SEX-SPECIFIC CONTROL OF SEX-LETHAL IS A CONSERVED MECHANISM FOR SEX DETERMINATION IN THE GENUS DROSOPHILA

In D. melanogaster the binary switch gene Sex-lethal (Sri) plays a piv otal role in somatic sex determination - when the Sri gene is on the f emale pathway is followed, while the male pathway is followed when the gene is off, In the present study we have asked whether the Sri gene is present in other species of the genus Drosophila and whether it is subject to a similar sex-specific on-off regulation, Sri proteins were found in all of the drosophilids examined, and they display a sex-spe cific pattern of expression, Furthermore, characterization of the Sri gene in the distant drosophilan relative, D. virilis, reveals that the structure and sequence organization of the gene has been well conserv ed and that, like melanogaster, alternative RNA processing is responsi ble for its sex-specific expression. Hence, this posttranscriptional o n-off regulatory mechanism probably existed before the separation of t he drosophilan and sophophoran subgenera and it seems likely that Sri functions as a sex determination switch gene in most species in the Dr osophila genus, Although alternative splicing appears to be responsibl e for the on-off regulation of the Sri gene in D. virilis, this specie s is unusual in that Sri proteins are present not only in females but also in males, The D. virilis female and male proteins appear to be id entical over most of their length except for the aminoterminal approx, 25 aa which are encoded by the differentially spliced exons, In trans criptionally active polytene chromosomes, the male and female proteins bind to the same cytogenetic loci, including the sites corresponding to the D. virilis Sri and tra genes, Hence, though the male proteins a re able to interact with appropriate target pre-mRNAs, they are appare ntly incapable of altering the splicing pattern of these pre-mRNAs.