title: Relationships between vertebrate ZW and XY sex chromosome systems.
abstract: The peculiar cytology and unique evolution of sex chromosomes raise many fundamental questions. Why and how sex chromosomes evolved has been debated over a century since H.J. Muller suggested that sex chromosome pairs evolved ultimately from a pair of autosomes. This theory was adapted to explain variations in the snake ZW chromosome pair and later the mammal XY. S. Ohno pointed out similarities between the mammal X and the bird/reptile Z chromosomes forty years ago, but his speculation that they had a common evolutionary origin, or at least evolved from similar regions of the genome, has been undermined by comparative gene mapping, and it is accepted that mammal XY and reptile ZW systems evolved independently from a common ancestor. Here we review evidence for the alternative theory, that ZW<-->XY transitions occurred during evolution, citing examples from fish and amphibians, and probably reptiles. We discuss new work from comparative genomics and cytogenetics that leads to a reconsideration of Ohno's idea and advance a new hypothesis that the mammal XY system may have arisen directly from an ancient reptile ZW system.
authors.affiliation: Research School of Biological Sciences, The Australian National University, Canberra, ACT 2601, Australia. Tariq.Ezaz@anu.edu.au
authors.forename: Tariq
authors.initials: T
authors.lastname: Ezaz
authors.forename: Rami
authors.initials: R
authors.lastname: Stiglec
authors.forename: Frederic
authors.initials: F
authors.lastname: Veyrunes
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Marshall Graves
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005260:Female; D008297:Male; D012730:Sex Chromosomes; D019849:Sex Determination Processes; D014714:Vertebrates / Q000235:genetics*