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FB2026_01
,
released March 12, 2026
Tandem duplication replacing the wild type 6.5kb EcoRI fragment with an 11.5kb EcoRI fragment. The duplication results in two copies of the RanGap gene (and the Hs2st gene nested within it) being present in RanGapSD. The organisation of the RanGap and Hs2st genes in the proximal half of the duplication is essentially the same as in RanGap+. The Hs2st gene in the distal half of the duplication encodes a wild-type protein. The remaining distal transcript encodes a mutant version of RanGap, whose C-terminal portion differs from wild type beginning at the duplication junction. This results in a truncated RanGap protein that lacks the C-terminal 234 amino acids.
A 5kb tandem duplication is uniquely associated with all RanGapSD chromosomes.
Mutation is a duplication resulting in truncation of the last 234 amino acids. Mutation extends beyond the range indicated here.
A gain of function allele of RanGap associated with a 5-kb tandem duplication (Dp(2;2)RanGapSD), produces, in addition to a normal RanGap protein, one that is truncated. This mutant protein has a dramatic effect on chromosome behavior in males, causing a "Segregation Distortion" (SD) phenotype. Many chromosomes carrying RanGapSD alleles have been isolated from natural populations and were formerly named as different alleles of the "Sd" locus. However, it is now clear that this mutation had a single origin (see FBrf0054130) and that all of these chromosomes are the same, at least with respect to their RanGap allele. RanGapSD/+ males transmit their SD-bearing homolog, to the virtual exclusion of the +-bearing homolog; as many as 99% of the functional sperm may carry SD. Segregation distortion requires the presence of a 'sensitive' Rsp allele in the genome. In addition, the degree of segregation distortion is affected by alleles at a number of other interacting loci: E(SD), M(Sd), St-SD and Su(SD)X.
Electron microscopic studies of spermatogenesis (FBrf0024467, FBrf0024468) demonstrate that spermiogenesis of SD/+ males is defective; chromatin in half of the spermatid nuclei fails to condense properly, leading in some cases to a failure of the spermatids to become individually invested in membrane, remaining syncytial instead and in all cases to incomplete maturation of half the sperm.
Autosomal translocations involving either the segregation distorter chromosome or its sensitive homologue have no significant effect on the expression of segregation distortion.
Segregation distorter.
Spermatogenesis in heterozygous SD-NH males is normal until post-elongation, when approximately half of the nuclei in the spermatid bundle fail to undergo the histone transition and have cytologically abnormal nuclei. Most of these spermatids fail to be individualised and remain in syncytial groups. During the coiling process these non-individualised spermatids are pushed caudally along the cyst.
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by aubAHN/aub[+]
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by aubAWE13/aub[+]
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by aub[+]/aubHR-HN
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by aubHR-E721A/aub[+]
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by aub[+]/aubQC42
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by aub[+]/aubHN2
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by aub[+]/aubN11
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by piwi06843/piwi[+]
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by cuffWM25/cuff[+]
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by squ[+]/squPP32
RanGAPSd has abnormal meiotic cell cycle phenotype, enhanceable by zuc[+]/zucHM27
RanGAPSd has abnormal meiotic cell cycle phenotype, non-enhanceable by zuc[+]/zucHM27
RanGAPSd has abnormal meiotic cell cycle phenotype, suppressible by squ[+]/squPP32
RanGAPSd has abnormal meiotic cell cycle phenotype, suppressible by Su(Sd)2a-12a-1/Su(Sd)2a-1[+]
RanGAPSd has abnormal meiotic cell cycle phenotype, suppressible by Su(Sd)4a-24a-2/Su(Sd)4a-2[+]
RanGAPSd has phenotype, suppressible by Rcc1βTub85D.PK
RanGAPSd has phenotype, suppressible by RanβTub85D.PK
The segregation distortion seen in males carrying either the SD-5star or SD-LosArenos Dp(2;2)RanGAPSD chromosome variant (in a sensitive Rsp background) is enhanced (the k[[c]] value, or proportion of Dp(2;2)RanGAPSD-bearing progeny as the fraction of total progeny (corrected for viability), is significantly increased) if the males are also carrying a single copy of either aubAHN, aubHN2, piwi06843 or cuffWM25.
The segregation distortion seen in males carrying the SD-5star or SD-72 Dp(2;2)RanGAPSD chromosome variant (in a sensitive Rsp background) is suppressed (the k[[c]] value, or proportion of Dp(2;2)RanGAPSD-bearing progeny as the fraction of total progeny (corrected for viability), is significantly decreased) if the males are also carrying a single copy of squPP32.
The segregation distortion seen in males carrying the SD-LosArenos Dp(2;2)RanGAPSD chromosome variant (in a sensitive Rsp background) is enhanced (the k[[c]] value, or proportion of Dp(2;2)RanGAPSD-bearing progeny as the fraction of total progeny (corrected for viability), is significantly increased) if the males are also carrying a single copy of squPP32.
