Expression of the sex determining cascade genes Sex-lethal and doublesex in the phorid fly Megaselia scalaris

Sex-lethal (Sxl) and doublesex (dsx) are known to represent parts of the sex-determining cascade in Drosophila melanogaster. We generated cDNA probes of the homologous genes from Megaselia scalaris, a fly species with an epistatic maleness factor as the primary sex determining signal. In Northern blot hybridization of poly(A)(+) RNA, the M. scalaris dsx probe detected two bands, one of which had a sex-specific size difference, while the Sri probe bound to RNAs of equal size in females and males. RT-PCR showed Sri to be transcribed in gonads of adult females and males but not in somatic tissues. Thus, while dsr appears to have a similar function in M. scalaris and D. melanogaster, Sxl does not. The results suggest that the sex-determining pathway of M. scalaris joins that of D. melanogaster between the Sri and dsx steps.