THE EVOLUTION OF INSECT‐MEDIATED SELF‐POLLINATION FROM AN OUTCROSSING SYSTEM IN CALYPTRIDIUM (PORTULACACEAE)

Abstract

Calyptridium monospermum (Portulacaceae) comprises two biological races visited by different groups of insects. One race, pollinated by a bumblebee, Bombus vosnesenskii (Apidae), is outcrossing, while the other race is pollinated casually by a variety of other insects whose visits result in insect‐mediated self‐pollination and geitonogamy within the inflorescence. Insect‐mediated selfing is also the mode of pollination in the closely related species, C. umbellatum. While all plants of both species are self‐compatible, insect visitation is necessary for good seed set. Selfing populations of C. monospermum exhibit many floral adaptations for insect‐mediated self‐pollination which parallel those of the selfing C. umbellatum, while retaining vegetative characteristics typical of C. monospermum. These floral traits involve flower color and odor, style and stamen orientation, size of stigmatic surfaces, number of pollen grains produced, density of open flowers on inflorescences, and presence or absence of protogyny. Artifical pollination experiments show that these floral adaptations are effective in increasing the probability of insect‐mediated self‐pollination. Insect‐mediated selfing in C. umbellatum and in some populations of C. monospermum is encouraged and probably made necessary by the inconstant pollinating behavior of the insects which visit these plants, as shown by the large percentage of non‐Calyptridium pollen recovered from the bodies of insects captured while visiting these plants.