Ecological constraints on the maintenance of gynodioecy in Sidalcea malviflora ssp. Malviflora (Malvaceae)

Selection for separate genders in the flowering plants has long been of interest to evolutionary biologists. In the gynodioecious sexual system, there is the question of how females are maintained among hermaphrodites, given that they are genetically disadvantaged by their loss of male function. While much work has focused on physiological and genetic mechanisms of female advantage, little work has looked at the role that ecological process and pattern play in selecting for the maintenance of females. In particular, interspecific interactions including plant-pollinator and plant-herbivore relationships may select for or against one gender. Population-level patterns such as the spatial distribution of females and hermaphrodites may also impact individual fitness if mating success is frequency-dependent. In this thesis, I use observational and experimental techniques to look at the combined and interactive effect that pollinators, herbivores and population sex structure have on the balance of maternal fitness between the genders in Sidalcea malviflora ssp. malviflora (Malvaceae), a gynodioecious perennial. Contrary to theory and empirical results from other gynodioecious species, hermaphrodites were found to be pollen-limited, while females were resource-limited following artificial pollen augmentation. Pollen limitation in hermaphrodites was due to the low selfing rates imposed by protandry, as well as pollinator avoidance of female phase hermaphrodite flowers. The frequency of pollen donors in local neighborhoods also limited seed set in both genders, showing that females are pollen-limited under natural variation in pollen loads. Hence female fitness is negatively frequency-dependent, while hermaphrodite fitness is positively frequency-dependent. Using various kinds of spatial analyses, I show that there is a patchy distribution of the genders in this species. Given frequency-dependent fitness, this kind of population structure advantages hermaphrodites and disadvantages females. Floral herbivory by pollen specialists interacts with pollinator behavior to select against hermaphrodites. Seed set is depressed following herbivory, partly due to pollinator avoidance of damaged flowers. Since herbivores primarily target hermaphrodites, pollen production becomes a liability from which females escape. These combined results serve as an example of how ecological processes and patterns can act as evolutionary mechanisms in plant sexual systems, sometimes with surprising results.