Gonadal differentiation in amphibians is sensitive to steroids. The phenotypic sex can be changed by hormonal treatments, but the molecular mechanism for gonadal differentiation is not known. Up to date, many genes involved in gonadal differentiation have been isolated in vertebrates. Dmrt1, a gene that contains the DM-domain (Doublesex/Mab-3 DNA-binding motif), is considered to be one of the essential genes involved in the testicular differentiation cascade in mammals, birds, reptiles, and fish. However, this gene has not been isolated in amphibians yet. To elucidate its role(s) for gonadal differentiation in vertebrates, a molecular cloning of Dmrt1 in amphibians is urgent. In this study, we have successfully isolated a Dmrt1 homolog from the frog Rana rugosa testis cDNA library and examined its expression during gonadal differentiation and in sex-reversed gonads. The Dmrt1 mRNA was exclusively detected in testis among adult tissues by the RT-PCR analysis. The Dmrt1 was first expressed in the differentiating testis at stage XXV in which spermatogonia are only germ cells, and became stronger at later stages. Moreover, the Dmrt1 transcript was not detected during ovarian differentiation. However, this gene was clearly expressed in XX sex-reversed gonads caused by injection of testosterone into all-female tadpoles that have well-differentiated ovaries. Taken together, the results suggest that Dmrt1 is closely implicated in testicular, but not ovarian differentiation in amphibians.
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