Sex-Specific Trade-Offs and Responses to Foliar Shade in the Gynodioecious Species Silene Vulgaris (Caryophyllaceae)

Allocation trade-offs are predicted to affect evolutionary dynamics, including the evolution of sexual dimorphism. In gynodioecious species, where populations have both females and hermaphrodites, selection may result in sexual dimorphism in individual traits or trait correlations because, in contrast to females, hermaphrodites acquire fitness through both male and female function. Using the gynodioecious species Silene vulgaris, we measured reproductive traits (ovule number, ovule size, anther size, and floral traits related to display) and a vegetative trait (internode length) among plants growing under full sun and foliar shade treatments. We tested for sex-specific correlation structures as well as trait plasticity and the sensitivity of correlation structures to the light environment. Hermaphrodites exhibited a fixed trade-off between anther size and ovule size, whereas females exhibited an ovule size-number trade-off. For a few traits, we detected plasticity to light treatment and plasticity that differed significantly between sexes across light treatments. Exposure to foliar shade resulted in a sex-specific trade-off between vegetative and reproductive allocation; hermaphrodites exhibited a negative correlation (trade-off) between internode elongation and ovule number, whereas females showed a marginally significant, positive correlation between these two traits. The trade-offs observed in hermaphrodites are of particular evolutionary relevance, suggesting that hermaphrodites will evolve to become increasingly male and populations increasingly sexually dimorphic.