Mating systems in sympatric Hedyotis species (Rubiaceae) in Hong Kong : the breakdown of distyly and its impact on hybridization and self-fertilization

Abstract

Many species in Hedyotis and related genera (Rubiaceae tribe Spermacoceae) exhibit heterostyly, a floral polymorphism in which the anthers and stigmas occupy reciprocal positions in two different ‘morphs’ within populations. This polymorphism is typically associated with a biochemically mediated self-incompatibility system that prevents autogamy and intra-morph fertilization. This study focuses on the reproductive biology, pollination ecology and population genetics of the Hedyotis acutangula-shiuyingiae complex in Hong Kong (comprising the often sympatric species H. acutangula, H. vachellii, H. shiuyingiae, H. bodinieri and H. loganioides). Interspecific hybridization is investigated as a possible explanation for the gene tree incongruence identified previously, with the breakdown of self- and interspecific incompatibility hypothesized to have enabled hybridization within the species complex. Correlations observed in self- and interspecific mate recognition systems may be associated with unilateral incompatibility that acts as a barrier to hybridization: the breakdown of specific self- and interspecific pollen recognition system might allow successful

interspecific mating. Hedyotis acutangula is demonstrated to be self- and interspecific compatible, whereas H. vachellii, H. bodinieri and H. loganioides are self-compatible and interspecific incompatible; H. shiuyingiae, in contrast, is strictly self-incompatible and interspecific incompatible. The rapid growth of pollen tubes from heterospecific pollen in H. acutangula indicates their ability to compete with conspecific pollen and undergo interspecific hybridization. Population genetics data based on microsatellite markers provide compelling evidence of the hybrid origin of H. bodinieri and H. loganioides and their conspecificity. Reproductive biology and population genetic results confirmed the maternal (H. acutangula) and paternal (H. shiuyingiae) parentage of these hybrids. Apis cerena, the Asian honey bee, was identified as a pollinator, common to all the Hedyotis species studied. The simultaneous flowering, overlapping anthesis and reproductive activity, shared pollinators and interspecific pollen flow among sympatric Hedyotis species provide compelling evidence for interspecific hybridization in the complex. Selection to avoid interspecific hybridization can increase self-pollination in sympatric populations. The functional breakdown of distyly and the presence of selfing syndromes in the sympatric Hedyotis species studied suggests that self-pollination is favored, with anther dehiscence and stigmatic receptivity prior to anthesis, self-compatibility and thrum morph-specific spontaneous self-pollination in H. acutangula; self-incompatibility and relaxed stylar dimorphism in H. vachellii; and stigmatic receptivity and anther dehiscence prior to anthesis and high rates of spontaneous selfing in H. bodinieri. The success of self-pollination prior to anthesis in H. bodinieri represents the first reported case of the co-occurrence of distyly and pre-anthesis cleistogamy in Hedyotis. The functional breakdown of heterostyly and the emergence of pre-anthesis cleistogamy represents an initial step in the evolutionary transition from xenogamy to autogamy. The ability of cleistogamous pollen to compete with inter-morph pollen provides the potential for pre-anthesis cleistogamy in H. bodinieri to break the stability of distyly by reducing opportunities for outcrossing. Pre-anthesis cleistogamy is likely a derived mechanism that ensures reproductive assurance despite potential mate and pollinator limitation in the fragmented habitats in Hong Kong.