Roughly half of the species of bryophytes have separate sexes (dioecious) and half are hermaphroditic (monoecious). This variation has major consequences for the ecology and evolution of the different species. In some sexually reproducing dioecious bryophytes, sex ratio has been shown to vary with environmental conditions.
Bisang et al. use a specifically designed molecular marker to assign sex in individual shoots, collected from a natural environment, of the dioecious moss Drepanocladus trifarius, which has rare sexual reproduction and lacks apparent secondary sex characteristics. They find that although the sexes do not differ with regard to annual growth, spatial distribution or habitat requirements, the genetic sex ratio is nevertheless significantly female-biased. This supports the notion that factors other than sex-related differences in reproductive costs and sexual dimorphism can also drive the evolution of biased sex ratios in plants.