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.     

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 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.     

Sex Chromosome Meiotic Drive in DROSOPHILA MELANOGASTER Males

In Drosophila melanogaster males, deficiency for X heterochromatin causes high X-Y nondisjunction and skewed sex chromosome segregation ratios (meiotic drive). Y and XY classes are recovered poorly because of sperm dysfunction. In this study it was found that X heterochromatic deficiencies disrupt recovery not only of the Y chromosome but also of the X and autosomes, that both heterochromatic and euchromatic regions of chromosomes are affected and that the "sensitivity" of a chromosome to meiotic drive is a function of its length. Two models to explain these results are considered. One is a competitive model that proposes that all chromosomes must compete for a scarce chromosome-binding material in Xh(-) males. The failure to observe competitive interactions among chromosome recovery probabilities rules out this model. The second is a pairing model which holds that normal spermiogenesis requires X-Y pairing at special heterochromatic pairing sites. Unsaturated pairing sites become gametic lethals. This model fails to account for autosomal sensitivity to meiotic drive. It is also contradicted by evidence that saturation of Y-pairing sites fails to suppress meiotic drive in Xh(- ) males and that extra X-pairing sites in an otherwise normal male do not induce drive. It is argued that meiotic drive results from separation of X euchromatin from X heterochromatin.     

The Y chromosomes of Drosophila simulans are highly polymorphic for their ability to suppress sex-ratio drive

The sex-ratio trait, known in several species of Drosophila including D. simulans, results from meiotic drive of the X chromosome against the Y. Males that carry a sex-ratio X chromosome produce strongly female-biased progeny. In D. simulans, drive suppressors have evolved on the Y chromosome and on the autosomes. Both the frequency of sex-ratio X and the strength of the total drive suppression (Y-linked and autosomal) vary widely among geographic populations of this worldwide species. We have investigated the pattern of Y-linked drive suppression in six natural populations representative of this variability. Y-linked suppressors were found to be a regular component of the suppression, with large differences between populations in the mean level of suppression. These variations did not correspond to differences in frequency of discrete types of Y chromosomes, but to a more or less wide continuum of phenotypes, from nonsuppressor to partial or total suppressor. We concluded that a large diversity of Y-linked suppressor alleles exists in D. simulans and that some populations are highly polymorphic. Our results support the hypothesis that a Y-chromosome polymorphism can be easily maintained by a balance between meiotic drive and the cost of drive suppression.     

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.     

The Sex-Ratio Trait in Drosophila Simulans: Genetic Analysis of Distortion and Suppression

The sex-ratio trait described in several Drosophila species is a type of naturally occurring X-linked meiotic drive that causes males bearing a sex-ratio X chromosome to produce progenies with a large excess of females. We have previously reported the occurrence of sex-ratio X chromosomes in Drosophila simulans. In this species, because of the co-occurrence of drive suppressors, the natural populations and the derived laboratory strains show an equal sex-ratio even when sex-ratio X chromosomes are present at a high frequency. The presence of sex-ratio X chromosomes is established via crosses with a standard strain that is devoid of drive suppressors. In this article, we show first that the sex-ratio trait in D. simulans results from the action of several X-linked loci. Second we describe drive suppressors on each major autosome as well as on the Y chromosome. The Y-linked factors suppress the drive partially whereas the autosomal suppression can be complete.     

The Origin of the Achiasmatic XY Sex Chromosome System in Cacopsylla peregrina (Frst.) (Psylloidea, Homoptera)

The status of an extra univalent, if it is a B chromosome or an achiasmatic Y chromosome, associating with the X chromosome in male meiosis of Cacopsylla peregrina (Frst.) (Homoptera, Psylloidea) was analysed. One extra univalent was present in all males collected from three geographically well separated populations, it was mitotically stable, and showed precise segregation from the X chromosome. These findings led us to propose that the univalent represents in fact a Y chromosome. The behaviour of the X and Y chromosomes during meiotic prophase suggested that their regular segregation was based on an achiasmatic segregation mechanism characterised by a 'touch and go' pairing of segregating chromosomes at metaphase I. To explain the formation of the achiasmatic Y within an insect group with X0 sex chromosome system, it was suggested that the Y chromosome has evolved from a mitotically stable B chromosome that was first integrated into an achiasmatic segregation system with the X chromosome, and has later become fixed in the karyotype as a Y chromosome.     

