Figure 5.

Major sex chromosome changes associated with divergence of mammal lineages. De novo evolution of SRY and the XY sex chromosome system (green star) occurred early in therian evolution and may have triggered divergence of Theria from Prototheria. At the same time, the ancient ZW system was bolstered by translocation with an autosome (blue cross), which may also have promoted divergence of Prototheria from Theria. Two further translocations occurred before divergence of platypus and echidna; different final translocations may have triggered divergence of Families Ornithorhynchidae (platypus) and Tachyglossidae (echidna). In the Theria, fusion of the XY with an autosome (denoted by +) occurred before eutherian radiation, and may have triggered divergence of Metatheria and Eutheria. Fusions are evident in some marsupial lineages, and many changes in sex determination (symbols in other colours) are observed in different rodent lineages which may signify a recent burst of sex chromosome turnover and speciation.