The segregation distortion seen in males carrying either the SD-LosArenos Dp(2;2)RanGAPSD chromosome variant (in a sensitive Rsp background) is enhanced (the k[[c]] value, or proportion of Dp(2;2)RanGAPSD-bearing progeny as the fraction of total progeny (corrected for viability), is significantly increased) if the males are also carrying a single copy of zucHM27, while segregation distortion of the SD-5star or SD-72 Dp(2;2)RanGAPSD chromosome variants is not significantly affected.
The segregation distortion seen in males carrying either the SD-5star, SD-LosArenos, SD-72 or SD-Mad-ltcn Dp(2;2)RanGAPSD chromosome variant (in a sensitive Rsp background) is enhanced (the k[[c]] value, or proportion of Dp(2;2)RanGAPSD-bearing progeny as the fraction of total progeny (corrected for viability), is significantly increased) if the males are also carrying a single copy of aubQC42.
The segregation distortion seen in males carrying either the SD-5 or SD-Mad-ltcn Dp(2;2)RanGAPSD chromosome variant is enhanced (the k[[c]] value, or proportion of Dp(2;2)RanGAPSD-bearing progeny as the fraction of total progeny (corrected for viability), is significantly increased) if the males are also carrying a single copy of aubN11.
The segregation distortion seen in males carrying either the SD-5star, SD-LosArenos, SD-Mad-bw3 or SD-Mad-ltcn Dp(2;2)RanGAPSD chromosome variant (in a sensitive Rsp background) is enhanced (the k[[c]] value, or proportion of Dp(2;2)RanGAPSD-bearing progeny as the fraction of total progeny (corrected for viability), is significantly increased) if the males are also carrying a single copy of aubAWE13.
The segregation distortion seen in males carrying the SD-LosArenos Dp(2;2)RanGAPSD chromosome variant is enhanced (the k[[c]] value, or proportion of Dp(2;2)RanGAPSD-bearing progeny as the fraction of total progeny (corrected for viability), is significantly increased) if the males are also carrying a single copy of either aubHR-HN or aubHR-E721A.
Flies carrying RanGapSD (SD chromosomes tested are SD-5 and SD-Roma) show a high level of segregation distortion against target chromosomes carrying Rsps or Rspss. ranβTub85D.PK or Bj1βTub85D.PK completely suppress the high level of segregation distortion against target chromosomes carrying Rsps or Rspss which is normally caused by RanGapSD (SD chromosomes tested are SD-5 and SD-Roma).
In the presence of the Su(SD)X chromosome males show strong positive segregation distortion at low temperatures only. At high temperatures SD-72 are dysfunctional resulting in negative segregation distortion. Negative distortion is temperature sensitive, the temperature sensitive period is during spermiogenesis. SD-72 sperm raised at 27.5oC are dysfunctional during spermiogenesis, those that are functional may lose their fertilizing ability relatively quickly after ejaculation into the female. The mechanism of negative distortion is based on differential recovery of dysfunction in two homologs during spermiogenesis, therefore causing an absolute reduction in the number of SD-72 chromosomes, the overall number of progeny remains unchanged.
Under certain genotypic conditions, the chromosome carrying the "sensitive" Rsps allele in a RanGapSD heterozygous Rspi/Rsps male can be transmitted to the progeny at frequencies greater than 0.5, or correspondingly, the chromosome carrying the "insensitive" Rspi allele can be distorted with respect to the chromosome carrying the "sensitive" Rsps allele. This "negative segregation distortion" indicates that the relative sensitivity and insensitivity of Rsps and Rspi to distortion in a male are not absolute, but relative, and they may be reversed depending on the residual genotype.
Analysis of different SD chromosomes containing the Dp(2;2)RanGapSD duplication suggests that the duplication arose only once; all SD chromosomes (9 independently isolated chromosomes have been tested) have the same constellation of polymorphic restriction sites in a 50kb region that extends 20kb distal and 28kb proximal to the duplication, and have one of two conserved haplotypes in the region that is 28-43kb proximal to the duplication. Two different SD chromosomes, SD-Roma and SD-VO17, both lack the pericentric and/or 2R inversions that are typically associated with SD chromosomes and were isolated from Mediterranean populations. If the duplication arose only once, it may have originated in the Mediterranean region on a chromosome that was free of inversions. The specific inversions associated with the different SD chromosomes would then have arisen subsequently after the geographic spread of these chromosomes.
Analysis of the level of polymorphism at six loci in the pericentromeric region of chromosome 2 reveals that SD chromosomes harbor much less polymorphism than SD+ chromosomes. The results suggest that the SD system evolved recently. The large genomic region hitchhiking with RanGapSD indicates that a multilocus, epistatically selected system could affect the levels of DNA polymorphism observed in regions of reduced recombination.
All naturally occurring SD chromosomes carry RanGapSD and Rspi. Typical SD+ chromosomes carry Rsps and its presence is sufficient to render a chromosome sensitive to the action of RanGapSD. Some SD+ chromosomes carry Rspi. To cause high levels of distortion, RanGapSD requires the presence of a series of upward modifiers known as E(SD)unspecified, St-SDunspecified and M(Sd)unspecified.
The nature of variation of the segregation ratio among RanGapSD males has been studied.