A Possible Case of Negative Segregation Distortion in the Sd System of Drosophila Melanogaster

Models proposed to explain segregation distortion in Drosophila melanogaster are based partly upon the observation that, in the Sd heterozygous Rspi/Rsps male, the chromosome carrying the sensitive Rsps allele is distorted, but the chromosome carrying the insensitive Rspi allele is not. The results of the present study suggest that this may not always be the case. Under a certain genotypic condition, the chromosome carrying the Rsps allele can be transmitted to the progeny in frequencies of more than 0.5 (about 0.6), or correspondingly, the chromosome carrying the Rspi allele may be distorted with respect to the one with the Rsps allele. Thus, the relative sensitivity and insensitivity of the two Rsp alleles in a male are not absolute, but relative; and they may be reversed depending upon the residual genotype. If this is true, a major modification of the current models or a proposal of an entirely new model may become necessary to explain the mechanism of segregation distortion satisfactorily.     

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.     

Cytogenetic Analysis of Segregation Distortion in Drosophila Melanogaster: The Cytological Organization of the Responder (Rsp) Locus

The segregation distortion phenomenon occurs in Drosophila melanogaster males carrying an SD second chromosome and an SD+ homolog. In such males the SD chromosome is transmitted to the progeny more frequently than the expected 50% because of an abnormal differentiation of the SD+-bearing sperms. Three major loci are involved in this phenomenon: SD and Rsp, associated with the SD and SD+ chromosome, respectively, and E(SD). In the present work we performed a cytogenetic analysis of the Rsp locus which was known to map to the centromeric heterochromatin of the second chromosome. Hoechst- and N-banding techniques were used to characterize chromosomes carrying Responder insensitive (Rspi), Responder sensitive (Rsps) and Responder supersensitive (Rspss) alleles. Our results locate the Rsp locus to the h39 region of 2R heterochromatin. This region is a Hoechst-bright, N-banding negative heterochromatic block adjacent to the centromere. Quantitative variations of the h39 region were observed. The degree of sensitivity to Sd was found to be directly correlated with the physical size of that region, demonstrating that the Rsp locus is composed of repeated DNA.     

On the Components of Segregation Distortion in Drosophila Melanogaster. IV. Construction and Analysis of Free Duplications for the Responder Locus

Male Drosophila heterozygous for an SD-bearing second chromosome and a normal homolog preferentially transmit the SD chromosome to their offspring. The distorted transmission involves the induced dysfunction of the sperm that receive the SD+ chromosome. The loci on the SD chromosome responsible for causing distortion are the Sd locus the the E(SD) locus. Their target of action on the SD+ chromosome is the Rsps locus. Previous studies of Rsps indicated that deletion of this locus rendered a chromosome insensitive to the action of SD and mapped Rsps physically within the centric heterochromatin of 2R. In this study we have constructed a collection of marked free duplications for the centromeric region of a second chromosome that carried Rsps. The heterochromatic extent of each duplication as well as its sensitivity to distortion was determined. We found that Rsps is the most proximal known locus within the 2R heterochromatin. Furthermore, our results demonstrate that the presence of Rsps is not only necessary but sufficient to confer sensitivity to distortion irrespective of its association with an intact second chromosome or one that pairs meiotically with an SD chromosome. By use of these duplications we increased the usual dosage of Rsps relative to SD to determine whether there was any competition for limited amounts of SD [and/or E(SD)] product. When two Rsps-bearing chromosomes are present within the same spermatocyte nucleus an SD chromosome is capable of causing efficient distortion of both. However, at least in some cases the degree of distortion against a given Rsps was reduced by the presence of an extra dose of Rsps indicating that there was some competition between them. The bearing of these results on present models of segregation distortion are discussed.     

Temperature Sensitivity of Negative Segregation Distortion in Drosophila Melanogaster

Previous work has shown that the direction of segregation distortion in the SD (Segregation Distorter) system in Drosophila melanogaster can sometimes be reversed, but this was found only with rather weak distorters and the effect was not large. The present study reports large negative segregation distortion in a strong distorter, SD-72 chromosome. In the presence of a specific X chromosome, supp-X(SD), the proportion, k, of SD-72 chromosomes recovered from the SD-72/cn bw males ranges from 0.99 at 20° to 0.11 at 28.5°, whereas with a standard-X chromosome, k ranges from 0.99 to 0.95 for the same temperature range. The temperature-sensitive period is during spermiogenesis. Using a mating system in which the sperm supply is nearly exhausted, it was shown that the negative distortion at high temperatures is due to an absolute reduction in the number of SD-72 chromosomes and an absolute increase in the number of cn bw chromosomes recovered. After adjusting for non-SD-related temperature effects, the amount of decrease in the number of SD-72 progeny is nearly the same as the amount of increase in the number of cn bw progeny, suggesting that the dysfunction switches from a spermatid carrying one homolog to one carrying the other. Negative distortion requires a radical revision of current hypotheses for the mechanism of segregation distortion and a possible modification of the current model is suggested, based on differential recovery of dysfunction in the two homologs during spermiogenesis.     

