The origin of female-biased sex ratios in intertidal seagrasses (Phyllospadix spp.).

Flowering sex ratios of dioecious plants are commonly male-biased but rarely female-biased. While greater costs of reproduction from females have been repeatedly demonstrated and explain male biases, male reproductive costs almost never exceed female costs, making the origins of female biases enigmatic. I investigated the seagrasses Phyllospadix scouleri and P. serrulatus (surfgrasses), which have some of the most extreme female-biased sex ratios documented (>90% female), to identify the mechanisms driving sex ratio bias. I developed sex-linked amplified fragment length polymorphism (AFLP) markers and applied them to three P. scouleri life stages at four sites to determine when during the life cycle sex ratio bias arises. Sex ratios were even among seedlings but became more female-biased at later life stages, indicating that sex ratios were driven by male-biased mortality. To identify when during the life cycle sex ratio bias developed, I examined sex differences in survival among seedlings and three aspects of reproductive costs that could potentially generate biased sex ratios under field conditions. No differences in seedling survival between the sexes were detected, and there was no evidence of substantial sex differences in costs of reproduction. I found no support for a trade-off between current and future reproduction or between reproductive investment and growth. Thus, costs of reproduction appear unlikely to drive sex ratio bias in surfgrass. Instead, small sex differences in growth and survival spread across the life cycle appear to be responsible for female-biased sex ratios and suggest that life history trade-offs other than reproductive costs drive sex ratio bias.