Sex Determination in Reptiles: An Update

Sex determination and sex differentiation are two separate butrelated phenomena. Sex differentiation is a programmed cascadeof events in which the indifferent gonad develops as a testisor an ovary with the appropriate urogenital and secondary sexcharacters. Sex determination is the event that sets this cascadein motion. In placental mammals, there is good evidence thatsex is determined by a gene on the Y chromosome (SRY) that initiatestestis formation. In the absence of SRY an ovary develops. Thereare, however, examples of placental mammal that develop as normalmales with no detectable SRY. In reptiles, sex differentiationappears to be similar to mammals (i.e., the same genes and hormonesact ina similar manner), but sex determination is clearly verydifferent. Ovarian differentiation in placental mammals canoccur in the absence of estrogen or an estrogen receptor. Ovariandifferentiation in reptiles requires the presence of estrogen.In the absence of estrogen a testis develops. In TSD reptiles,embryos will develop as females when treated with estrogen evenif eggs are incubated at male-inducing temperatures, and conversely,will develop as males when estrogen synthesis is blocked ineggs incubated at female-inducing temperatures. A number ofother genes have also been shown to be important in mammaliansex determination. One of these genes, Sox9, which is expressedin differentiating mouse testis, has recently been found tobe expressed in embryonic reptile testis. Other genes that appearto be common to both mammals and reptiles in the sex determiningcascade are SF- 1, MIH, and possibly DAX-1. Current researchis now focused on how the gene that produces the enzyme necessaryfor estrogen synthesis (aromatase) is regulated in the embryosof reptiles with genetic or environmental sex determination.Controversial issues in reptilian sex determination are 1) therole of the brain in gonadal sex determination, and 2) the roleof steroid hormones in the yolk prior to sex determination