Citation for Study 13986
Kissling J., & Barrett S. 2013. Variation and evolution of herkogamy in Exochaenium (Gentianaceae): implications for the evolution of distyly. Annals of Botany, 112: 95-102.
- Kissling J.
- Barrett S.
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. We examined this problem 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, we reconstructed the evolutionary history of herkogamic and non-herkogamic conditions 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. We determined ancestral characters states and used the reconstructions 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 pendant 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 Our 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 conditions.
distyly, Exochaenium, floral evolution, Gentianaceae, herkogamy, heterostyly, phylogeny
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