Segregation Distorter

In Drosophila melanogaster, Segregation Distorter (SD) haplotypes are found at a few percent in most natural populations. Heterozygous males transmit the SD allele to more than 90% of their offspring. However, a single copy of SD is strongly deleterious to diploids of both sexes. The main components are a distorter locus, Sd, approximately 1 cM from the centromere of the second chromosome, and a responder locus, Rsp, very close to the centromere. In nature, the driving allele at Sd is strongly associated with the insensitive Rspⁱ allele. Thus, the SD haplotype carries Sd Rspⁱ, and the wild-type haplotype SD⁺ carries Sd⁺Rsp⁺. (In other words, these alleles are in almost complete linkage disequilibrium [Fig. WN21.1].)

This association is maintained by strong epistasis. Male recombinants carrying the driving allele Sd with the sensitive Rsp⁺ allele destroy their own sperm, whereas the opposite haplotype (Sd⁺ Rspⁱ) suffers the deleterious effects of Rsp but gains no advantage from segregation distortion (Fig. WN21.1A). Although the SD region includes about 10% of the Drosophila genome, this centromeric region is heterochromatic and so experiences little recombination (Fig. WN21.1B). In addition, several inversions are associated with SD, which further reduce recombination. Chromosomes carrying the SD haplotype are almost identical across the centromeric region and show no appreciable divergence from the more diverse SD⁺ haplotypes. This implies that the SD haplotype arose very recently, and it provides one of very few examples of a selective sweep in which we actually know the causes of selection (pp. 536–538).

Palopoli and Wu (1996) give evidence that the SD haplotype has recently increased to high frequency. Both components of this system have now been identified, although the precise mechanism is unclear. Sd encodes a protein that is inappropriately localized to the nucleus and interferes with Ran signaling there, and the Rspⁱ allele corresponds to less of a highly repeated satellite DNA array.