The original autosome from which the mammal X and Y diverged can be deduced from the relationship of the X pair to other vertebrate sex chromosome systems.
2005 Fraser and Heitman 2005 Steinemann and Steinemann 2005 Graves 2006). The acquisition of a sex-determining gene on one member of an autosome pair and the accumulation of sex-specific alleles nearby were followed by suppression of recombination between the new proto-sex chromosomes, which resulted in vulnerability to mutation and deletions of the sex-specific chromosome (Y or W) and, therefore, its degradation ( Charlesworth and Charlesworth 2000 Charlesworth et al. The similarities shared by sex chromosomes are thus the results of convergent evolutionary forces in different vertebrate lineages. The sex-determining gene that initiates testis development is also different for instance, there is no SRY in non-mammal vertebrates, and sex determination in birds seems to be largely controlled by dosage of a Z-borne gene (possibly DMRT1) ( Raymond et al. However, comparative gene mapping shows that the sex chromosomes of mammals (XY), birds (ZW), and snakes (ZW) are entirely nonhomologous, implying that they evolved from different pairs of autosomes ( Fridolfsson et al. The paired chromosome (X or Z) in the homogametic sex is generally well conserved, large, and gene-rich, whereas the sex-specific Y or W is usually small, heterochromatic, and almost devoid of active genes. Sex chromosomes are named XY when the heterogametic sex is the male (female XX and male XY as in mammals) and ZW when heterogametic sex is the female (female ZW and male ZZ as in birds or snakes).Īmniote XY and ZW sex chromosome pairs are superficially similar. Genetic sex determination is the most widespread within lineages and commonly involves a pair of differentiated sex chromosomes (for review, see Ezaz et al. In other amniotes (reptiles and birds), sex is determined by a variety of different mechanisms that belong in two broad classes: genetic or environmental, although recent data show that some species combine both ( Quinn et al. Mapping orthologs of mammal X-borne genes in other vertebrates will help our understanding of how the mammal XY and the sex-determining ( SRY) gene evolved, and also how they function.
The mammal X and Y evolved from a pair of autosomes after the proto-Y acquired a male-determining gene, X–Y recombination between male-advantage genes was suppressed, and the Y degenerated ( Charlesworth et al. Marsupial mammals have a smaller X and Y that represent an ancestral therian mammal sex pair and define the ancient region of the human sex chromosomes. The X chromosome of most placental mammals has a virtually identical gene content, whereas the degenerate Y chromosome contains overlapping subsets of only a few active genes (for review, see Graves 2006). In mammals, with a few notable exceptions, sex is determined by an XX female:XY male sex chromosome system in which the Y chromosome bears the male-dominant testis-determining gene SRY.