Genetics of mammalian sex determination: some unloved exceptions.

The genetics of sex determination is a child of the twentieth century, which overturned the previously held view that sex was determined by the environment. The last quarter of the century witnessed an active search for sex-determining genes in mammals. Although successful, the modus operandi of these genes remained unknown, and the relationship between the sex-determining systems of mammals and other vertebrates remained enigmatic. To overcome these problems, scientists in the 21st century should heed William Bateson's counsel to treasure exceptions, for they point the way to progress. One exception to conventional concepts of sex determination is the bilaterally asymmetrical distribution of ovaries and testes in true hermaphroditism. Ovaries favour the left side in humans and the right side in mice. Observations suggesting that a reversal of asymmetry may occur with increasing organ size may point to a possible explanation. A reevaluation is also required regarding the beginning of sex differentiation, in view of mounting evidence of a sex difference in growth rates of early embryos. Another question to be settled is whether the function of SRY is confined to the fetal gonad. The recent demonstration that Sry induces cell proliferation in the fetal mouse gonad (Schmahl et al., 2000) further emphasizes the importance of differential growth in sex determination and differentiation. It is suggested that SRY represents an additional growth-promoting gene sequestered by mammals to enable the XY embryo to undergo male sex differentiation in the female hormonal environment of the uterus. An increased awareness of the relationship between growth and gonadal differentiation should lead to a better understanding of sex determination in mammals and an ability to relate the function of sex-determining genes to the effects of environmental factors. J. Exp. Zool. 290:484-489, 2001.