Consequences of Stigma Receptivity Schedules for Sexual Selection in Flowering Plants

Abstract

In flowering plants, pollen-tube density and pollen-donor diversity should influence the likelihood of sexual selection in the arena of the recipient pistil. We propose high pollen-tube density as a prerequisite for gametophytic male competition, which does not broadly expand the range of genotypes available for female choice. Pollen diversity (e.g., increases in the number of different pollen donors sampled) should allow recipient styles a broader choice of potential mates and may, by increasing variance in pollen-tube growth rates, strengthen competitive regimens if sufficient numbers of grains have germinated. Thus, when coupled, high pollen-tube density and pollen-donor diversity could encourage both types of sexual selection. Stigma-receptivity schedules, by structuring the rate of pollentube recruitment and the length of time in which tubes may be recruited prior to the fertilization of all the ovules, have the potential to affect both pollen-tube density and donor diversity. Using quantitative data on receptivity from Clintonia borealis, we modeled pollen-tube recruitment under observed and hypothetical, continuous and restricted receptivity schedules. Because adhesion and germination of pollen on stigmas are correlated in these schedules, the model predicts that a time course of receptivity favoring the highest pollen-tube density (continuous) restricts both the opportunities for female choice during pollen germination and the potential diversity of pollen donors. Conversely, opportunities for female choice are maximized by restricting pollen-tube recruitment until late in anthesis. Receptivity timing in C. borealis appears to produce a pattern of pollen-tube recruitment closest to that for the continuous schedule, but allowing for slightly greater potential for female choice. Thus, the model suggests that in C. borealis, male competition and female choice may occasionally be coupled, and explores the general conditions under which mate quality in flowering plants will be regulated by one or both of these mechanisms.