SHARED DNA-SEQUENCES BETWEEN THE X-CHROMOSOME AND Y-CHROMOSOME IN THETAMMAR WALLABY - EVIDENCE FOR INDEPENDENT ADDITIONS TO EUTHERIAN AND MARSUPIAL SEX-CHROMOSOMES

Marsupial sex chromosomes are smaller than their eutherian counterpart s and are thought to reflect an ancestral mammalian X and Y. The gene content of this original X is represented largely by the long arm of t he human X chromosome. Genes on the short arm of the human X are autos omal in marsupials and monotremes, and represent a recent addition to the eutherian X and Y. The marsupial X and Y apparently lack a pseudoa utosomal region and show only end-to-end pairing at meiosis. However, the sex chromosomes of macropodid marsupials (kangaroos and wallabies) are larger than the sex chromosomes of other groups, and a nucleolus organizer is present on the X and occasionally the Y. Chromosome paint ing using DNA from sorted and microdissected wallaby X and Y chromosom es reveals homologous sequences on the tammar X and Y chromosomes, con centrated on the long arm of the Y chromosome and short arm of the X. Ribosomal DNA sequences were detected by fluorescence in situ hybridiz ation on the wallaby Xp but not the Y. Since no chiasmata have been ob served in marsupial sex chromosomes, it is unlikely that these shared sequences act as a pseudoautosomal region within which crossing over m ay occur, but they may be required for end-to-end associations. The sh ared region of wallaby X and Y chromosomes bears no homology with the recently added region of the eutherian sex chromosomes, so we conclude that independent additions occurred to both sex chromosomes in a euth erian and macropodid ancestor, as predicted by the addition-attrition hypothesis of sex chromosome evolution.