In humans, the sex chromosomes are very different in size and gene content; the X is a mid-sized chromosome bearing about genes in Mb , whereas the Y is roughly a third that size and highly heterochromatic, containing only 45 active genes coding for unique proteins. The homology of the bird and monotreme sex chromosomes infers an ancestral gray lines , superseded by SRY in therian mammals dashed taken from [, ]. This will help us to gain an under- standing of how SRY evolved in the first therian mammals, and of the pathway of which SRY only recently assumed control. In light of this startling new information, here we review for the first time data from which we can deduce the sex-determining state of the ancestral mammal, which may have utilized extremely ancient sex chromosomes predating the divergence of reptiles and mammals. The XY pair still share small terminal regions, known as pseudoautosomal regions PARs , that are homologous and continue to recombine at meiosis. The differentiation of the therian sex chromosomes from this ancient autosome began when one member of the ancestral autosome pair acquired a new sex- determining allele, defining a proto-Y and proto-X . Y genes are highly differentiated and specialized copies of the widely expressed X homologues from which they evolved.
Comparisons between the Y chromosomes of placen- tal species reveals that this process has occurred in parallel, as different, although overlapping, subsets of genes have survived on each Y reviewed in . Other sex-specific alleles accumulated near this locus, and suppression of recombination between the nascent sex chromosomes ensured that the male- specific genes stayed together. In mice,the X-borne Sox3 gene, from which Sry evolved, is widely expressed in the gonad and central nervous system, whereas Sry expression has a much more limited profile in the testis and brain . In the absence of recombination, selection is inefficient; a favorable allele can promote an otherwise poor Y hitchhiking , or be lost due to being on a poor Y background selection. In light of this startling new information, here we review for the first time data from which we can deduce the sex-determining state of the ancestral mammal, which may have utilized extremely ancient sex chromosomes predating the divergence of reptiles and mammals. A small separate region of the ancient X with homology to chicken chromosome 12, als o separate in fish, was identified from examination of the database , but it is not clear whether this is truly orthologous. A considerable shock to our understanding of mam- mal sex chromosomes and sex determination was dealt by our recent discovery that the basal monot- reme mammals subclass Prototheria , which di- verged from Theria MYA, do not share the same XY system as therians, and lack an SRY orthologue [10—13]. The Y retains only a small fraction of its original gene content, and uniquely in the genome, is specialized for reproductive function. Two main processes have been implicated with the attrition of the therian Y: In addition, most genes on the Y chromosome have paralogous copies on the X, from which they obviously diverged. Comparative mapping in marsupials, however, reveals two evolu- tionary layers . In humans, the sex chromosomes are very different in size and gene content; the X is a mid-sized chromosome bearing about genes in Mb , whereas the Y is roughly a third that size and highly heterochromatic, containing only 45 active genes coding for unique proteins. A bombshell was the finding that genes on monotreme sex chromosomes map to the bird ZW pair [11, 13, 15, 16]. Sex-determining sys- tems of amniotes. Although not obvious from morphology, gene map- ping reveals that thehuman Yis a highly differentiated copy of the X. In the absence of recombination, the Y chromosome progressively Cell. Humans and mice retain different subsets of genes on the Y, as does the tammar wallaby. Cellular and Molecular Life Sciences are heteromorphic: Gene mapping experiments show that the therian XY pair has no homology to the bird ZW , and the snakeZW pair is different again . All groups of therian mammals possess an SRY gene on the Y chromosome except for a few exceptional rodents such as the mole voles  and Japanese country rats [3, 4], which have lost their Y and SRY recently. The higher mutation rate of the Y, com- pared to the X and the autosomes, is due to its continual passage through the testis, which is a highly mutagenic oxidative environment where germ cells undergo many rounds of replication in the absence of repair enzymes in sperm . The gene content of the Y varies between species, but SRY orthologues have been identified on the Y in both branches of therian mammals, placentals infraclass Eutheria and marsupials infraclass Metatheria , implying that this sex-determining mechanism was present before their divergence million years ago MYA reviewed in . The therian sex chromosomes are homologous to auto- somes in non-mammalian vertebrates, and the bird and snake ZW pairs are homologous to autosomes in therian mammals [5—7]. Genes on the long arm and pericentric region of the human X lie on the marsupial X, whereas genes distal to Xp SRY is therefore unique to the therian mammals, and the evolution of SRY, as a new sex-determining switch, presumably initiated the differentiation of the therian X and Y sex chromosome pair — MYA Fig. The homology of the avian and monotreme sex chromosomes raises the possibility that these systems had a common origin in the ancestor of amniotes.
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