Summary

Some seeds of dioecious plants develop into male plants and others become females. Brothers and sisters can grow close together in the seed shadow of the maternal plant, which promotes sib-mating, and classical sex-allocation theory predicts a slight female bias among the seeds produced. We describe different ways of examining seed sex ratios and some of the pitfalls involved. The available direct (seed sex ratio) and indirect (proportions of male and female plants in the field) evidence suggests that the seed sex ratio is often close to 0.5, despite the fact that there is genetic variation in the seed sex ratio in some cases. The combination of significant sib-mating and an unbiased seed sex ratio is at odds with classical sex-allocation theory. Genetic conflict theory might provide new insights and should be a central theme in future research. The adult sex ratio can also become male or female biased due to sexually differential mortality, but this does not influence the seed sex ratio.

Introduction

Like most animals, but unlike the great majority of plant species, dioecious plants have separate male and female individuals. Both male and female organs develop in each of their flowers, in separate floral whorls, but the development of one type is halted before maturity (Grant et al. 1994), with the timing of the arrest differing between species.