From brain determination to testis determination: evolution of the mammalian sex-determining gene.
In mammals, sex is determined by an XY male:XX female sex chromosome system in which a male-dominant gene on the Y chromosome (SRY) determines testis formation. Sex chromosomes evolved from an ordinary autosome pair as the Y chromosome was progressively degraded. The Y chromosome has lost nearly all of its 1500 original genes, and those that survived did so because they evolved a critical role in male determination or differentiation. SRY is typical of Y-borne genes. Comparative gene mapping and sequencing shows that SRY arose quite recently as a degraded version of the SOX3 gene on the X chromosome. SOX3 is expressed predominantly in brain, and so is more likely to be a brain-determining than a testis-determining gene. The male-dominant action of SRYmay be an illusion, as its structure suggests that it works by interfering with the action of a related gene, which in turn inhibits testis development. This hypothesis can give a good account of how a brain-determining gene acquired a role in testis determination via differential dosage of SOX3. SRYhas no central role in sex determination and it can be replaced as a trigger and loft, as have many other Y-borne genes in recent evolutionary history. The absence of SRY in two species of the mole vole (Ellobius) suggests that its useful life is already running out.
Graves
J A
JA
Research School of Biological Sciences, Australian National University, Canberra, ACT. graves@rsbs.anu.edu.au
D000818:Animals; D005075:Biological Evolution; D001921:Brain; D002454:Cell Differentiation; D002874:Chromosome Mapping; D004268:DNA-Binding Proteins / Q000235:genetics; D024282:Genes, sry; D006609:High Mobility Group Proteins / Q000235:genetics; D008297:Male; D055748:SOXB1 Transcription Factors; D019849:Sex Determination Processes; D013737:Testis; D014157:Transcription Factors; D014960:X Chromosome; D014998:Y Chromosome
D004268:DNA-Binding Proteins; D006609:High Mobility Group Proteins; D055748:SOXB1 Transcription Factors; D014157:Transcription Factors