Figure 2.

Overview of the steps in sex chromosome evolution with empirical support in vertebrates. This hypothetical network of evolutionary trajectories may branch off to potentially endless outcomes with a possibility to freeze for a long time and even reverse to certain states. Steps (1–6) are the same as in figure 1 but sexually antagonistic genes are not necessarily involved and can be replaced here by general sex-linked genes; (7)—switch to hermaphroditism or ESD, where no sex chromosomes are present; (8)—long-term evolution without emergence of sex chromosomes; (9)—long-term persistence of poorly differentiated sex chromosomes; (10)—reversal to stages with less differentiated sex chromosomes; (11)—expansion of repeats on sex-specific sex chromosome causing its change in size; (12)—accumulation of repeats on both sex chromosomes; (13)—fusion of the sex chromosomes with an autosome leading to expansion of the pseudoautosomal region; (14)—emergence of a new sex-determining locus on another chromosome; (15)—emergence of a new sex-determining locus within existing sex chromosomes; (16a,b)—two translocations of the same sex-determining locus to other chromosomes; (17)—origin of a new system of sex determination by involvement of B chromosome; (18)—emergence of sex-determining systems with three homologous sex chromosomes; (19)—fusion of sex chromosomes with an autosome leading to multiple neo-sex chromosomes; (20)—introgression of a sex-determining gene from a different population or species; (21)—allopolyploidization connected with emergence of a new sex-determining system in a genome of hybrid-origin. For simplification, only the heterogametic sex is depicted.