Sex-biased seed predation in a gynodioecious species, Sidalcea campestris

The coexistence of females and hermaphrodites, or gynodioecy, requires that females have a reproductive advantage over hermaphrodites in offspring quality and/or quantity. Female advantage can be mediated by a variety of ecological factors, including interactions with antagonists such as pre-dispersal seed predators. We compared differential rates of seed predation on hermaphrodites and females in Sidalcea campestris to assess the potential role of seed predators in gynodioecy maintenance.

We evaluated rates of seed predation in 28 populations of S. campestris, a gynodioecious, insect-pollinated herbaceous perennial. By dissecting fruits of hermaphrodite and female plants in each site, we tested how plant sex and population female frequency influence the proportion of fertilized ovules consumed by pre-dispersal seed predators.

The specialist pre-dispersal weevil seed predator Macrorhoptus sidalceae was detected in 24 (86%) study populations. Across all sites, average predation levels ranged from zero to 85%. Seed predation rates were approximately 1.35× higher in hermaphrodites than in females. Seed predation rates were unaffected by population-level female frequency.

Hermaphrodite fruits experienced higher seed predation rates than female fruits, regardless of population female frequency. Pre-dispersal seed predators may influence sex-specific reproductive success and potentially contribute to female advantage in S. campestris, though additional ecological and genetic factors may regulate population female frequency in this system. Our study highlights the importance of considering how non-pollinator agents of selection, such as seed predators, potentially influence floral trait evolution and the evolution of sexual systems more broadly.