Floral traits and hermaphrodite abundance shape female advantage in a gynodioecious plant

Gynodioecy is maintained through female advantage in seed fitness that depends on the sex ratio of a population. We hypothesized that females of Silene acaulis would exhibit floral traits associated with elevated outcross pollination and that sex ratio would impact pollen limitation and inbreeding depression such that female advantage is greatest at high hermaphrodite frequency.

We examined how sex ratio affects female advantage by pairing observations of floral morphology and natural pollination with a pollination experiment in two sites that differed in sex ratio. We described the population structure of sex ratio by comparing site sex ratio to local scale sex ratio within 1 m of focal plants.

We found evidence for frequency dependence of female advantage through pollination. As local hermaphrodite abundance increased, females were more likely to receive pollen and set fruit. Sex ratio was spatially structured, and only local sex ratio had significant effects on reproductive processes. Both sexes were similarly severely pollen limited, but longer stigmas on female flowers increased chances of pollen deposition. Hermaphrodites experienced inbreeding depression expressed during seed germination, but inbreeding did not fully explain female advantage.

We confirm female reproductive advantage in a gynodioecious species and show that it is partly facilitated by superior pollination-mediated seed production due to longer stigmas which maximizes pollen receipt. Female advantage exhibits negative frequency dependence locally, such that female advantage is greatest when nearby hermaphrodites are abundant. These results advance our understanding of links between population reproductive processes and sexual selection.