Sex chromosomes and sex determination pathway dynamics in plant and animal models
In this review, we discuss and compare data obtained from animal and plant models, focusing our attention on the mechanisms that affect sex linkage and changes in sex-determining pathways. Patterns in data across taxa suggest that sex bias and the dynamics that occurs within hybrid zones can play an important role in these processes that enable the spread of some otherwise handicapped genotypes. We discuss the data obtained from several main plant model species in the light of the patterns demonstrated in animal models. In several plant models, we discuss possible differences in the age of their sex-determining pathways and the age of their current sex chromosomes. We also address an open question: how can an X/A ratio based sex-determining system evolve from a sex determining system based on two genes on the Y chromosome that control two separate sex-determining pathways (for the control of gynoecium suppression and anther promotion)? Taking inspiration from the well described mechanisms involved in sex determination dynamics in animals, we suggest a hypothetical stepwise scenario of change of the plant sex-determining system based on two separate sex-determining pathways (for the control of gynoecium suppression and anther promotion) into the other sex-determining systems. We suppose that an intermediate step occurs before shift to X/A based sex determination. At that phase, sex determination in plants is still based on an active Y chromosome, although there exists already a connected control of both sex determining pathways. We suggest that this connection is enabled by the existence of the genes that control sexual dimorphism in the vegetative state of plant development, and that, in some circumstances, these genes can become sex determining genes.