Pollinator-mediated fitness differences do not fully explain the maintenance of gynodioecy in Sidalcea campestris

The maintenance of gynodioecy, female and hermaphroditic coexistence in plants, requires females to have a reproductive advantage over hermaphrodites in offspring quality, quantity, or both. Pollinators can influence this by mediating the reproductive success of females and hermaphrodites. We measured differences in pollinator-mediated fitness components between female and hermaphroditic plants in Sidalcea campestris to evaluate the role of pollinator-mediated selection in maintaining gynodioecy.

We studied sex-biased pollinator dynamics across 28 gynodioecious populations of S. campestris and conducted pollen supplementation experiments in 20 of these populations. Using pollinator surveys and pollen supplementation experiments, we tested for differences in pollinator visitation rates, pollinator community composition, seed production, and pollen limitation between sexes.

Hermaphrodites received an average of 2.24 times more visits than females, although pollinator community composition was similar for both sexes. The estimated average probability of female seed set was 1.7 times higher than hermaphrodites for open-pollinated flowers and 2.0 times higher for hand-pollinated flowers, providing evidence of a female reproductive advantage. Seed set in both sexes was pollen limited, and females were not more limited than hermaphrodites. Female pollen limitation was unaffected by population-level female frequency.

These results revealed complex pollinator-mediated fitness differences between females and hermaphrodites. While our study demonstrated that females achieve the expected reproductive advantage for cytonuclear gynodioecy, the lack of differential pollen limitation and frequency-dependent female fitness are inconsistent with expectations of stable gynodioecy. Additional environmental, ecological, and genetic factors may regulate population dynamics in this gynodioecious system.