Journal Article

Variation and evolution of herkogamy in Exochaenium (Gentianaceae): implications for the evolution of distyly

Jonathan Kissling and Spencer C. H. Barrett

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 112, issue 1, pages 95-102
Published in print July 2013 | ISSN: 0305-7364
Published online May 2013 | e-ISSN: 1095-8290 | DOI: https://dx.doi.org/10.1093/aob/mct097
Variation and evolution of herkogamy in Exochaenium (Gentianaceae): implications for the evolution of distyly

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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.

Keywords: Distyly; Exochaenium; floral evolution; Gentianaceae; herkogamy; heterostyly; phylogeny

Journal Article.  4983 words.  Illustrated.

Subjects: Plant Reproduction and Propagation

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