Sex ratio variation and gene flow among populations of the gynodioecious geranium maculatum.
Christopher, Dorothy A
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Gynodioecy is a plant sexual system in which separate female and hermaphrodite individuals occur in a species. Population sex ratio is highly variable, which is unique among plant sexual systems with multiple sexes. We use Geranium maculatum, a gynodioecious species in which population female frequency varies from 0-50%, to investigate factors that may be important in determining the distribution of females. Non-selective forces such as gene flow and migration can be important in shaping the distribution of sexual phenotypes. Here we use a population genetic approach to identify patterns of genetic structure that may play a role in determining where females occur. We found decreased migration rates to and from populations with high female frequency. This suggests that frequency reintroduction of male sterility alleles is not the mechanism that maintains females in populations. We then investigated how fitness differences and patterns of sex allocation differ between the sexes and among populations that differ in their female frequency. Theory states that hermaphrodites should increase allocation to male function when females are present, however, we did not find evidence of this. Additionally, females did not have significantly higher seed production than hermaphrodites, however they do have larger rhizomes, which may allow females to increase lifetime fitness relative to hermaphrodites. Finally, we considered pollen dispersal distances, and examined how these differ between populations. Pollen dispersal has implications for gene flow and the distribution of genetic variation in a population. We found that populations of high female frequency had reduced pollen dispersal distances. This pattern may perhaps be related to pollinator foraging patterns: previous work has shown pollinators discriminate against females in Geranium maculatum.