US-AUSTRALIAN ACADEMIES JOINT WORKSHOP

US-AUSTRALIAN ACADEMIES JOINT WORKSHOP ON VERTEBRATE COMPARATIVE GENOMICS
Beckman Conference Centre, Irvine, California, 23-25 May 2007

Homology shared by bird and platypus sex chromosomes reveals a recent origin for mammalian sex chromosomes
by Paul Waters

Dr Paul Waters received his PhD working on mammalian Y chromosome evolution at The Australian National University, 2003. He has since completed a postdoctoral in South Africa working on the genomes of afrotherian mammals, and is currently a postdoctoral fellow in the Comparative Genomics Group, RSBS, ANU. His research interests include sex chromosome structure, function and evolution, X-chromosome inactivation, comparative genomics and transposable elements.

Therian mammals (placentals and marsupials) have an XX female: XY male sex chromosome system, in which a dominant gene on the Y (SRY) determines maleness. However, in platypus (a monotreme mammal) females have 10 X chromosomes and males have 5X and 5 Ys that form a chain at meiosis. One end of this chain (X5) contains genes orthologous to the chicken Z, and the other end (X1) was thought to bear genes orthologous to the human X, suggesting that monotreme sex chromosomes represent an intermediate evolutionary step between a bird like ZZ/ZW system and a human like XX/XY system. No SRY has been found.

However, regions orthologous to the human X all map to platypus chromosome 6, including SOX3, the gene from which SRY evolved. Chicken Z genes are distributed over several platypus Xs. Thus the monotreme sex chromosome complex arose from an ancient reptile ZW system and does not contain an SRY homologue. The therian X and Y evolved from an autosome pair (represented by platypus 6) after monotremes diverged 210 MYA but before marsupials and placentals split 180MYA, so are much younger than previously thought.

Contact details:
Comparative Genomics Group
Research School of Biological Sciences
The Australian National University
GPO Box 475, ACT 2601
Canberra, Australia

Tel: +61 2 6125 3612/2371
Fax: +61 2 6125 4891
Email: waters@rsbs.anu.edu.au