Autosomal location of genes from the conserved mammalian X in the platypus (Ornithorhynchus anatinus): implications for mammalian sex chromosome evolution Paul D. Waters 1,2 , Margaret L. Delbridge 1 , Janine E. Deakin 1 , Nisrine El-Mogharbel 1 , Patrick J. Kirby 1,3 , Denise R. Carvalho-Silva 1,4 & Jennifer A. Marshall Graves 1 1 Comparative Genomics Group, Research Group of Biological Science, The Australian National University, Canberra, ACT 2601, Australia; E-mail: pwaters@sun.ac.za; Tel: +27 021 8083229; Fax: +27 021 8082405; 2 Current address: Department of Zoology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa; 3 Current address: National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709, USA; 4 Current address: The Wellcome Trust Sanger Institute, Genome Campus, Hinxton, Cambs CB10 1SA, UK Received 21 January 2005. Received in revised form and accepted for publication by Adrian Sumner 8 April 2005 Key words: evolution, mammalian, Ornithorhynchus anatinus, platypus, X chromosome Abstract Mammalian sex chromosomes evolved from an ancient autosomal pair. Mapping of human X- and Y-borne genes in distantly related mammals and non-mammalian vertebrates has proved valuable to help deduce the evolution of this unique part of the genome. The platypus, a monotreme mammal distantly related to eutherians and marsupials, has an extraordinary sex chromosome system comprising five X and five Y chromosomes that form a translocation chain at male meiosis. The largest X chromosome (X 1 ), which lies at one end of the chain, has considerable homology to the human X. Using comparative mapping and the emerging chicken database, we demonstrate that part of the therian X chromosome, previously thought to be conserved across all mammals, was lost from the platypus X 1 to an autosome. This region included genes flanking the XIST locus, and also genes with Y-linked homologues that are important to male reproduction in therians. Since these genes lie on the X in marsupials and eutherians, and also on the homologous region of chicken chromosome 4, this represents a loss from the monotreme X rather than an additional evolutionary stratum of the human X. Introduction All mammals have an XX female: XY male sex chromosome system, or some variant of it. A dominant gene (SRY ) on the Y determines male- ness (Sinclair et al. 1990, Koopman et al. 1991). The large euchromatic X and small, hetero- chromatic and gene-poor Y evolved from a homologous autosomal pair (Ohno 1967) via a process of Y degradation (Charlesworth 1991) after it acquired a male-dominant sex-determining locus, and recombination with the X was sup- pressed. All that remains of once extensive recombination between the human X and Y is a small pseudoautosomal region (PAR1) at the ends of Xp and Yp, and a smaller PAR2 at the Chromosome Research 13: 401–410, 2005. 401 # 2005 Springer. Printed in the Netherlands