What Is the Evolutionary Basis of Sex Determination Variability in Fish?
Why is sex determination so diverse in fish compared to mammals and birds? It has been suggested that fish might have some developmental and genomic predisposing peculiarities, including a flexibility of differentiation of male and female gonads from a same precursor tissue, as well as a high number of gene duplicates and an important genomic plasticity [Volff, 2005; Froschauer et al., 2006; Mank et al., 2006]. However, the most important but so far largely unanswered question concerns the selection pressures behind the diversity of sex determination. Does the frequent switching between sex-determining systems provide any advantage to fish, or is it only a consequence of a peculiar mode of evolution? Particularly interesting is the fact that most Y and W sex chromosomes are poorly or not degenerate in fish despite an apparent suppression of recombination in the sex-determining region. Are fish sex chromosomes so poorly differentiated because they did not have time to degenerate, or bec