Genetic and developmental analysis of the sex-determining gene ‘double sex’ (dsx) ofDrosophila melanogaster

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Abstract
Summary: Sex determination inDrosophiladepends on the ratio ofXchromosomes to sets of autosomes (X:A). This chromosomal signal is used to regulate a few control genes whose state of activity selects either the male or the female sexual pathway. We have studied the structure and function ofdsx(double sex) which appears to be the last regulatory gene on whose function the sexual pathway eventually depends. We have mutagenized the locus, varied the doses of dominantdsx-mutations and wildtype alleles, and combined differentdsx-alleles with recessive mutations in other sex-determining genes, such asix,tra-2andtra.The locusdsxharbours two genetic functions,dsxmto implement the male program,dsxfto implement the female program. We found thatdsxmanddsxfcan mutate independently although most mutations abolish both functions. We conclude thatdsxmanddsxfeach have their specific domain, but also share a large region of DNA that is essential for both functions. We present evidence that the dominant mutations correspond to a constitutive expression of the male-determining functiondsxm, with the simultaneous abolishment of the female-determining functiondsxf. This effect can be counteracted by two doses of expresseddsxfso that a female phenotype results. The products of one dose of expresseddsxmand one dose of expresseddsxfin the same cell appear to neutralize each other which leads to a null phenotype. The mutant combinations suggest that the product ofdsxfrequires the products ofix+,tra-2+andtra+to become functional.