Exploring the stability of sex chromosome systems in <i>Anolis</i> and other iguanas

Sex determination was once thought to be unstable in ectothermic vertebrates, but several highly diversified groups of non-avian reptiles, such as iguanas sensu lato (Pleurodonta), appear to have conserved sex chromosomes. However, this statement has been criticized as being based on a parsimonious conclusion from limited sampling. Here, we tested sex chromosome homology in a further 35 species of pleurodont lizards, mainly of the family Anolidae. We demonstrated sex chromosome homology based on the comparison of the gene dosage of X-specific genes by quantitative PCR. Including these new data, the homology of sex chromosomes has so far been supported in 92 out of over 1,200 recently recognized species of Pleurodonta, representing all but one family of this clade. These very conserved sex chromosomes are at least as old as the basal split of Pleurodonta (estimated over 90 MY). In the case of the family Corytophanidae, all genera share different XX/XY sex chromosomes, which are thus over 25 million years old. We discuss the efficiency and limitations of the approach used for tests of the homology of sex chromosomes, as well as the reasons for the evolutionary stability of sex chromosomes in some lineages. We argue that to obtain a complete picture, all extant species should be tested for homology; however, until we reach this ambitious goal, parsimonious estimates in lineages where fragmentary data suggest stability of sex chromosomes, as in Pleurodonta, are substantiated.