On the components of segregation distortion in Drosophila melanogaster. III. Nature of enhancer of SD.

Analysis of X-ray-induced deletions in the Segregation Distorter (SD) chromosome, SD-5, revealed that this chromosome had a gene proximal to lt in the centric heterochromatin of 2L that strongly enhanced the meiotic drive caused by the SD chromosome. This Enhancer of Segregation Distortion [E(SD)] locus had not been characterized in earlier studies of SD chromosomes because it cannot be readily separated by recombination from the Responder (Rsp) locus in the proximal heterochromatin of 2R.--To determine whether E(SD) is a general component of all SD chromosomes and to examine further its effects on distortion, we produced deletions of E(SD) in three additional SD chromosomes. Analysis of these deletions leads to the following conclusions: (1) along with Sd and Rsp, E(SD) is common to all SD chromosomes; (2) the E(SD) allele on each SD chromosome enhances distortion by the same amount, which indicates that allelic variation at the E(SD) locus is not responsible for the different drive strengths seen among SD chromosomes; (3) E(SD) causes very little or no distortion by itself in the absence of Sd; (4) E(SD), like Sd, acts in a dosage-dependent manner; (5) E(SD) exerts its effect in cis or trans to Sd; and (6) if E(SD)+ exists, its function is not related to SD.