Marsupial Gene Mapping and the Evolution of Mammalian Sex-Chromosome Form and Function

Comparisons between the gene maps of distantly related mammalian species can provide information about the evolution of genome arrangement and function in mammals. Marsupial gene mapping is now being vigorously pursued, using newly developed cell and molecular techniques to complement classic breeding studies of model species. Gene associations and localisations established by all these techniques are tabulated, and the beginnings of gene maps, based on in situ hybridisation, are presented for a macropodid and a dasyurid species. The significance of marsupial gene mapping is apparent even from these limited data, which show that large autosomal regions have been conserved between marsupials and eutherians. However, an ancient X-autosome translocation is revealed, which either removed most of the human X short-arm markers (including the putative sex determining factor) to autosomes or added this region to a smaller ancestral X. The implications of these findings to theories of mammalian sex chromosome evolution and function are discussed, and a hypothesis proposed for a gradual differentiation of the mammalian X and Y chromosome, accompanied by progressive spreading of X chromosome inactivation.