Self-pollination in buds and homostyly in Mussaenda shikokiana (Rubiaceae), a monomorphic species in a distylous clade

Distyly is a genetically controlled flower dimorphism. Most species in Mussaenda are distylous or functionally dioecious. Mussaenda shikokiana Makino consisted of one floral morph only in the populations studied in southern China. The present study aimed at illuminating the characteristics of floral monomorphism in M. shikokiana, and elucidating the mechanism behind the evolution of monomorphism from distyly. We examined floral morphology in five natural populations across the species' distribution range. Experiments and observations on the floral biology, pollinating fauna, and breeding system were carried out in two to four natural populations. We also screened allozyme markers to estimate the degree of outcrossing. The results indicated that M. shikokiana was a self-compatible, partially autogamous species and pollinators were required to maximize seed set. Anthers dehisced and pollen showed the highest viability 8-12h before flower opening, and stigmas were receptive in bud, resulting in bud-selfing. Pollen viability decreased sharply after flower opening, whereas stigma receptivity lasted to the third day. Pollinators including bees, butterflies, and hawkmoths visited the flowers of M. shikokiana frequently at the study sites. There was no significant difference among pollen tube growth rates in selfing and outcrossing; pollen tubes reached ovules within 16h in all treatments, indicating lack of typical self-incompatibility or cryptic self-incompatibility. Our results suggested that M. shikokiana is a homostylous species, and that homostyly promoting self-pollination was selected in its evolutionary history.