Mapping fitness landscapes to interpret sex allocation in hermaphrodites
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Sex allocation theory successfully predicts sex-ratio variation among organisms with separate sexes, but it has been much less successful in explaining variation in sex allocation in hermaphrodites because the assumption of a direct tradeoff between male and female functions is often violated. Here, we show that sex-allocation theory can be applied to hermaphrodites simply by mapping components of seasonal reproductive success onto a fitness landscape defined by potentially independent measures of allocation to male and female functions on orthogonal axes. Taking this approach allowed us to interpret the complex variation in the reproductive strategy of a long-lived perennial herb ( Pulsatilla alpina ) that produces both male and bisexual flowers and that shifts between male and female allocation among seasons. We find that components of reproductive success for P. alpina map onto a rugged landscape with peaks that reflect an interactive effect of male and female allocations on self-fertilization and total reproductive success and that correspond to the observed sex-allocation strategies adopted by the species in nature. This simple approach should be widely applicable to problems in the study of hermaphroditic reproduction in other plants and animals.
Significance Statement
Sex allocation theory has helped to explain sex-ratio variation in numerous dioecious species, but it has been difficult to apply to hermaphrodites, in which male-female tradeoffs are often obscure. Here, we show that by mapping fitness estimates for plants with complex allocation patterns on a two-dimensional landscape defined by both male and female allocations, we sidestep the tradeoff assumption. Our analysis reveals fitness peaks that correspond precisely to the strategic allocation decisions adopted by the species in nature. Our simple but novel approach provides a rescue-line for a powerful body of theory that has been criticized for being too difficult to apply to the messy world of hermaphrodites, both in plants and animals.
