CREWS, D*; PARTESOTTI, S; PORTER, R; RAMSEY, M; SKIPPER, JK; WU, M; Unversity of Texas, Austin: Physiology of temperature-dependent sex determination in reptiles

The triggers that determine sexual differentiation of the bipotential gonad are inherited genes in some species or environmental signals in other species. In many reptiles it is the temperature experienced during the mid-trimester of embryogenesis that determines the type of gonad that develops. Temperature-dependent sex determination (TSD) is considered ancestral to genotypic determination and, indeed, except for the difference in initial triggers, many of the same components are involved in both developmental pathways. In the red-eared slider turtle relatively low incubation temperatures produce all males whereas relatively high incubation temperatures produce all females. The transition from an all-male to an all-female sex ratio is abrupt, occurring over a 1 �C range where mixed sex ratios are produced. During the critical period of sex determination, sex steroid hormones can act as the physiological equivalent of incubation temperature; administering exogenous steroid hormones or inhibiting their synthesis during incubation alters the normal temperature-induced sex determination outcome. Recent findings also indicate that in spite of a difference in triggers, the red-eared slider shares many characteristics of sexual development with the mammalian system. Besides steroidogenic enzymes and sex steroid hormone receptors, genes involved in male sex differentiation such as DMRT-1, SF-1, SOX 9, MIS have been identified in the turtle and other reptile models. The inherent lability of sex determination in this species coupled with the rarity of intersexes and the likelihood that mammalian sex determination mechanisms are evolutionarily conserved makes the red-eared slider an excellent model for dissecting the molecular mechanisms underlying testicular and ovarian development during embryogenesis.