Pollen Flow,Fecundity, and the Adaptive Significance of Heterostyly in Palicourea padifolia (Rubiaceae)1

Palicourea padifolia is a distlous shrub visited primarily by hummingbirds and bees in mid-elevation rain forests in Costa Rica. At Las Cruces Biological Station, the population of P. padifolia is composed of equal numbers of pin and thrum plants and morphs are randomly distributed. Like the majority of distylous species, P. padifolia exhibits a self and intramorph-incompatible mating system. The analysis of stigmatic pollen loads obtained from emasculated flowers shows that pollen flow between floral morphs is disassortative, which supports the Darwinian hypothesis concerning the adaptive significance of heterostyly. Pins experience greater disassortative pollination than thrums. Pins also set more seed than thrums, suggesting that the higher frequency of compatible pollen deposited on pin stigmas results in greater fecundity; alternatively, greater seed set in pins could be due to the differential allocation of resources to male and female function between morphs. Overall, seed set for the Las Cruces popularion of P. padifolia is much lower than the potential maximum. Factors which could contribute to seed set limitation include the frequency and/or spatial pattern of pollinator visits, pollen availability, and resource availability.