Multiple independent origins of sex chromosomes in amniotes
- Eric J. Vallender*,† and
- Bruce T. Lahn†,‡
- *New England Primate Research Center, Division of Neurochemistry, Harvard Medical School, Southborough, MA 01772-9102; and
- ‡Department of Human Genetics, Howard Hughes Medical Institute, University of Chicago, Chicago, IL 60637
The extraordinary diversity of sex-determination mechanisms has long been noted. Many species resort to environmental cues for the determination of sex. A well known example is temperature-dependent sex determination in many (although not all) reptiles. Yet, many other species use genetic mechanisms, i.e., sex chromosomes, for the determination of sex. The best-studied systems of genetic sex determination include the XX:XY system that arose independently in mammals and Drosophila, the XX:XO system in Caenorhabditis elegans, and the ZZ:ZW system in birds. It is generally accepted that environmental sex determination is the ancestral state and that genetic sex determination evolved as a derived condition. It is also recognized that genetic sex determination is evolutionarily highly labile, having evolved into existence on many independent occasions across diverse taxa. A case in point is sex-determination mechanisms in amniotes (a clade encompassing reptiles, birds, and mammals). The ancestral state in amniotes is likely temperature-dependent sex determination, which is still found in many extant reptilian species, such as crocodilians and some turtles and lizards (1). From this ancestral state, genetic sex determination evolved in birds, which utilize the ZZ:ZW system, and also independently in mammals, which use the XX:XY system. The ZZ:ZW system is also found in all snake species. The split of the mammalian lineage from the rest of amniotes occurred ≈315 …
†To whom correspondence may be addressed. E-mail: eric_vallender{at}hms.harvard.edu or blahn{at}bsd.uchicago.edu
