CREWS, David: Sex determination: Where environment and genetics meet

In mammals and birds the genetic constitution established at the time of fertilization determines the type of gonad that develops, whereas in all crocodilians and many turtles it is the temperature experienced during the mid-trimester of embryogenesis that initiates gonadal differentiation. There is considerable conservation in the genetic cascades that underlie the sex determination process in vertebrates and the patterns of expression of these genes appear to reflect phylogenetic relationships, with mammals being similar to turtles, and birds more similar to crocodilians. For example, the timing of SOX9 and MIS expression during testis development appears to fall along phylogenetic lines; in mammals and turtles SOX9 expression precedes MIS expression, while in the alligator and the chicken the reverse pattern applies. Estrogen is a fundamental regulator of sexual development, acting through transcription factors to modulate the respective genetic pathways. Natural products and synthetic chemicals can mimic or interfere with the mechanisms that govern vertebrate reproductive development and function. For example, early exposure to compounds that interfere with the sex determination process can compromise an individual�s reproductive success. Together these discoveries have stimulated a paradigm shift in environmental toxicology, creating an opportunity to do for wildlife what applied molecular genetic techniques promise for mankind. I will describe some of the recent advances in our understanding of the triggers of genetic pathways either inherited from the parents or derived from the environment that direct vertebrate sex determination. Study of reptilian model systems under environmentally relevant physical and chemical environments will provide information necessary for intelligent environmental policy decisions.