An experimental test of the effects of resources and sex ratio on maternal fitness and phenotypic selection in gynodioecious Fragaria virginiana

Abstract

Resources, sex ratio, and seed production by hermaphrodites covary among natural populations of many gynodioecious plant species, such that they are functionally "more dioecious" as resources become more limiting. Strong correlations among these three factors confound our understanding of their relative roles in maintaining polymorphic sexual systems. We manipulated resource availability and sex ratio and measured their effects on relative fertility and phenotypic selection through the maternal fitness of females and hermaphrodites of Fragaria virginiana. Two results were particularly surprising. First, hermaphrodites showed little variability in fecundity across resource treatments and showed strong positive and context-dependent selection for fruit set. This suggests that variation in hermaphrodite seed production along resource gradients in nature may result from adaptation rather than plasticity. Second, although females increased their fecundity with higher resources, their fertility was unaffected by sex ratio, which is predicted to mediate pollen limitation of females in natural populations where they are common. Selection on petal size of females was also weak, indicating a minimal effect of pollinator attraction on variation in the fertility of female plants. Hence, we found no mechanistic explanation for the complete absence of high-resource high female populations in nature. Despite strong selection for increased fruit set of hermaphrodites, both the strength of selection and its contribution to the maintenance of gynodioecy are severely reduced under conditions where females have high relative fecundity (i.e., low resources and high-female sex ratios). High relative fertility plus high female frequency means that the evolution of phenotypic traits in hermaphrodites (i.e., response to selection via seed function) should be manifested through females because most hermaphrodites will have female mothers. Fruit set was never under strong selection in females; hence, selection to increase fruit set hermaphrodites will be less effective in maintaining their fruiting ability in natural populations with low resources and high female frequency. In sum, both sex ratio and resource availability influence trait evolution indirectly-through their effects on relative fertility of the sexes and patterns of selection. Sex ratio did not impose strong pollen limitation on females but did directly moderate the outcome of natural selection by biasing the maternal sex of the next generation. This direct effect of sex ratio on the manifestation of natural selection is expected to have far greater impact on the evolution of traits, such as seed-producing ability in hermaphrodites and the maintenance of sexual polymorphisms in nature, compared to indirect effects of sex ratio on relative fertility of the sexes.