THE ROLE OF ESTROGEN IN TURTLE SEX DETERMINATION AND THE EFFECT OF PCBS

In the current model of vertebrate sex determination and sexual differ entiation, gonadal sex is fixed at fertilization by specific chromosom es, a process known as genotypic sex determination (GSD). Only after t he gonad is formed do hormones begin to exert an influence that modifi es specific structures that eventually will differ between the sexes. Many egg-laying reptiles do not exhibit GSD but rather depend on the t emperature of the incubating egg to determine the gonadal sex of the o ffspring, a process termed temperature-dependent sex determination (TS D). Research on TSD indicates that sex determination in these species is fundamentally different in at least one way. Gonadal sex is not irr evocably set by the genetic composition inherited at fertilization but depends ultimately on which genes encoding for steroidogenic enzymes and hormone receptors are activated during the midtrimester of embryon ic development by temperature. incubation temperature modifies the act ivity as well as the temporal and spatial sequence of enzymes and horm one receptors such that sex-specific hormone milieus, created in the u rogenital system of the developing embryo, determine gonad type. Estro gen is the physiologic equivalent of incubation temperature and the pr oximate cue that initiates female sex determination. There is increasi ng evidence that some polychlorinated biphenyl (PCB) compounds are cap able of disrupting reproductive and endocrine function in fish, birds, and mammals, including humans. Reproductive disorders resulting from exposure to these xenobiotic compounds may include reductions in ferti lity, hatch rate in fish and birds, and viability of offspring, as wel l as alterations in hormone levels or adult sexual behaviors, all of w hich have further implications, particularly in wildlife population dy namics. Research on the mechanism through which these compounds may be acting to alter reproductive function indicates estrogenic activity, by which the compounds may be altering sexual differentiation. In TSD turtles, the estrogenic effect of some PCBs reverses gonadal sex in in dividuals incubating at an otherwise male-producing temperature. Furth ermore, certain PCBs are synergistic in their effect at very low conce ntrations.