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

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Abstract
SUMMARY: IIIRa is a genetic modifier of Segregation Distortion (SD) in Drosophila melanogaster, which was discovered in the same natural population from Ranna (Sicily) that carried SDRa. It is located at 49·7 ± 0·8 on chromosome III. IIIRa was found to have a dominant effect on segregation distortion which varied with the origin of the SD chromosome tested. Thus it enhanced the level of distortion caused by 14 SD chromosomes from seven natural populations in Southern Italy and Sicily, but decreased the level of distortion caused by SDR−1, a chromosome from a natural population near Rome. Moreover, IIIRa determined or enhanced the distorting effect of SDRa in males heterozygous for SDRa and various SD+ wild chromosomes differently sensitive to SDRa. The frequency of chromosomes having an effect like IIIRa chromosome was very high (around 70%) in samples from two natural populations of Southern Italy tested-those of Ranna and Corato. No effects of IIIRa other than its ability to modify SD have been detected.