Fig. 1.

The evolution of sex-determining mechanisms in reptiles. Clades were included to highlight the major divisions of reptiles. Both trees are identical except the tree on the left has outgroups Mammalia and Xenopus whereas the tree on the right used no outgroup in ancestral state reconstruction. Choice of outgroup affects number and kind of changes in reptilian sex-determining mechanisms as reconstructed by BayesTraits (http://www.evolution.rdg.ac.uk/BayesTraits.html). Note that several species-rich groups within Aves and Serpentes are entirely genotypically sex-determined with a ZZ/ZW sex chromosome system while many lepidosauromorph groups show a variety of sex-determining mechanisms.