A factor on a wild third chromosome (IIIRa) that modifies the Segregation Distortion phenomenon inDrosophila melanogaster

SUMMARY IIIRais a genetic modifier of Segregation Distortion (SD) inDrosophila melanogaster, which was discovered in the same natural population from Ranna (Sicily) that carriedSDRa. It is located at 49·7 ± 0·8 on chromosome III.IIIRawas found to have a dominant effect on segregation distortion which varied with the origin of theSDchromosome tested. Thus it enhanced the level of distortion caused by 14SDchromosomes from seven natural populations in Southern Italy and Sicily, but decreased the level of distortion caused bySDR−1, a chromosome from a natural population near Rome. Moreover,IIIRadetermined or enhanced the distorting effect ofSDRain males heterozygous forSDRaand variousSD+wild chromosomes differently sensitive toSDRa. The frequency of chromosomes having an effect likeIIIRachromosome was very high (around 70%) in samples from two natural populations of Southern Italy tested-those of Ranna and Corato. No effects ofIIIRaother than its ability to modify SD have been detected.