Environmental Sex Determination with Overlapping Generations

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Abstract
A population-genetics model is developed for environmental sex determination (ESD) in age-structured (overlapping-generation) populations. The model allows opportunities for reproduction by either sex to vary with spatial location (patch type) but not with age. The model is then applied to Salix arctica, a dioecious plant that shows habitat segregation by sex and a female-biased sex ratio. To explain the sex-ratio data, the model predicts that male mortality in both patch types is higher than female mortality and that the mortality ratio is much higher (about 12 times) in the dry patches. The sex-ratio data cannot be explained by current (e.g., discrete-generation) ESD theory. The "non-adaptation" hypothesis that the habitat-specific sex ratios are generated by sex-differential mortality is also discussed. The current model may aid in making decisions about whether habitat-specific sex ratios arise from ESD or from differential mortality, be it sex-specific or patch-specific.