Female-biased sex allocation is commonly observed in hermaphroditic plants, though equal resource allocation to the female and the male functions is predicted to be evolutionarily stable in a simple model. I briefly reviewed factors that select for biased sex allocation in hermaphroditic plants; i.e., non-linear fitness, self-fertilization, non-linear trade-off in resource allocation to the female and the male functions, local mate competition, and pollen limitation. I also showed that the trade-off between growth and reproduction does not select for biased sex allocation unless constraints prevent simultaneous maximization of the amount of resources allocated to reproduction and equalization of resources allocated to the female and the male functions. Finally, I present a new hypothesis that can explain female-biased sex allocation. In this model, a source-sink relationship in flower/fruit growth was considered. The ESS sex allocation including fruits is female-biased if fruit growth is sink-limited at least during its early stage.
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