Molecular mechanisms of TSD in reptiles: a search for the magic bullet

Significant progress has been made in understanding mechanisms of genetic sex determination. The ZFY gene encodes a zinc finger protein but is not the primary signal in sex determination. The SRY gene is the testis determining gene in man, mouse, rabbit, and probably marsupial mouse and wallaby. Temperature dependent sex determination probably involves a modification of development of the indifferent gonad due to differential expression of one or more specific DNA sequences whose behavior is controlled by some temperature sensitive process or to differential action of a gene product such as a protein. There are ZFY and SRY-like genes in reptiles. We cloned and sequenced a portion of the ZFY gene (Zft) from snapping turtle (Chelydra serpentina) that is found in both sexes. We cloned and sequenced portions of SRY-like genes (Sra for SRY-related-autosomal) from snapping turtle. Similar genes are found in alligator (Alligator mississippiensis) and lizards. Cladistic analysis suggests that there are two or three major families of SRY-like genes in vertebrates in addition to sex specific SRY genes located on the Y chromosome of eutherian and marsupial mammals. When placed on a phylogenetic tree these data indicate that Sras were present in early tetrapods. Sequestering of the SRY gene on the Y chromosome probably happened only once and this may have been the defining moment that set the mammalian line of Therapsid reptiles apart from other reptilian groups.