09-P010 A nonsense mutation in the TFIIH p52 gene causes abnormal rRNA processing in the zebrafish intestinal mutant, sycorax

Sex in birds is chromosomally based, as in mammals, but the sex chromosomes are different and the mechanism of avian sex determination is a long-standing mystery. In the chicken and other birds, the homogametic sex is male (ZZ) and the heterogametic sex is female (ZW). Two hypotheses have been proposed for the mechanism of avian sex determination. The W (female) chromosome may carry a dominant-acting ovary determinant. Alternatively, the dosage of a Z-linked gene may mediate sex determination, two doses being required for male development (ZZ). A strong candidate avian sex-determinant under the dosage hypothesis is the conserved Z-linked gene, DMRT1. DMRT1 encodes a transcription factor related to male sex regulators in the fly and worm. Here, we used a novel RNA interference approach to deliver microRNA directed against DMRT1 into chicken embryos. Knockdown of DMRT1 in ovo leads to feminisation of the embryonic gonads in genetically male (ZZ) embryos. Affected males show partial sex reversal, characterised by a feminised left gonad and a right testis. The feminised gonad shows reduced DMRT1 protein expression, disorganised testis cords and a decline in the testicular marker, SOX9. The ovarian marker, Aromatase, is ectopically activated. Germ cells also show a female pattern of distribution in the feminised male gonads. This is the first functional test of any candidate avian sex-determinant. Our results indicate that DMRT1 plays a key role in chicken testis determination. The data support the Z dosage hypothesis for bird sex determination, with DMRT1 representing the elusive avian sex determinant.