THE EVOLUTION OF SEX IN FLOWERING PLANTS

Summary 1. The importance of combined genetic and systematic analysis of sex for its evolutionary interpretation has long been understood in animals. It now seems possible to make ah evolutionary interpretation of sex in plants by similar methods. 2. From the sporadic distribution of dioecious species throughout mainly hermaphrodite families of flowering plants, it is inferred that dioecious species have evolved from hermaphrodites. 3. From the more frequent association of dioecious species with monoecious species than with hermaphrodites, it is inferred that they have evolved more frequently by means of a monoecious intermediary. 4. Highly organized sex‐determining mechanisms of dioecious plants can evolve from single gene segregation, as shown (a) by the synthesis of dioecious strains of hermaphrodite species, and (b) by the analysis of sex in dioecious species of plants. 5. This evolution is illustrated by the study of sex chromosomes and shows the transition from the type where the two chromosomes are indistinguishable to highly complex systems. Lower stages are commonest in plants, higher stages in insects and mammals. 6. Male sterility which has become established in wild populations of hermaphrodite species is cytoplasmically inherited in all known cases. This system is not considered to be a transitional stage in the development of dioecy but rather as another stable outbreeding mechanism. 7. Sporadic distribution and lower differentiation of sex chromosomes agree in showing that sex differentiation is short‐lived in flowering plants. This is to be expected since sex separation in sessile organisms, in comparison with other outbreeding systems in plants, suffers from two shortcomings: (a) that the degree of outbreeding is inflexible, and (b) that utilization of gametes is wasteful. Male sterility is not subject to these limitations. 8. This gametic wastage is mitigated by changes in the sex ratio. Such changes are effected by the differential pollen tube growth of male and female determining pollen grains. For this reason it is the male that usually becomes the heterogametic sex. 9. We can thus have an intelligible evolutionary interpretation of sex in plants when we consider (a) the sporadic occurrence of sex separation, (b) the simplicity of the mechanism determining this separation, and (c) the great diversity of other sexual breeding systems.