Based on its Z-sex-chromosomal location and its structural homology to male sexual regulatory factors in humans (DMRT1 and DMRT2), Drosophila (Dsx), and Caenorhabditis elegans (Mab-3), chicken DMRT1 is an excellent candidate for a testis-determining factor in birds. The data we present provide further strong support for this hypothesis. By whole mount in situ hybridization chicken DMRT1 is expressed at higher levels in the male than in the female genital ridges during early stages of embryogenesis. Its expression becomes testis-specific after onset of sexual differentiation. Northern blot and RT PCR analysis showed that in adult birds DMRT1 is expressed exclusively in the testis. We propose that two gene dosages are required for testis formation in ZZ males, whereas expression from a single Z chromosome in ZW females leads to female sexual differentiation.
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