Segregation Distorter is a genetic system in which males heterozygous for a SD second chromosome and a Rsp-sensitive homolog transmit the SD chromosome to greater than 50% of progeny, a phenomenon known generally as meiotic drive. Factors on the SD chromosome induce a failure of chromatin condensation in haploid spermatids receiving the Rsp-sensitive homolog.
SD chromosomes from nature carry a dominant, gain-of-function mutation in RanGAP called RanGAP[Sd]. They also carry dominant alleles of M(SD), E(SD) and St(SD), which strengthen the SD effect. Homologous second chromosomes susceptible to the SD effect carry a sensitive version of the Responder (Rsp) locus, a region of heterochromatin consisting of 120 bp A-T rich repeats. The number of repeats correlates with the degree of sensitivity, which ranges from insensitive (Rsp[i]) through sensitive (Rsp[s]) to supersensitive (Rsp[ss]). SD chromosomes from nature are Rsp-insensitive, while wild type chromosomes can be Rsp-insensitive, -sensitive or -supersensitive.
The following stocks have second chromosomes isolated from natural populations that induce the SD effect when combined in males with Rsp-sensitive or -supersensitive chromosomes.
The chromosomes carry inversions unrelated to the components of the SD system. The RanGAP[Sd] allele is associated with the tandem duplication Dp(2;2)RanGAP[Sd]. SD-5, SD-72 and SD-Mad are symbols for whole SD chromosomes including chromosomal rearrangements and allelic variants (similar to the way balancer symbols describe particular combinations of inversion breakpoints and marker mutations).
Wild type tester chromosome
This stock carries a second chromosome that has been used in many studies and is sensitive (Rsp[s]) to the effects of SD chromosomes.
Rsp loss-of-function mutation
This stock carries a deletion of Rsp from a chromosome that was originally sensitive to SD, making the chromosome insensitive.
This stock carries the dominant M(SD) allele on a chromosome lacking other SD components
This stock carries the dominant E(SD) allele on a chromosome lacking other SD components. The chromosome is Rsp-sensitive.
This stock carries the dominant, X-linked SD suppressor Su(SD)1-M3.
Stocks for male- or female-biased progeny
The SD components can be manipulated to eliminate most X-bearing or Y-bearing sperm. Because the genetics is complex, we describe these stocks on a separate webpage here.