Aberrant spermatogenesis and the peculiar mechanism of sex determination in Symphypleonan Collembola (Insecta)

Light and electron microscopy evidence have been obtained to describe the peculiar spermatogenesis in the collembolan species Sminthurus viridis and Allacma fusca (Sminthuridae). In these two species, the two sexes differ for the lack of two chromosomes (the sex chromosomes) in males (males, 2n = 10; females, 2n = 12). While oogenesis seems to proceed normally, spermatogenesis is peculiar because the two daughter cells of the first meiotic division have different chromosome numbers (six and four). The cell receiving four chromosomes degenerates, while the cell receiving six chromosomes completes meiosis and produces identical spermatozoa (n = 6). At fertilization, pronuclei with six chromosomes fuse together to form zygotes with 2n = 12. Male embryos must lose two sex chromosomes during the first zygotic mitosis, as all male cells have 2n = 10 chromosomes. The sex chromosome system of these species can be identified as X1X1X2X2:X1X20. Electron microscopy observations show that the same peculiar spermatogenesis occurs also in two others species of the same family, Caprainea marginata and Lipothrix lubbocki. The peculiar sex determination system described is similar but not identical to what is observed in other insect orders, and it may represent an evolutionary step toward parthenogenesis. It is suggested that this peculiar spermatogenesis is common to all Symphypleona.     

Cytomolecular analysis of the male sex chromosome of Tropidacris cristata grandis (Thunberg, 1824) (Orthoptera: Romaleidae)

Many species of grasshopper have an XX/XO sex chromosome system, including Tropidacris cristata grandis (23, XX/XO). The X chromosome behaves differently from the autosomes, but little is known about its origin and molecular composition. To better understand the genomic composition and evolutionary processes involved in the origin of the sex chromosomes, we undertook an analysis of its meiotic behavior, heterochromatin distribution and microdissection in T. c. grandis. Analysis of meiotic cells revealed a difference in the behavior of the X chromosome compared to the autosomes, with different patterns of condensation and cellular arrangement. Heterochromatic terminal blocks were predominant. The chromosome painting revealed a bright block in the centromeric/pericentromeric region of the X chromosome and slight markings in the other regions. In the autosomes, the X chromosome probe hybridized in the centromeric/pericentromeric region, and hybridization signals on terminal regions corresponding to the heterochromatic regions were also observed. The results showed that the X chromosome contains a significant amount of repetitive DNA. Based on the hybridization pattern, it is possible that the autosomes and sex chromosomes of T. c. grandis have a similar composition of repetitive DNAs, which could mean that the X chromosome has an autosomal origin.     

Genetic Analysis of Sex Chromosomal Meiotic Mutants in DROSOPHILA MELANOGASTER

A total of 209 ethyl methanesulfonate-treated X chromosomes were screened for meiotic mutants that either (1) increased sex or fourth chromosome nondisjunction at either meiotic division in males; (2) allowed recombination in such males; (3) increased nondisjunction of the X chromosome at either meiotic division in females; or (4) caused such females, when mated to males heterozygous for Segregation-Distorter (SD) and a sensitive homolog to alter the strength of meiotic drive in males.-Twenty male-specific meiotic mutants were found. Though the rates of nondisjunction differed, all twenty mutants were qualitatively similar in that (1) they alter the disjunction of the X chromosome from the Y chromosome; (2) among the recovered sex-chromosome exceptional progeny, there is a large excess of those derived from nullo-XY as compared to XY gametes; (3) there is a negative correlation between the frequency of sex-chromosome exceptional progeny and the frequency of males among the regular progeny. In their effects on meiosis these mutants are similar to In(1)sc(4L)sc(8R), which is deleted for the basal heterochromatin. These mutants, however, have normal phenotypes and viabilities when examined as X/0 males, and furthermore, a mapping of two of the mutants places them in the euchromatin of the X chromosome. It is suggested that these mutants are in genes whose products are involved in insuring the proper functioning of the basal pairing sites which are deleted in In(1)sc(4L)sc(8R), and in addition that there is a close connection, perhaps causal, between the disruption of normal X-Y pairing (and, therefore, disjunction) and the occurrence of meiotic drive in the male.-Eleven mutants were found which increased nondisjunction in females. These mutants were characterized as to (1) the division at which they acted; (2) their effect on recombination; (3) their dominance; (4) their effects on disjunction of all four chromosome pairs. Five female mutants caused a nonuniform decrease in recombination, being most pronounced in distal regions, and an increase in first division nondisjunction of all chromosome pairs. Their behavior is consistent with the hypothesis that these mutants are defective in a process which is a precondition for exchange. Two female mutants were allelic and caused a uniform reduction in recombination for all intervals (though to different extents for the two alleles) and an increase in first-division nondisjunction of all chromosomes. Limited recombination data suggest that these mutants do not alter coincidence, and thus, following the arguments of Sandler et al. (1968), are defective in exchange rather than a precondiiton for exchange. A single female mutant behaves in a manner that is consistent with it being a defect in a gene whose functioning is essential for distributive pairing. Three of the female meiotic mutants cause abnormal chromosome behavior at a number of times in meiosis. Thus, nondisjunction at both meiotic divisions is increased, recombinant chromosomes nondisjoin, and there is a polarized alteration in recombination.-The striking differences between the types of control of meiosis in the two sexes is discussed and attention is drawn to the possible similarities between (1) the disjunction functions of exchange and the process specified by the chromosome-specific male mutants; and (2) the prevention of functional aneuploid gamete formation by distributive disjunction and meiotic drive.     

