On the Components of Segregation Distortion in DROSOPHILA MELANOGASTER. II. Deletion Mapping and Dosage Analysis of the SD Locus.

Segregation distorter (SD) chromosomes are preferentially transmitted to offspring from heterozygous SD/SD(+) males owing to the induced dysfunction of the SD(+)-bearing sperm. This phenomenon involves at least two major loci: the Sd locus whose presence is necessary for distortion to occur and the Rsp locus which acts as the site of Sd action. Several additional loci on SD chromosomes enhance distortion.-In a previous study deletions were used to map the Sd locus and to determine some of its properties. We have extended this analysis with the isolation and characterization of 14 new deletions in the Sd region. From our results we conclude (1) SD chromosomes contain a single Sd locus located in region 37D2-6 of the salivary gland chromosome map. Deletion of this locus in any of three SD chromosomes now studied results in complete loss of ability to distort a sensitive chromosome; (2) the reduced male fecundity observed in many homozygous SD or SD(i)/SD(j) combinations is due at least in part to the action of the Sd locus. The fecundity of these males can be substantially increased by deletion of one Sd locus. Thus, it is the presence of two doses of Sd rather than the absence of Sd(+) that produces the lowered male fecundity in SD homozygotes; (3) Sd behaves as a neomorph, whereas Sd(+), if it exists at all, is amorphic with respect to segregation distortion; (4) these results support a model in which the Sd product is made in limiting amounts and the interaction of this product with the Rsp locus causes sperm dysfunction. The Sd product appears to act preferentially at Rsp(s) (sensitive-Responder) but may also act at Rsp(i) (insensitive-Responder).