Sex determination in reptiles: An update

Abstract

SYNOPSIS. Sex determination and sex differentiation are two separate but related phenomena. Sex differentiation is a programmed cascade of events in which the indifferent gonad develops as a testis or an ovary with the appropriate urogenital and secondary sex characters. Sex determination is the event that sets this cascade in motion. In placental mammals, there is good evidence that sex is determined by a gene on the Y chromosome (SRY) that initiates testis formation. In the absence of SRY an ovary develops. There are, however, examples of placental mammals that develop as normal males with no detectable SRY. In reptiles, sex differentiation appears to be similar to mammals (i.e., the same genes and hormones act in a similar manner), but sex determination is clearly very different. Ovarian differentiation in placental mammals can occur in the absence of estrogen or an estrogen receptor. Ovarian differentiation 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 even if eggs are incubated at male-inducing temperatures, and conversely, will develop as males when estrogen synthesis is blocked in eggs incubated at female-inducing temperatures. A number of other genes have also been shown to be important in mammalian sex determination. One of these genes, Sox9, which is expressed in differentiating mouse testis, has recently been found to be expressed in embryonic reptile testis. Other genes that appear to be common to both mammals and reptiles in the sex determining cascade are SF1, MIH, and possibly DAX-1. Current research is now focused on how the gene that produces the enzyme necessary for estrogen synthesis (aromatase) is regulated in the embryos of reptiles with genetic or environmental sex determination. Controversial issues in reptilian sex determination are 1) the role of the brain in gonadal sex determination, and 2) the role of steroid hormones in the yolk prior to sex determination.