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Abstract
Gynodioecy is thought to be the most common transition from hermaphroditism to dioecy, which is a major evolutionary transition in plants. There are three major stages in this transition: the initial invasion of females, the maintenance of females, and the masculinization of hermaphrodites. The research presented here addresses each stage to examine this process in Geranium maculatum, an herbaceous perennial. I found that the initial invasion requires a large seed fitness increase by females. This is partly due to the nuclear control of sex, which requires that females have at least twice the seed fitness as hermaphrodites to successfully invade, and to increased pollinator discrimination against females when they are rare. The maintenance of females is influenced by seed fitness differences between the sexes. Due primarily to differences in seed production and flowering frequency, I found that females are expected to be maintained in all populations examined, but only when including sex differences throughout the entire lifecycle of a plant. Seed production differences in part may be due to the lower selfing and biparental inbreeding found in females or due to the sexes living in different environments within populations. On the other hand, I found that pollinator discrimination may decrease seed production, lowering females relative fitness. Thus, the maintenance of females is influenced by several different factors. The last step to dioecy, the masculinization of hermaphrodites, does not appear to occur in this species due to selection favoring more flowers, which increases both pollen and seed fitness. Without a tradeoff between pollen and seed fitness, it seems unlikely that dioecy will evolve in G. maculatum. Thus, in this species, the initial invasion of females may be difficult, but once established gynodioecy appears to be stable, despite sex being under nuclear control.