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 t he sex-determining cascade in Drosophila melanogaster. We generated cD NA probes of the homologous genes from Megaselia scalaris, a fly speci es with an epistatic maleness factor as the primary sex determining si gnal. In Northern blot hybridization of poly(A)(+) RNA, the M. scalari s dsx probe detected two bands, one of which had a sex-specific size d ifference, 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 fema les and males but not in somatic tissues. Thus, while dsr appears to h ave a similar function in M. scalaris and D. melanogaster, Sxl does no t. The results suggest that the sex-determining pathway of M. scalaris joins that of D. melanogaster between the Sri and dsx steps.