Gender-specific selection during early life history stages in the dioecious grass Distichlis spicata

Bias in the adult sex ratio of a population, if not due to a biased primary sex ratio, may be due to differential mortality of males and females, which can occur during any life history stage. Yet, although adult sex-ratio bias is common in dioecious plant species, little is known about the demography of prereproductive individuals. Using a RAPD-PCR (randomly amplified polymorphic DNA) marker that co-segregates with female phenotype, I studied juvenile demography in the saltmarsh grass Distichlis spicata, which exhibits within-population sex-ratio variation (spatial segregation of the sexes) and has genetic sex determination. First. I surveyed populations of D. spicata to ascertain whether any environmental factor co-varies with local sex ratio, and I found that habitats with female majorities are located at significantly lower elevations in the marsh than are habitats with male majorities. In randomly collected seeds, the sex ratio was significantly biased toward females. but male and female seeds did not differ in mass. Using a subsample of these seeds. I conducted greenhouse and field experiments to examine whether differences in seed germination or seedling survival between the sexes explain sex-ratio variation along the micro-elevation gradient in the marsh. In greenhouse treatments designed to mimic field habitats with female majorities, more male seeds germinated than did female seeds. In habitats with female majorities in the field, significantly more female seedlings survived an extreme high tide than did male seedlings (25.17% of females vs. 13.49% of males). These results suggest that, in habitats with female majorities. gender-specific bias in seedling survivorship, rather than seed germination, is a causal factor in the underlying sex-ratio bias. In habitats with male majorities, seed germination and seedling survival did not differ between the sexes. This study is the first to document environment-dependent differential seed germination and seedling mortality between males and females in a dioecious plant species.