X-linked elements associated with negative segregation distortion in the SD system of Drosophila melanogaster.

Three elements, M1, M2 and M3, found in a special X chromosome, supp-X(SD), modify the degree and direction of segregation distortion in the SD system of Drosophila melanogaster. The first element, M1, is located between the y and the cv loci, probably close to the y locus. The second element, M2, is located near the cv locus and the third element, M3, is located between the y and the car loci. The M1 element appears to cause a relatively small amount of reduction in the rate of recovery of the SD-72, but not the cn bw, chromosome from SD-72/cn bw males, when raised at 27.5 degrees. The M2 and the M3 elements cause considerable decrease in the recovery rate of the SD-72 chromosome, whereas they increase the recovery rate of the cn bw chromosome. The amount of decrease is nearly the same as the amount of increase for each element. Some type of "switch" mechanism in the directions of distortion is suggested for each of these two elements and their effects appear to be approximately additive.