The effect of 6-dimethylaminopurine (6-DMAP) and cycloheximide (CHX) on the development and chromosomal complement of sheep parthenogenetic and nuclear transfer embryos

The effects of activation by 6-dimethylaminopurine (6-DMAP) and cycloheximide (CHX) on the development and chromosomal complement of sheep parthenogenetic and SCNT embryos were investigated. The results revealed that the blastocyst development of parthenogenetic embryos was significantly higher (P < 0.05) in 6-DMAP activated oocytes, compared to those activated with CHX (21.0 +/- 0.9 vs. 14.9 +/- 0.5, respectively). In contrast, the blastocyst frequencies did not significantly differ (P > 0.05) between the two activation treatment groups for SCNT embryos. The 6-DMAP or CHX treatment did not result in any significant difference in the blastocyst total cell number in either parthenote or SCNT embryos. The chromosomal analysis revealed that all the parthenogenetic embryos (100.0%) derived from 6-DMAP treatment, were chromosomally abnormal whereas in CHX-treated embryos, it was significantly lowered (93.6%, P < 0.05). Conversely, the proportions of chromosomally abnormal SCNT embryos did not significantly differ (P > 0.05) among the 6-DMAP and CHX- treated embryo groups (60.0% vs. 56.2%, respectively). This study demonstrated that oocyte activation agents such as DMAP and CHX have differing effects on meiotic or mitotic nuclei. The study also highlighted the feasibility of using bovine X and Y chromosome specific painting probes in sheep embryos.     

中国黑腹果蝇中分离变相因子[Segregation Distorter(SD)]的初步研究

黑腹果蝇（Ｄｒｏｓｏｐｈｉｌａ ｍｅｌａｎｏｇａｓｔｅｒ）中的分离变相因子「Ｓｅｇｒｅｇａｔｉｏｎ Ｄｉｓｔｏｒｔｅｒ（ＳＤ）」是一种非常典型的具有减数分裂驱动性质的特殊遗传因子，ＳＤ在世界不同地区的黑腹果蝇群体中广泛存在，通过杂交的方法测得其频率在１％－５％范围之内。首次在中国北京，青岛采集大量野生黑腹果蝇样本，对ＳＤ进行频率测定，发现ＳＤ在中国野生黑腹果蝇群体中也广泛存在，且频率与世界其他地区     

Reduced polymorphism associated with X chromosome meiotic drive in the stalk-eyed fly Teleopsis dalmanni

