Reptilian embryos: A model for sex determination and sexual differentiation in amniotes

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Researchers realize that the selection of organisms for use as models requires both practical (e.g., abundance, availability, ease of care, and size) and scientific (appropriateness of the model) considerations. Based upon both of these considerations the use of embryonic reptiles as models for studies focusing on sex determination and sexual differentiation in amniotes is suggested. Phylogenetically, the class Reptilia gave rise both to birds and mammals, and ontogenic-ally, reptiles are very similar to these groups. Among amniote vertebrates, reptiles are unique in that they exhibit two distinct kinds of sex-determining mechanisms: genotypic sex determination (GSD) and temperature-dependent sex determination (TSD). In addition representatives of this group exhibit diverse patterns of sex chromosome morphology ranging from male heteromorphic to homomorphic, to female heteromorphic. This diversity has provided researchers with unique opportunities to address questions regarding the physiology of gonadal determination. This paper reviews recent studies which have demonstrated that 1) GSD and TSD may have a similar molecular basis; 2) steroid hormones can override the normal sex-determining mechanism both in species with TSD and GSD; 3) the embryonic period of hormone sensitivity coincides with the period of normal gonadal differentiation; and 4) once a gonad has differentiated it is apparently committed and cannot be “sex-reversed.” In addition, conflicting reports regarding the effects of exogenous hormones on species exhibiting TSD and those with heteromorphic sex chromosomes are considered from the point of view that such variations are the result of differences in mechanisms controlling the various levels of sexuality.