Figure 4.

Phylogeny displaying the different sex chromosome systems of major vertebrate groups. Homology between genomes, and with ancestral autosomes, is indicated by the same color. Key events are marked on the tree. (Green) A chicken-like ZW system appears to be ancestral to all amniotes, which was retained in birds and caught up in the meiotic chain of monotremes. There was a switch from ZW to XY systems independently in both therian and monotreme mammals. (Red) The region added to the sex chromosomes of placental mammals was broken independently in opossum and monotremes. (Green) The autosome orthologous to the bird Z chromosome was broken in the therian ancestor and subsequently scrambled onto several autosomes in mouse; therefore only the chromosome carrying Dmrt1 is displayed in that species.