Sex chromosome meiotic drive has been suggested as a cause of several evolutionary genetic phenomena, including genomic conflicts that give rise to reproductive isolation between new species. In this paper we present a population genetic analysis of X chromosome drive in the stalk-eyed fly, Teleopsis dalmanni, to determine how this natural polymorphism influences genetic diversity. We analyzed patterns of DNA sequence variation at two X-linked regions (comprising 1325 bp) approximately 50 cM apart and one autosomal region (comprising 921 bp) for 50 males, half of which were collected in the field from one of two allopatric locations and the other half were derived from lab-reared individuals with known brood sex ratios. These two populations are recently diverged but exhibit partial postzygotic reproductive isolation, i.e. crosses produce sterile hybrid males and fertile females. We find no nucleotide or microsatellite variation on the drive X chromosome, whereas the same individuals show levels of variation at autosomal regions that are similar to field-collected flies. Furthermore, one field-caught individual collected 10 years previously had a nearly identical X haplotype to the drive X, and is over 2% divergent from other haplotypes sampled from the field. These results are consistent with a selective sweep that has removed genetic variation from much of the drive X chromosome. We discuss how this finding may relate to the rapid evolution of postzygotic reproductive isolation that has been documented for these flies.     

Chromosome rearrangement patterns of an SD chromosome (SDKona-2) in Drosophila melanogaster caused by hybrid dysgenesis.

The types and frequencies of spontaneous chromosome rearrangements caused by hybrid dysgenesis were studied in a second chromosome autosome of Drosophila melanogaster. This second chromosome, being an SD chromosome, had two important advantages over other autosomes for this study: (i) it had the two inversions characteristic of a standard SD-72 chromosome type, which distinguished it from its homolog in polytene chromosome spreads, and (ii) because of the meiotic drive associated with the segregation distorter system, it was preferentially transmitted to the next generation. The chromosome mutation frequency of this chromosome (given the name SDKona-2) was 8.3 and 11.7% in the F2 and F3 generations, respectively. The types of new chromosome rearrangements observed in the first four generations included paracentric inversions, pericentric inversions, duplications, deletions, reciprocal translocations (involving the third chromosome), and transpositions. Small paracentric inversions were the most common type of new rearrangement. Later, over 35 generations, some of these new rearrangements changed, either by becoming more complex or by being replaced with yet another new chromosome rearrangement. Duplications were unstable and were replaced by paracentric inversions whose breakpoints were on either side of the duplication. Transpositions arose both from a single multibreak event and from a series of two-break events.     

Genetic Analysis of Stellate Elements of Drosophila Melanogaster

Repeated elements are remarkably important for male meiosis and spermiogenesis in Drosophila melanogaster. Pairing of the X and Y chromosomes is mediated by the ribosomal RNA genes of the Y chromosome and X chromosome heterochromatin, spermiogenesis depends on the fertility factors of the Y chromosome. Intriguingly, a peculiar genetic system of interaction between the Y-linked crystal locus and the X-linked Stellate elements seem to be also involved in male meiosis and spermiogenesis. Deletion of the crystal element of the Y, via an interaction with the Stellate elements of the X, causes meiotic abnormalities, gamete-genotype dependent failure of sperm development (meiotic drive), and deposition of protein crystals in spermatocytes. The current hypothesis is that the meiotic abnormalities observed in cry(-) males is due to an induced overexpression of the normally repressed Ste elements. An implication of this hypothesis is that the strength of the abnormalities would depend on the amount of the Ste copies. To test this point we have genetically and cytologically examined the relationship of Ste copy number and organization to meiotic behavior in cry(-) males. We found that heterochromatic as well as euchromatic Ste repeats are functional and that the abnormality in chromosome condensation and the frequency of nondisjunction are related to Ste copy number. Moreover, we found that meiosis is disrupted after synapsis and that cry-induced meiotic drive is probably not mediated by Ste.     

Sex chromosome inactivation in the male

Mammalian females have two X chromosomes, while males have only one X plus a Y chromosome. In order to balance X-linked gene dosage between the sexes, one X chromosome undergoes inactivation during development of female embryos. This process has been termed X-chromosome inactivation (XCI). Inactivation of the single X chromosome also occurs in the male, but is transient and is confined to the late stages of first meiotic prophase during spermatogenesis. This phenomenon has been termed meiotic sex chromosome inactivation (MSCI). A substantial portion (~15-25%) of X-linked mRNA-encoding genes escapes XCI in female somatic cells. While no mRNA genes are known to escape MSCI in males, ~80% of X-linked miRNA genes have been shown to escape this process. Recent results have led to the proposal that the RNA interference mechanism may be involved in regulating XCI in female cells. We suggest that some MSCI-escaping miRNAs may play a similar role in regulating MSCI in male germ cells.