Experimental And Theoretical Studies Of The Pollination Ecology Of Gynodioecy

Abstract

Gynodioecy, a breeding system with females and hermaphrodites, is the most common dimorphic system in plants and found in more species than all other polymorphic systems combined. One unresolved question in gynodioecy evolution is how dramatic sex ratio variation (0-100% female) is maintained among populations. To address this gap, I used complementary empirical and theoretical approaches to ellucidate pollinators' role in the evolution and maintenance of gynodioecy and variation in gynodioecious sex ratios. I conducted two field studies restricting the types of pollinators available to artificial populations of Silene vulgaris. The first study contrasted pollination by pollen collectors, strongly favoring hermaphrodites, with nectar collectors that readily visit both sexes. Hermaphrodites' relative fitness was greatest in the context of pollen collectors whereas females had dramatically higher relative fitness in the context of nectar collectors, demonstrating pollinators' potential to restrict or facilitate gynodioecy. The second study measured fitness in artificial populations of S. vulgaris with either pollen or nectar collecting pollinators in the context of a large natural population, providing access to more pollen sources and more pollinators. Female relative fitness was constant across pollination contexts, unlike in the previous study, as an abundance of pollinators and pollen sources diminished the differences in pollination contexts arising from pollinator bias. On the theoretical front, I developed mathematical models that describe female and hermaphrodite fitness in terms of pollinator abundance and behavior. Then, a single locus nuclear model of gynodioecious sex ratio evolution was used to describe equilibrium female frequency as well as the conditions permitting gynodioecy in terms of pollinator behavior. As in the field studies, I found that pollinators' influence could range from subtle to dramatic. More specifically, under realistic parameter values, where pollinators prefer hermaphrodites to females, incorporation of pollination ecology generally reduces female frequency, and the conditions for the evolution of gynodioecy become more stringent than if pollination processes were ignored. Together, these studies bolster the surprisingly overlooked idea that evolution of gynodioecious populations is directly influenced by pollinator context.