SEX DETERMINATION AND PRIMARY SEX-DIFFERENTIATION IN AMPHIBIANS - GENETIC AND DEVELOPMENTAL MECHANISMS

Most amphibians lack morphologically distinguishable sex chromosomes, but a number of experimental techniques have shown that amphibian sex determination is controlled genetically. The few studies suggesting th at environment influences sex determination in amphibians have all bee n conducted at temperatures outside of the range normally experienced by the species under study, and these effects probably do not occur un der natural conditions. No sex-determining genes have been described i n amphibians, and sex differentiation can be altered by treatment with exogenous steroid hormones. The effects of sex steroids vary extensiv ely between species, and a variety of steroids can alter the sex ratio s of treated larvae. The role of endogenous sex steroids in gonadal di fferentiation has not been fully explored; thus the natural role of st eroids in amphibian gonadal differentiation is unknown. Sex steroid re ceptors have not been examined in amphibian gonads, and the mechanism of steroid action on the gonad is unclear. In addition to steroids, th e thyroid hormones may play a role in gonadal differentiation. Pituita ry gonadotrop(h)ins affect gonadal growth, but not differentiation or maturation of gonads. In addition to the issue of resolving the mechan isms underlying hormone action in gonadal differentiation, other debat es concerning interactions between the developing gonads and the invad ing germ cells, and even the origin of the medullary and cortical port ions of the developing gonads, remain unresolved. Studies examining li nks between sex determination and gonadal differentiation are needed. In addition, examinations of variation in steroidal effects on gonadal development in a phylogenetic context are lacking. (C) 1998 Wiley-Lis s, Inc.