Androgen receptor of the frog Rana rugosa: molecular cloning and its characterization

The androgen receptor(AR) gene is located on the Z and W sex chromosomes in the frog Rana rugosa, designated Z- and W-AR, respectively. Among various tissues of an adult frog, AR expression levels were highest in the testis and brain. In the testis, AR was expressed in germ cells. AR expression occured in developing embryos from stage 21 and was very high in the gonad of a male tadpole before the onset of sex determination. When Z- and W-AR were expressed in Xenopus A6 cells, they activated androgen-dependent transcription of a luciferase reporter gene. By contrast, estrogen receptor (ER) alpha and beta showed no sexually dimorphic expression during sex determination, but their expressions became much higher in the gonad of a female tadpole after sex determination. In addition, AR transcripts in the ZZ-tadpoles were twice as abundant as in the ZW genotype. In contrast, W-AR expression was extremely low although when W-AR was expressed in A6 cells, it activated transcription in the luciferase assay. In this regard it is worth noting that the promoter regions of Z- and W-AR are not identical. The results suggest that Z-AR plays an important role in the testis formation in a R. rugosa tadpole, whereas ERbeta is involved in ovary differentiation. Very low expression of W-AR may be due to its promoter region having mutations in key transcription factor binding sites, although these remain to be identified. Thus, it is proposed that AR could be a candidate for a male-determining gene in R. rugosa.