The Independent Distorting Ability of the Enhancer of Segregation Distortion, E(sd), in Drosophila Melanogaster

Segregation distortion is a meiotic drive system, discovered in wild populations, in which males heterozygous for an SD chromosome and a sensitive SD+ homolog transmit the SD chromosome almost exclusively. SD represents a complex of three closely linked loci in the centromeric region of chromosome 2: Sd, the Segregation distorter gene; E(SD), the Enhancer of Segregation Distortion, required for full expression of drive; and Rsp, the target for the action of Sd, existing in a continuum of states classifiable into sensitive (Rsps) and insensitive (Rspi). In an SD/SD+ male which is Sd E(SD) Rspi/Sd+ E(SD)+ Rsps, the Sd and E(SD) elements act jointly to induce the dysfunction of those spermatids receiving the Rsps chromosome. By manipulating the number of copies and the position of the Enhancer region, I demonstrated that: (1) E(SD), whether in its normal position or translocated to the Y chromosome, is able to enhance the degree of Sd-caused distortion in a dosage-dependent manner; (2) even in the absence of Sd, the E(SD) allele in two doses can cause significant distortion, in Sd+ or Df(Sd)-bearing genotypes; (3) quantitative differences among Enhancers of different sources suggest allelic variation at E(SD), which could account at least in part for differences among wild SD chromosomes in strength of distortion; (4) E(SD)/E(SD)-mediated distortion, like that of Sd, is directed at the Rsp target, whether Rsp is on the second or the Y chromosome; (5) E(SD), like Sd, is suppressed by an unlinked dominant suppressor of SD action. These results show that E(SD) is independently capable of acting on Rsp and is not a simple modifier of the action of Sd. E(SD) provides an example of a trans-acting gene embedded in heterochromatin that can interact with another heterochromatic gene, Rsp, as well as parallel the effect of a euchromatic gene, Sd.     

The Selfish Segregation Distorter Gene Complex of Drosophila melanogaster

Segregation Distorter (SD) is an autosomal meiotic drive gene complex found worldwide in natural populations of Drosophila melanogaster. During spermatogenesis, SD induces dysfunction of SD(+) spermatids so that SD/SD(+) males sire almost exclusively SD-bearing progeny rather than the expected 1:1 Mendelian ratio. SD is thus evolutionarily "selfish," enhancing its own transmission at the expense of its bearers. Here we review the molecular and evolutionary genetics of SD. Genetic analyses show that the SD is a multilocus gene complex involving two key loci-the driver, Segregation distorter (Sd), and the target of drive, Responder (Rsp)-and at least three upward modifiers of distortion. Molecular analyses show that Sd encodes a truncated duplication of the gene RanGAP, whereas Rsp is a large pericentromeric block of satellite DNA. The Sd-RanGAP protein is enzymatically wild type but mislocalized within cells and, for reasons that remain unclear, appears to disrupt the histone-to-protamine transition in drive-sensitive spermatids bearing many Rsp satellite repeats but not drive-insensitive spermatids bearing few or no Rsp satellite repeats. Evolutionary analyses show that the Sd-RanGAP duplication arose recently within the D. melanogaster lineage, exploiting the preexisting and considerably older Rsp satellite locus. Once established, the SD haplotype collected enhancers of distortion and suppressors of recombination. Further dissection of the molecular genetic and cellular basis of SD-mediated distortion seems likely to provide insights into several important areas currently understudied, including the genetic control of spermatogenesis, the maintenance and evolution of satellite DNAs, the possible roles of small interfering RNAs in the germline, and the molecular population genetics of the interaction of genetic linkage and natural selection.     

Cytogenetic Analysis of an SD Chromosome from a Natural Population of DROSOPHILA MELANOGASTER

The discovery and the cytogenetic characterization of a new SD (Segregation Distorter) chromosome 2 from a natural population in Ranna (Sicily, Italy), SD(Ra), are reported. The main features of this chromosome are as follows: (a) it contains an Sd(Ra) gene with a moderate degree of segregation distortion (k = 0.72), (b) a recessive female sterile gene, fs(2)(TLM), responsible for modifications of the morphology and structure of the tests and ovaries is located at 89.7, (c) SD(Ra)/SD(Ra) males and females are viable but sterile, the females due to homozygosis of fs(2)(TLM) and the males because of homozygosis of a region containing the Sd locus, and (d) SDi/SDj combinations are fertile, thus suggesting that the different Sd factors found in natural populations constitute a multiple allelic series.-These data may indicate that each population containing SD chromosomes has evolved its own genetic architecture for the complex SD system, with specific modifiers and perhaps different Sd genes. The possibility of reconstructing the evolutionary pattern of the SD(Ra) chromosome in the natural Ranna population after the model of Charlesworth and Hartl (1978) and Crow (1979) is considered.     

