Variation in self-incompatibility and selfing syndromes in sympatric Hedyotis species (Rubiaceae)

Abstract

The majority of Hedyotis species (Rubiaceae) are morphologically distylous, with distinct pin (exserted stigma and inserted stamens) and thrum (exserted stamens and inserted stigma) morphs, and are suitable models for investigating the functional roles of distyly. Significant levels of discordance among gene trees based on chloroplast and nuclear DNA markers suggest the possibility of interspecific hybridisation. Selection to prevent interspecific mating can cause an increase in self-pollination in sympatric populations. In this study, we investigate the breeding systems of three phylogenetically closely related species: H. acutangula, which is widespread in Hong Kong, and two narrow endemics (H. vachellii and H. bodinieri). All three species are locally sympatric, have overlapping flowering seasons, and reproductive active stages, and share pollinators, thereby enhancing opportunities for interspecific pollen transfer. Controlled pollination experiments were performed on the three taxa in the field, including spontaneous and artificial self-pollination, geitonogamy, inter- and intra-morph xenogamy, and the controls (natural conditions and emasculated). Our results suggest a lack of strict self-incompatibility. The capacity of the thrum morph (Ss) to undergo autogamy in H. acutangula ensures dimorphic founder populations. The functional break-down of distyly and relaxed style dimorphism in H. vachellii favours autogamy. Anther dehisce prior to anthesis, pre-anthetic receptive stigmas and high rates of selfing provides rich opportunities for “pre anthesis” cleistogamy in H. bodinieri. Our study demonstrates that distyly and self-incompatibility are not tightly associated in these three Hedyotis species.