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Platypus Special/Letter

Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes

¹ Research School of Biological Sciences, Australian National University, Canberra 2601, Australia; ² Department of Veterinary Medicine, Cambridge Resource Centre for Comparative Genomics, University of Cambridge, Cambridge CB3 OES, United Kingdom; ³ School of Molecular and Biomedical Science, University of Adelaide, Adelaide 5005, Australia; ⁴ Faculty of Veterinary Science, University of Sydney, Sydney 2006, Australia; ⁵ School of Medicine, Genome Sequencing Center, Washington University, St. Louis, Missouri 63130, USA

In therian mammals (placentals and marsupials), sex is determined by an XX female: XY male system, in which a gene (SRY) on the Y affects male determination. There is no equivalent in other amniotes, although some taxa (notably birds and snakes) have differentiated sex chromosomes. Birds have a ZW female: ZZ male system with no homology with mammal sex chromosomes, in which dosage of a Z-borne gene (possibly DMRT1) affects male determination. As the most basal mammal group, the egg-laying monotremes are ideal for determining how the therian XY system evolved. The platypus has an extraordinary sex chromosome complex, in which five X and five Y chromosomes pair in a translocation chain of alternating X and Y chromosomes. We used physical mapping to identify genes on the pairing regions between adjacent X and Y chromosomes. Most significantly, comparative mapping shows that, contrary to earlier reports, there is no homology between the platypus and therian X chromosomes. Orthologs of genes in the conserved region of the human X (including SOX3, the gene from which SRY evolved) all map to platypus chromosome 6, which therefore represents the ancestral autosome from which the therian X and Y pair derived. Rather, the platypus X chromosomes have substantial homology with the bird Z chromosome (including DMRT1) and to segments syntenic with this region in the human genome. Thus, platypus sex chromosomes have strong homology with bird, but not to therian sex chromosomes, implying that the therian X and Y chromosomes (and the SRY gene) evolved from an autosomal pair after the divergence of monotremes only 166 million years ago. Therefore, the therian X and Y are more than 145 million years younger than previously thought.

⁷ Corresponding author.

E-mail waters{at}rsbs.anu.edu.au; fax 61-2-61254891.

Article published online before print. Article and publication date are at http://www.genome.org/cgi/doi/10.1101/gr.7101908.

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