Does sexual dimorphism in plants promote sex chromosome evolution?

Many plant species with separate sexes (dioecious species) show sexual dimorphism, although this is less often recognised than in animals. It seems likely that evolutionary changes will follow after dioecy evolves, and adaptations after such changes can potentially be studied in dioecious plants, which have often recently changed from cosexuality. Developmental processes that evolved long before the species' current sex-determining switch evolved may undergo fundamental changes during the evolution of dioecy, affecting not only flowers and inflorescences, but probably the physiology of the whole plant, to support the different roles of the two sexes (secondary sex differences). Some changes may benefit one sex, but be deleterious in the other. Such sexually antagonistic conflicts in the initial stages of the evolution of dioecy are thought to be important in favouring changes in genes linked to the primary sex-determining loci, potentially leading to recombination becoming suppressed in the genome region carrying these genetic factors (evolution of a greater degree of sex-linkage). Sexual antagonism may also select for the evolution of sex-specific expression of such genes. This review discusses approaches for obtaining evidence for adaptive changes and distinguishing secondary sex differences from primary differences (including their pleiotropic effects).