Does genetic conflict drive rapid molecular evolution of nuclear transport genes in Drosophila?

The Segregation Distorter (SD) system of Drosophila melanogaster is one the best-characterized meiotic drive complexes known. SD gains an unfair transmission advantage through heterozygous SD/SD(+) males by incapacitating SD(+)-bearing spermatids so that virtually all progeny inherit SD. Segregation distorter (Sd), the primary distorting locus in the SD complex, is a truncated duplication of the RanGAP gene, a major regulator of the small GTPase Ran, which has several functions including the maintenance of the nucleocytoplasmic RanGTP concentration gradient that mediates nuclear transport. The truncated Sd-RanGAP protein is enzymatically active but mislocalizes to the nucleus where it somehow causes distortion. Here I present data consistent with the idea that wild-type RanGAP, and possibly other loci able to influence the RanGTP gradient, has been caught up in an ancient genetic conflict that predates the SD complex. The legacy of this conflict could include the unexpectedly rapid evolution of nuclear transport-related proteins, the accumulation of chromosomal inversions, the recruitment of gene duplications, and the turnover of repetitive sequences in the centric heterochromatin.     

Detection of Rsp and Modifier Variation in the Meiotic Drive System SEGREGATION DISTORTER (SD) of DROSOPHILA MELANOGASTER

Identification of allelic variability at the two major loci (Sd and Rsp) that interact to cause sperm dysfunction in Segregation distorter (SD) males of D. melanogaster has been hampered by the difficulty in separating the elements recombinationally. In addition, small differences in the strength of Sd alleles or sensitivities of Rsp alleles to Sd are difficult to measure against background genetic or environmental variation. Viability effects of the markers used to score progeny classes may also introduce a bias. Removal of Sd and E(SD) from their second chromosome location to create a Dp(2;Y)Sd E(SD) chromosome eliminates these problems, since any combination of Rsp alleles can be easily tested without resorting to recombinational techniques. Further, since these pairs of Rsp alleles are compared in their response to Dp Sd E(SD) in the same individual males, background variation and viability effects can be easily removed to allow fine-scale resolution of Rsp differences. Tests of all possible pairwise combination of six laboratory chromosomes in this way revealed at least three and possibly four different Rsp allelic classes. In addition, the hierarchical nature of the tests further allowed for determination of the presence of linked suppressors or enhancers of Sd activity. A sample of 11 second chromosomes selected from a group recently isolated from a natural population was also unambiguously ordered as to Rsp allelic status using this approach. The resultant pattern was similar to that obtained for the laboratory chromosomes, except for the not unexpected observation that the natural population apparently harbored more drive suppressors. The pattern of results obtained from these pairwise combinations of Rsp alleles supports the notion that there are no dominance interactions within the group, but that each responds more or less independently to Sd in giving sperm dysfunction.     

Rapid Evolution of a Coadapted Gene Complex: Evidence from the Segregation Distorter (Sd) System of Meiotic Drive in Drosophila Melanogaster

Segregation Distorter (SD) is a system of meiotic drive found in natural populations of Drosophila melanogaster. Males heterozygous for an SD second chromosome and a normal homologue (SD(+)) produce predominantly SD-bearing sperm. The coadapted gene complex responsible for this transmission advantage spans the second chromosome centromere, consisting of three major and several minor interacting loci. To investigate the evolutionary history of this system, we surveyed levels of polymorphism and divergence at six genes that together encompass this pericentromeric region and span seven map units. Interestingly, there was no discernible divergence between SD and SD(+) chromosomes for any of these molecular markers. Furthermore, SD chromosomes harbored much less polymorphism than did SD(+) chromosomes. The results suggest that the SD system evolved recently, swept to appreciable frequencies worldwide, and carried with it the entire second chromosome centromeric region (roughly 10% of the genome). Despite its well-documented genetic complexity, this coadapted system appears to have evolved on a time scale that is much shorter than can be gauged using nucleotide substitution data. Finally, the large genomic region hitchhiking with SD indicates that a multilocus, epistatically selected system could affect the levels of DNA polymorphism observed in regions of reduced recombination.     

Segregation Distortion in Drosophila Melanogaster: Genomic Organization of Responder Sequences

The heterochromatic Responder (Rsp) locus of Drosophila melanogaster is the target of the two distorter loci Sd and E(SD). Rsp is located in a specific heterochromatic region of the second chromosome and is made up of AT-rich satellite sequences whose abundance is related to its sensitivity to the distorter chromosomes. Here we report that a cluster of Rsp sequences is also located in the third chromosome. The third-chromosome cluster has the same flanking sequences as the clone originally used to identify the Rsp elements, and one of the flanking sequences is a rearranged 412 retrotrsansposon. The presence of a second, unlinked Rsp-sequence cluster makes re-interpretation necessary for some earlier experiments in which segregation of the third chromosome had not been followed and raises interesing possibilities for the origin of the Rsp locus.     

