the evolutionary relationships among the three main forms of herkogamy - approach, reverse and reciprocal herkogamy (distyly) - or about transitions to and from a non-herkogamous condition. This problem was examined in Exochaenium (Gentianaceae), a genus of African herbs that exhibits considerable variation in floral morphology, including the three forms of herkogamy.


METHODS
Using maximum parsimony and maximum likelihood methods, the evolutionary history of herkogamic and non-herkogamic conditions was reconstructed from a molecular phylogeny of 15 species of Exochaenium and four outgroup taxa, based on three chloroplast regions, the nuclear ribosomal internal transcribed spacer (ITS1 and 2) and the 5·8S gene. Ancestral character states were determined and the reconstructions were used to evaluate competing models for the origin of reciprocal herkogamy.


KEY RESULTS
Reciprocal herkogamy originated once in Exochaenium from an ancestor with approach herkogamy. Reverse herkogamy and the non-herkogamic condition homostyly were derived from heterostyly. Distylous species possessed pendent, slightly zygomorphic flowers, and the single transition to reverse herkogamy was associated with the hawkmoth pollination syndrome. Reductions in flower size characterized three of four independent transitions from reciprocal herkogamy to homostyly.


CONCLUSIONS
The results support Lloyd and Webb's model in which distyly originated from an ancestor with approach herkogamy. They also demonstrate the lability of sex organ deployment and implicate pollinators, or their absence, as playing an important role in driving transitions among herkogamic and non-herkogamic conditions.
['Kissling|Jonathan|J|', 'Barrett|Spencer C H|SC|']
D017954:Africa South of the Sahara D005075:Biological Evolution D002736:Chloroplasts D035264:Flowers D014644:Genetic Variation D029965:Gentianaceae D016013:Likelihood Functions D008954:Models, Biological D008969:Molecular Sequence Data D010802:Phylogeny D054817:Pollination
herkogamy Exochaenium Gentianaceae Distyly phylogeny heterostyly floral evolution