Figure 3.

Orthology of sex chromosomes and autosomes in amniotes. Four conserved regions, containing orthologous genes, are represented as green, dark blue, red and yellow regions in a variety of amniotes, lying on sex chromosomes (background shaded in yellow, pink or blue) or autosomes (numbered where known). The human (eutherian) XY pair consists of the blue and green ancestral regions that are separate in reptiles and birds and in monotremes and marsupials; the blue represents the ancestral therian XY (XCR) and the green blocks an addition in eutherians (XAR or YAR). SOX3 in the dark-blue block evolved into the mammalian sex-determining gene SRY. The dark-blue and the green blocks are autosomal in birds, reptiles and monotreme mammals. In birds, a different ancestral block (red), containing the sex-determining gene DMRT1, forms the ZW sex-chromosome pair; this gene block is also present in the ZW sex-chromosome pair in a gekko lizard (not shown), and on the sex-chromosome complex of platypus. In eutherians and marsupials, this block is autosomal. The red block is also autosomal in snakes, in which a different chromosome (yellow) has taken on a sex-determining role; the snake ZW is autosomal in all other lineages. All these genome regions are autosomal in a turtle that has temperature sex determination (TSD).