Closing the (Ran)GAP on segregation distortion in Drosophila

Segregation Distorter (SD) is a meiotic drive system in Drosophila that causes preferential transmission of the SD chromosome from SD/SD(+) males owing to induced dysfunction of SD(+) spermatids. Since its discovery in 1956, SD and its mode of action have baffled biologists. Recently, substantial progress has been made in elucidating this puzzle. Sd, the primary gene responsible for distortion encodes a mutant RanGAP, a key protein in the Ran signaling pathway required for nuclear transport and other nuclear functions. The mutant protein is enzymatically active but mislocalized to nuclei, which apparently disrupts Ran signaling by reducing intranuclear Ran-GTP levels. Some evidence suggests that a defect in nuclear transport may be the main cause of sperm dysfunction. Although important questions remain, the basic mechanism of distortion is now understood sufficiently well that specific hypotheses can be formulated and tested. This previously mysterious genetic system may now offer unique insights into novel aspects of regulation by Ran.     

Genetic instability inDrosophila melanogaster: tandem duplications as monitors of intrastrand exchange

Two tandem duplications of the X chromosome associated with the white eye color locus are described. In heterozygous females both revert to a nonduplicated chromosome without detectable meiotic recombination. Clustering of revertants suggests the reversion event occurs in the germ line prior to meiosis. Similarly in males one duplication also reverts with clustering implying a premeiotic event. Revertant X chromosomes derived from males are either nonduplicated or deleted. Intrastrand exchange can account for some but not all revertants recovered.Dedicated to Professor Bauer on the occasion of his seventy-fifth birthday     

Examination of a Mutable System Affecting the Components of the Segregation Distorter Meiotic Drive System of Drosophila Melanogaster

Segregation distortion in Drosophila melanogaster is the result of an interaction between the genetic elements Sd, a Rsp sensitive to Sd, and an array of modifiers, that results in the death of sperm carrying Rsp. A stock (designated M-5; cn bw) has been constructed which has the property of inducing the partial loss of sensitivity from previously sensitive cn bw chromosomes, the partial loss of distorting ability from SD chromosomes, and a concomitant acquisition of modifiers on the X chromosome and possibly also on the autosomes. By several criteria the changes exhibited under the influence of M-5; cn bw are characteristic of the transposable-element systems which produce hybrid dysgenesis. In the first place, the magnitude of these effects depends on the nature of the crosses performed. The analogy is further strengthened by the observation that the changes induced by M-5; cn bw share other stigmata of Drosophila transposable-element systems, including high sterility among the progeny of outcrosses, and the production of chromosomal rearrangements. The possible relationship of this system to the P, I and hobo transposable element systems is discussed, as well as its bearing on aspects of the Segregation Distorter phenomenon which have yet to be explained.     

The Effect of Novel Chromosome Position and Variable Dose on the Genetic Behavior of the Responder (Rsp) Element of the Segregation Distorter (Sd) System of Drosophila Melanogaster

In the Segregation distorter (SD) system of meiotic drive, a minimum of two trans-acting elements [Sd and E(SD)] act in concert to cause a certain probability of dysfunction for sperm carrying a sensitive allele at the Responder (Rsp) target locus. By employing a number of insertional translocations of autosomal material into the long arm of the Y chromosome, Rsp can be mapped as the most proximal locus in the 2R heterochromatin as defined both by cytology and lethal complementation tests. Several of these insertional translocations result in the transposition of Rsp to the Y chromosome, where its sensitivity remains virtually unaltered. This argues that Rsp is separable from the second chromosome centromere, that its behavior does not depend on its gross chromosomal position, and that meiotic pairing of the chromosomes carrying the various SD elements is not a prerequisite for sperm dysfunction. Several other translocations apparently leave both resulting chromosomes at least partially sensitive to SD action, suggesting that Rsp is a large subdivisible genetic element. This view is compatible with observations published elsewhere that suggest that Rsp is a cytologically large region of highly repetitive AT-rich DNA. The availability of Y-linked copies of Rsp also allows the construction of SD males carrying two independently segregating Rsp alleles; this in turn allows the production of sperm with zero, one or two Rsp copies from the same male. Examination of the relative recovery proportions of progeny arising from these gametes suggests that sperm with two Rsp copies survive at much lower frequencies than would be predicted if each Rsp acted independently in causing sperm dysfunction. Possible explanations for such behavior are discussed.     

