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Location and properties of some of the major loci affecting the segregation distortion phenomenon in Drosophila Melanogaster Sharp, Cecil Bert


There has recently been renewed interest concerning the location of the major loci responsible for the Segregation Distortion phenomenon in Drosophila melanogaster. Hartl (1974) has shown that two major sites are involved: Sd and Rsp. Rsp confers insensitivity to SD chromosomes, while Sd is considered to be the major locus that initiates distortion, Sd is located to the left of Rsp and both are located between Tft and cn. Ganetzky (1977) has extended these findings by showing that just distal to pr there is a locus that, if deleted on a SD chromosome, eliminates distortion and he argues that this is the Sd site. Ganetzky (1977) also uncovered another important locus, in or near the heterochromatin of 2L, that, if deleted from a SD chromosome, greatly reduces the ability of that chromosome to distort and he argued that this site is an enhancer of SD, E(SD). Ganetzky (1977) , also suggests that Rsp might be located very close to the centromere in the proximal heterochromatin of 2R. The results presented here demonstrate the presence of an important component of SD located within the proximal heterochromatin of 2L. These results also show that there is another important site located just distal to pr. However, when this site is removed by recombination from a SD chromosome, a certain level of residual distortion remains. It is argued that the site that Ganetzky (1977) called E(SD) is likely responsible for this residual distortion in the absence of the site just distal to pr. Thus the site near pr is called Sd₁ and the site near 1t is called Sd₂,. Loss of either site results in a large reduction, but not complete elimination, of the distorting ability of a SD chromosome. Other data are presented that, on the whole, agree with Ganetzky's (1977) proposal that Rsp is located in the centromeric heterochromatin of 2R, very close to the centromere. Miklos and Smith-White (1971) have suggested that k (the segregation ratio observed from a given mating) is a deceptive measure of the degree of distortion and they have proposed another method of measuring distortion based on their model of sperm dysfunction. Some of the weak assumptions of this model are discussed and a simpler alternative is presented. The alternative model assumes that the potential segregation ratios of a population of SD males follow a truncated normal distribution. Data are presented that are not necessarily inconsistent with this assumption. The same data show that it is likely that certain SJJ chromosomes differ in their susceptibility to modifiers of It is concluded that at present k provides the clearest measure of distortion.

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