MALE AND FEMALE POLLINATION SUCCESS IN A DECEPTIVE ORCHID, A SELECTION STUDY

We carried out phenotypic selection analyses to assess the relative im portance of several Horal traits, as well as microhabitat, in determin ing both male and female pollination success in a nonrewarding orchid (Cypripedium acaule Ait.) in two Nova Scotia populations. Fruit produc tion in these one-flowered, cosexual plants was strongly pollen limite d within a season: 100% of individuals set fruit following hand-pollin ation, but only 5% and 13% did so under natural circumstances in the t wo populations. Male and female reproductive success were highly corre lated, with >90% of plants that received a pollinium also dispersing o ne. Despite very low visitation rates, the chance of a. removed pollin ium fertilizing another ovary was 36% and 51%, respectively, in the tw o populations. There was no evidence for selection acting differently, either through the two sexual functions or in the two populations. In contrast, the strength of directional selection on several traits dif fered among microhabitats within each population. Univariate and multi variate regressions indicated statistically significant directional se lection acting on flower-opening date through male, female, and total (combined) pollination success in one population. For female, male, an d total success in this population, we also detected correlational sel ection acting on opening date and flower height. There was also negati ve correlational selection acting on flower height and labellum length through female success. Microhabitats within the populations had a la rger effect on both female and male success than did floral traits In particular the presence of ericaceous shrubs and an open canopy appear ed to be closely linked with higher pollination rates.