The maintenance of gynodioecy in sidalcea hendersonii

I investigated the genetic and ecological factors contributing to the maintenance of females in populations of the gynodioecious plant, Sidalcea hendersonii. Crossing experiments indicated that male sterility is controlled by a dominant nuclear allele. High frequencies of female plants in the majority of populations surveyed, in combination with nuclear determination of sex, elevates the theoretical requirements for female fitness in this species. Females did have higher fitness, producing more surviving offspring than hermaphrodite plants in an experimental population, and outcrossed hermaphrodite plants out-performed self-pollinated hermaphrodites. These results suggest that female advantage is the product of both maternal effect and obligate outcrossing. However, no inherent fitness advantages were evident in natural populations where females and hermaphrodites did not differ in viable seed production. Ecological factors may play an important role in the maintenance of gynodioecy. Though flower size differences between the sexes (thought to be related to pollinator visitation) did not affect seed production, seed predation did. In populations where females were abundant, weevil larvae destroyed significantly more seeds from hermaphrodite plants and substantially reduced hermaphrodite seed production overall. This study provides the first evidence that sex-related predation may be responsible for high female frequencies in natural populations of a gynodioecious species.Science, Faculty ofBotany, Department ofGraduat