TEMPERATURE-DEPENDENT SEX DETERMINATION IN REPTILES - PROXIMATE MECHANISMS, ULTIMATE OUTCOMES, AND PRACTICAL APPLICATIONS

In many egg-laying reptiles, the incubation temperature of the egg det ermines the sex of the offspring, a process known as temperature-depen dent sex determination (TSD). In TSD sex determination is an ''all or none'' process and intersexes are rarely formed. How is the external s ignal of temperature transduced into a genetic signal that determines gonadal sex and channels sexual development? Studies with the red-eare d slider turtle have focused on the physiological, biochemical, and mo lecular cascades initiated by the temperature signal. Both male and fe male development are active processes-rather than the organized/defaul t system characteristic of vertebrates with genotypic sex determinatio n-that require simultaneous activation and suppression of testis- and ovary-determining cascades for normal sex determination. It appears th at temperature accomplishes this end by acting on genes encoding for s teroidogenic enzymes and steroid hormone accepters and modifying the e ndocrine microenvironment in the embryo. The temperature experienced i n development also has long-term functional outcomes in addition to se x determination. Research with the leopard gecko indicates that incuba tion temperature as well as steroid hormones serve as organizers in sh aping the adult phenotype, with temperature modulating sex hormone act ion in sexual differentiation. Finally, practical applications of this research have emerged for the conservation and restoration of endange red egg-laying reptiles as well as the embryonic development of reptil es as biomarkers to monitor the estrogenic effects of common environme ntal contaminants. (C) 1994 Wiley-Liss, Inc.