Sex allocation, inbreeding, and measures of population differentiation in hermaphroditic metapopulations

ABSTRACT Selection in inbreeding populations is expected to favour female-biased sex ratios in dioecious or gonochoristic species as a result of local mate competition, a prediction that finds strong support in populations in which females have control of the proportion of their sons versus daughters. Local mate competition due to inbreeding should also promote female-biased sex allocation in hermaphrodites, with reduced emphasis on the production of sperm or pollen relative to eggs, ovules or seeds. While inbreeding can be the direct result of the mating system in local populations, it can also result from demographic causes such as population turnover in metapopulations with frequent local extinction and recolonization. This effect of the turnover of demes has previously been analysed under the ‘haystack model’ for species with separate sexes. Here, we use quantitative genetic simulations to ask how population turnover affects the evolution of sex allocation in hermaphroditic metapopulations, and we assess the extent to which different genetic measures of inbreeding and population differentiation, especially FST and Jost’s D, predict the equilibrium sex allocation. We find that population turnover may dramatically enhance the female bias of hermaphroditic metapopulations, particularly where the inter-deme migration rate is low, even where local inbreeding, measured by FIS, is low or absent. In such situations, FST is a good predictor of the equilibrium sex allocation, and much better than Jost’s D. Our study extends predictions for sex allocation in subdivided populations that might experience population turnover to hermaphroditic species, and draws attention in general to the power of Wright’s hierarchical inbreeding statistics to predict the sex allocation in metapopulations at equilibrium