Virus Deoxyribonucleic Acid Sequences in Subdiploid and Subtetraploid Revertants of Polyoma- Transformed Cells

Polyoma-transformed cells can revert in the properties characteristic of transformation, although they maintain the polyoma-specific T antigen. Transformed cells contain the same number of copies of polyoma virus deoxyribonucleic acid (DNA) per cell (eight) as revertants with a subdiploid or a subtetraploid chromosome number. The results indicate that the duplication of chromosomes in the subtetraploid revertants did not include the chromosomes that carry the viral genome. The virus DNA in both transformed and revertant cells was associated with high-molecular-weight cell DNA. Reversion of the properties of transformed cells was, therefore, not associated either with a decrease in number of virus DNA copies per cell or with a lack of association of the virus DNA with cell DNA.     

Mechanism of reversion of the gal3 mutation of Escherichia coli

The gal3 mutation is an insertion of a DNA sequence in the operator-promoter region of the galactose operon of E. coli. It reverts spontaneously to produce three kinds of gal+ revertants, which are: (i) stable and inducible, (ii) stable and constitutive, and (iii) unstable and constitutive. The constitutive revertants also show drastically reduced frequencies of transduction with lambda. The mechanism by which these reversions occur has remained unknown. It is proposed that the stable and inducible revertants arise by accurate excision of the insertion sequence. The unstable and constitutive revertants arise by tandem duplications of the gal operon in such a way that the structural genes of the extra copy of gal operon become connected to a different promoter. The resulting tandem configuration (gal3 ETK...P'E'T'K') permits constitutive expression and gal3 segregation (by internal recombination) simultaneously. The proposal was tested by comparison of the buoyant densities in CsCl of derivatives of a lambdagal phage carrying gal+, gal3, and the inducible and constitutive revertants. The densities of the inducible revertants were identical to the wild type, and the slight increase in density found to be associated with the gal3 insertion was missing. It was concluded that inducible revertants arise by excision of the inserted sequence. In contrast, lysates of a constitutive revertant exhibited several anomalous properties. The lysates contained a small quantity of phage whose density was identical to lambdagal3, produced few gal+ transductants (10(-3)-10(-4) of a normal HFT lysate), and the transductants were stable and constitutive. In turn, these abnormal transductants produced lysates which showed no lambdagal particles on centrifugation, and no transducing activity whatsoever. These anomalous properties of the constitutive revertant were attributed to the failure of lambda to package the DNA duplication efficiently. Transduction experiments with P1 (which can package more DNA than lambda) show that the unstable, constitutive reversions were located adjacent to prophage lambda. Segregation of the gal and lambda markers among the gal+ transductants was in accordance with the pattern expected for a duplication. Introduction of a recA marker resulted in stabilization of the reversion without affecting its constitutive expression. It was concluded that the unstable, constitutive reversion was a tandem duplication. It is further proposed that the stable, constitutive class of revertants might represent inverted (gal3 ETK...K'T'E'P') or partial tandem (gal3 ET...E'T'K') duplications of the gal operon.     

Targeted Tandem Duplication of a Large Chromosomal Segment in Aspergillus oryzae

We describe here the first successful construction of a targeted tandem duplication of a large chromosomal segment in Aspergillus oryzae. The targeted tandem chromosomal duplication was achieved by using strains that had a 5′-deleted pyrG upstream of the region targeted for tandem chromosomal duplication and a 3′-deleted pyrG downstream of the target region. Consequently, strains bearing a 210-kb targeted tandem chromosomal duplication near the centromeric region of chromosome 8 and strains bearing a targeted tandem chromosomal duplication of a 700-kb region of chromosome 2 were successfully constructed. The strains bearing the tandem chromosomal duplication were efficiently obtained from the regenerated protoplast of the parental strains. However, the generation of the chromosomal duplication did not depend on the introduction of double-stranded breaks (DSBs) by I-SceI. The chromosomal duplications of these strains were stably maintained after five generations of culture under nonselective conditions. The strains bearing the tandem chromosomal duplication in the 700-kb region of chromosome 2 showed highly increased protease activity in solid-state culture, indicating that the duplication of large chromosomal segments could be a useful new breeding technology and gene analysis method.     

Genetic events associated with an insertion mutation in yeast

The his4-912 mutation shares similar genetic properties with mutations promoted by procaryotic insertion elements. This mutation lacks all three his4 functions. Many different classes of His+ revertants have been obtained from his4-912. The most frequent class of His+ revertants results from a site mutation which confers a cold-sensitive His- phenotype. Other classes of revertants contain translocations (one between chromosomes I and III and the other between chromosomes III and XII), a transposition of the his4 region to chromosome VIII, and an inversion of most of the left arm of chromosome III. Another class contains deletions which extend from his4-912 into the his4 region. In each of these classes of revertants, the his4 region is closely linked to the chromosomal aberration. Many of these revertants contain additional changes in chromosome structure (duplication, deletion and aneuploidy) that are unrelated to the reversion of his4-912 to His4+.