Biological parameters and the segregation distortion (SD) phenomenon in Drosophila melanogaster

The relationship between some biological parameters (mortality, longevity, fertility, fecundity and sex ratio) and segregation of second chromosomes in heterozygous and homozygous SD males has been analyzed. The results obtained in SD/SD+ heterozygous males show: (1) their reduced fertility with respect to that of control males, (2) an alteration in the sex ratio in the SD+ progeny only, and (3) inversely related sex-ratio and segregation distortion values. In SDi/SDj combinations: (1) surprisingly, fertility is intermediate between that of SD/SD+ heterozygous males and that of control males, (2) the segregation ratios of the second chromosomes are normal (0.50), and (3) the sex ratio = 0.50 in both classes of SD progeny. The relationship between mortality (and therefore longevity) and fertility of the different genotypes and fecundity per male indicates that the total productivity of heterozygous males is less than that so far claimed. Indeed, their productivity depends not only on the mechanism of nonformation of the SD+ sperm, but also on their reduced longevity. The k = 0.50 and the high fecundity of SDi/SDj combinations indicated that in these males the SD phenomenon is partially suppressed, the SD chromosomes being insensitive to each other, thus implying that particular Rsp alleles are sensitive to given Sd alleles. The complementation pattern for male fertility of SD homozygous males again supports previous evidence that Sd factors from natural populations are, in effect, different Sd genes.     

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.     

X-Linked Elements Associated with Negative Segregation Distortion in the Sd System of Drosophila Melanogaster

Three elements, M(1), M(2) and M(3), found in a special X chromosome, supp-X(SD), modify the degree and direction of segregation distortion in the SD system of Drosophila melanogaster. The first element, M(1), is located between the y and the cv loci, probably close to the y locus. The second element, M(2), is located near the cv locus and the third element, M(3), is located between the y and the car loci. The M(1) element appears to cause a relatively small amount of reduction in the rate of recovery of the SD-72, but not the cn bw, chromosome from SD-72/ cn bw males, when raised at 27.5°. The M(2) and the M(3) elements cause considerable decrease in the recovery rate of the SD-72 chromosome, whereas they increase the recovery rate of the cn bw chromosome. The amount of decrease is nearly the same as the amount of increase for each element. Some type of ``switch' mechanism in the directions of distortion is suggested for each of these two elements and their effects appear to be approximately additive.     

Genetic Dissection of Segregation Distortion II. Mechanism of Suppression of Distortion by Certain Inversions

In(2L+2R)Cy and In(2LR)Pm² are inversion-bearing chromosomes, the former carrying a paracentric inversion in each arm and the latter carrying a long pericentric. Both chromosomes produce normal segregation ratios when present in heterozygous males with certain segregation distorter chromosomes. The apparent suppression of distortion by these chromosomes was long attributed to a failure of synapsis, but this hypothesis has fallen out of favor recently because a large number of chromosome aberrations, particularly translocations and inversions, suppress distortion even though their breakpoints fall into no recognizable pattern. Although failure of synapsis does not appear to be the mechanism of suppression of distortion, what is responsible for the suppression remains unknown. In this paper it is shown that In(2L+2R)Cy and In(2LR)Pm² suppress segregation distortion because they carry Rsp, a component of the segregation distorter system that renders a chromosome insensitive to distortion. Both chromosomes induce "suicide" of chromosomes carrying Sd Rsp⁺.     

