Pleiotropic effects of sex-determining genes in the evolution of dioecy in two plant species.

One reason for studying sex chromosomes of flowering plants is that they have often evolved separate sexes recently, and the genomes of dioecious species may not yet have evolved adaptations to their changes from the ancestral state. An unstudied question concerns the relative importance of such adaptation, versus the effects of the mutations that led to separate sexes in the first place. Theoretical models for such an evolutionary change make the prediction that the mutations that created males must have sexually antagonistic effects, not only abolishing female functions, but also increasing male functions relative to the ancestral functional hermaphrodites. It is important to test this critical assumption. Moreover, the involvement of sexual antagonism also implies that plant sex-determining genes may directly cause some of the sexual dimorphisms observed in dioecious plants. Sex-determining genes are starting to be uncovered in plants, including species in the genera Diospyros and Actinidia (families Ebenaceae and Actinidiaceae, respectively). Here, we describe transgenic experiments in which the effects of the very different male-determining genes of these two dioecious species were studied in a non-dioecious plant, Nicotiana tabacum. The results indeed support the critical assumption outlined above.