Fig. 1.

Independent origins of sex chromosomes in birds, snakes, and mammals. In ancestral amniotes, which presumably used temperature-dependent sex determination, there were no sex chromosomes. Sex chromosomes then evolved from autosomes on three independent occasions in birds, snakes, and mammals. A different autosome was converted to sex chromosomes in each of these three lineages. The ZZ:ZW system emerged twice (once in birds and once in snakes), whereas the XX:XY system emerged once in mammals.