On the Models of Segregation Distortion in DROSOPHILA MELANOGASTER

The Segregation Distorter system of Drosophila melanogaster consists of two major elements, Sd and Rsp. There are two allelic alternatives of Rsp-sensitive (Rsp(s)) and insensitive (Rsp(i)); a chromosome carrying Rsp(i) is not distorted. According to the model proposed by Hartl (1973), these two elements interact to cause segregation distortion. For a sperm to complete the maturation process, it is assumed that the Rsp locus has to be complexed with the product of the Sd locus. This product is assumed to be a multimetric regulatory protein. Three kinds of regulatory multimers may be distinguished: Sd(+)/Sd(+), which is assumed to complex with both Rsp(s) and Rsp(i); Sd(+)/Sd heteromultimers, which complex preferentially with Rsp(i); and Sd/Sd homomultimers, which complex with neither Rsp(s) nor Rsp(i). Most of the regulatory protein in the Sd(+)/Sd heterozygous male is assumed to be the Sd(+)/Sd heteromultimer.--Some modifications of Hartl's model were made by Ganetzky (1977). Rather than the binding of a product of Sd at the Rsp locus being a necessary condition for normal spermigenesis, this binding causes sperm dysfunction. It is assumed that the product of Sd complexes more readily with Rsp(s) than with Rsp(i) and that the amount of Sd product is limited with respect to the number of binding sites available. No function is ascribed to the Sd(+) locus. In order to explain reduced male fertility of some genotypes, Ganetzky further assumes that the Sd product, when not competed for by an Rsp(s) locus, can bind to an Rsp(i) locus.--Two consequences of these models were critically examined: according to these models (1) an Sd Rsp(s)/Sd(+)Rsp(s) male should not show any segregation distortion, and (2) an Sd Rsp(s)/Sd Rsp(s) male should show much reduced fertility, if not complete sterility.--The results of the present study bear on these two points. (1) Rsp(s) locus seems to consist of multiple alleles, each having a different degree of ability to interact with the product of the Sd locus. An Sd Rsp(s)/Sd(+)Rsp(s) male shows a certain degree of segregation distortion when the two Rsp(s) alleles are different, but it shows a normal Mendelian segregation ratio when the Rsp(s) alleles are homozygous. The first prediction of the models is supported by actual observation when the two Rsp(s) alleles are the same. (2) There is a suggestion of slight reduction in fertility, but generally Sd Rsp(s)/Sd Rsp(s) males are quite fertile. Thus, the second prediction is not supported by actual observation. The mechanism of segregation distortion is still open for future studies.     

Segregation ratios within Segregation Distorter lines of Drosophila melanogaster conform to a beta-binomial distribution

Segregation Distorter (SD) chromosomes are preferentially recovered from SD/SD+ males due to the dysfunction of sperm bearing the SD+ chromosome. The proportion of offspring bearing the SD chromosome is given the symbol k. The nature of the frequency distribution of k was examined by comparing observed k distributions produced by six different SD chromosomes, each with a different mean, with k distributions predicted by two different statistical models. The first model was one where the k of all males with a given SD chromosome were considered to be equal prior to the determination of those gametes which produce viable zygotes. In this model the only source of variation of k would be binomial sampling. The results rigorously demonstrated for the first time that the observed k distributions did not fit the prediction that the only source of variation was binomial sampling. The next model tested was that the prior distribution of segregation ratios conformed to a beta distribution, such that the distribution of k would be a beta-binomial distribution. The predicted distributions of this model did not differ significantly from the observed distributions of k in five of the six cases examined. The sixth case probably failed to fit a beta-binomial distribution due to a major segregating modifier. The demonstration that the prior distribution of segregation ratios of SD lines can generally be approximated with a beta distribution is crucial for the biometrical analysis of segregation distortion.     

Transposition of the Responder Element (Rsp) of the Segregation Distorter System (Sd) to the X Chromosome in Drosophila Melanogaster

In order to test whether the meiotic drive system Segregation distorter (SD) can operate on the X chromosome to exclude it from functional sperm, we have transposed the Responder locus (Rsp) to this element. This was accomplished by inducing detachments of a compound-X chromosome in females carrying a Y chromosome bearing a Rsps allele. Six Responder-sensitive-bearing X chromosomes, with kappa values ranging from 0.90 to 1.00, were established as permanent lines. Two of these have been characterized more extensively with respect to various parameters affecting meiotic drive. SD males with a Responder-sensitive X chromosome produce almost exclusively male embryos, while those with a Rsp-Y chromosome produce almost exclusively female embryos. This provides a genetic system of great potential utility for the study of early sex-specific differentiation events as it allows the collection of large numbers of embryos of a given sex.     

Genetic Dissection of Segregation Distortion. III. Unequal Recovery of Reciprocal Recombinants

The genetic structure of a segregation distorter chromosome (a derivative of SD-36) has been analyzed in a system in which recombination in the second chromosome is blocked by inversions except for the critical region around the centromeric heterochromatin. The results confirm the map order and characteristics of four loci known to be involved in segregation distortion, namely Sd, E(SD), Rsp^ins, M(SD). However, SD-36 carries a fifth major locus involved in distortion. This locus is near pr in 2L and has the effect of enhancing the degree of distortion. In addition, reciprocal recombinant products from SD-36 are recovered unequally. All recombinants carrying the pr region from SD-36 seem also to carry Sd, although Sd has previously been mapped 1.6 units to the left of pr. Both the enhancement of distortion and the unequal recovery of reciprocal products can be explained if it is assumed that the new locus near pr in SD-36 is actually a duplication of Sd.     

