Genetic and environmental control of temporal and size-dependent sex allocation in a wind-pollinated plant

Sex allocation in hermaphrodites can be affected by spatial and temporal variation in resources, especially in plants where size-dependent gender modification is commonplace. The evolution of sex allocation will depend on the relative importance of genetic and environmental factors governing patterns of investment in female and male function. In wind-pollinated plants, theoretical models predict a positive relation between size and male investment because of the fitness advantages associated with more effective pollen dispersal. Theory also predicts that the timing and allocation to each sex function should depend on available resources. We grew maternal half-sibling families of annual, wind-pollinated, Ambrosia artemisiifolia in sun and shade treatments to investigate these predictions. There was significant genetic variation for female and male flower production in both sun and shade treatments. Size-dependent sex allocation occurred in the direction predicted by theory, with male flower production increasing more rapidly in larger plants. The timing of sex function also varied, with significant genetic variation for dichogamy within environments and plasticity of this trait between environments. Protandry was expressed more commonly in the sun and protogyny in the shade. The occurrence of dynamic sex allocation with changing size and experimental treatment indicates the potential for adaptive responses under different ecological conditions.