Female-predominant sex ratios in angiosperms

A model is presented which expresses the seed production of dioecious and gynodioecious animal-pollinated Angiosperms in terms of the relative seed-fecundity of the sexes, the number of pollinator visits to each flower and the sex ratio. The model predicts that the maximum seed set occurs when females predominate, providing the pollinators visit each flower more than once and the seed set of males is not high. There is strong evidence that a marked preponderance of females in species of four genera is due to more frequent fertilisation by female-determining pollen nuclei than by male-determining nuclei. Two hypotheses have been proposed to explain this differential fertilisation. Several objections are raised to the hypothesis of Lewis (1942), Mulcahy (1967) and Kaplan (1972) that female-predominant sex ratios have been selected because they maximise the total seed production of populations. It is considered that the hypothesis of Smith (1963) that the differential fertilisation is a consequence of the genetic differentiation of sex chromosomes offers a more likely explanation of female-predominant sex ratios.