Segregation distortion in hybrids between the Bogota and USA subspecies of Drosophila pseudoobscura

We show that, contrary to claims in the literature, "sterile" males resulting from the cross of the Bogota and USA subspecies of Drosophila pseudoobscura are weakly fertile. Surprisingly, these hybrid males produce almost all daughters when crossed to females of any genotype (pure Bogota, pure USA, hybrid F1). Several lines of evidence suggest that this sex ratio distortion is caused by sex chromosome segregation distortion in hybrid males. We genetically analyze this normally cryptic segregation distortion and show that it involves several regions of the Bogota X chromosome that show strong epistatic interactions with each other. We further show that segregation distortion is normally masked within the Bogota subspecies by autosomal suppressors. Our analysis shows that the genetic basis of hybrid segregation distortion is similar to that of hybrid male sterility between the same subspecies. Indeed the severity of segregation distortion is correlated with the severity of sterility among hybrids. We discuss the possibility that hybrid sterility in this paradigmatic case of incipient speciation is caused by segregation distortion.     

A genetic test to determine the origin of maternal transmission ratio distortion. Meiotic drive at the mouse Om locus

We have shown previously that the progeny of crosses between heterozygous females and C57BL/6 males show transmission ratio distortion at the Om locus on mouse chromosome 11. This result has been replicated in several independent experiments. Here we show that the distortion maps to a single locus on chromosome 11, closely linked to Om, and that gene conversion is not implicated in the origin of this phenomenon. To further investigate the origin of the transmission ratio distortion we generated a test using the well-known effect of recombination on maternal meiotic drive. The genetic test presented here discriminates between unequal segregation of alleles during meiosis and lethality, based on the analysis of genotype at both the distorted locus and the centromere of the same chromosome. We used this test to determine the cause of the transmission ratio distortion observed at the Om locus. Our results indicate that transmission ratio distortion at Om is due to unequal segregation of alleles to the polar body at the second meiotic division. Because the presence of segregation distortion at Om also depends on the genotype of the sire, our results confirm that the sperm can influence segregation of maternal chromosomes to the second polar body.     

Factors Influencing the Effect of Segregation Distortion in Natural Populations of DROSOPHILA MELANOGASTER

The major components of the SD system have been examined in two natural populations of D. melanogaster to investigate how SD behaves and is maintained in nature and to estimate its impact and efficiency. A twofold approach was used: (1) direct measurements of segregation distortion in wild males and (2) measurement of sensitivity of wild SD (+) chromosomes to SD action. Characterization of newly isolated SD chromosomes and of a large number of SD( +) chromosomes from nature demonstrated that (1) SD can operate efficiently in the wild genome: 45% of SD/SD(+) males collected from nature had k values larger than 0.70. (2) Forty-three of 44 newly recovered SD chromosomes are of the SD-72 type, having a small pericentric inversion that maintains tight linkage among the Sd, E(SD) and Rsp loci in the SD complex. In 1956, most SD chromosomes in Madison lacked this inversion. (3) Only 12 of the 44 SD chromosomes carried a recessive lethal (compared with five of six in 1956), and many of the viable SD chromosomes were fertile as homozygotes, indicating that SD homozygotes need not have obvious reductions in fitness. (4) Among more than 500 wild chromosomes assayed for response to distortion by a strong SD, at least 40-50% were sensitive, about 33% were partially sensitive and 17% were insensitive. This frequency of sensitives is higher than in reports from some other populations. An estimated 12% of the wild chromosomes were classified as true Rsp(i) by their constellation of effects, including a special test of ability to cause self-distortion of a "suicide" chromosome, R(cn)-10. In a direct assay with R(cn)-10, an independent sample of 99 chromosomes from nature gave 30% putative Rsp(i). Thus, these populations contain in the range of 12-30% Rsp(i). (5) Chromosomes supersensitive to SD, previously described for certain laboratory stocks, were also found to coexist in nature with SD. (6) Profiles of wild chromosomes with a panel of three or four different SD testers suggest a series of allelic alternatives at the Rsp locus including supersensitive, sensitive, semisensitive and insensitive, and that loci other than Rsp may also be important in determining the effect of SD in nature.     

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.