Meiotic checkpoints and the interchromosomal effect on crossing over in Drosophila females

During prophase of meiosis I, genetic recombination is initiated with a Spo11-dependent DNA double-strand break (DSB). Repair of these DSBs can generate crossovers, which become chiasmata and are important for the process of chromosome segregation. To ensure at least one chiasma per homologous pair of chromosomes, the number and distribution of crossovers is regulated. One system contributing to the distribution of crossovers is the pachytene checkpoint, which requires the conserved gene pch2 that encodes an AAA+ATPase family member. Pch2-dependent pachytene checkpoint function causes delays in pachytene progression when there are defects in processes required for crossover formation, such as mutations in DSB-repair genes and when there are defects in the structure of the meiotic chromosome axis. Thus, the pachytene checkpoint appears to monitor events leading up to the generation of crossovers. Interestingly, heterozygous chromosome rearrangements cause Pch2-dependent pachytene delays and as little as two breaks in the continuity of the paired chromosome axes are sufficient to evoke checkpoint activity. These chromosome rearrangements also cause an interchromosomal effect on recombination whereby crossing over is suppressed between the affected chromosomes but is increased between the normal chromosome pairs. We have shown that this phenomenon is also due to pachytene checkpoint activity.     

Meiotic checkpoints and the interchromosomal effect on crossing over in Drosophila females

During prophase of meiosis I, genetic recombination is initiated with a Spo11-dependent DNA double-strand break (DSB). Repair of these DSBs can generate crossovers, which become chiasmata and are important for the process of chromosome segregation. To ensure at least one chiasma per homologous pair of chromosomes, the number and distribution of crossovers is regulated. One system contributing to the distribution of crossovers is the pachytene checkpoint, which requires the conserved gene pch2 that encodes an AAA+ATPase family member. Pch2-dependent pachytene checkpoint function causes delays in pachytene progression when there are defects in processes required for crossover formation, such as mutations in DSB-repair genes and when there are defects in the structure of the meiotic chromosome axis. Thus, the pachytene checkpoint appears to monitor events leading up to the generation of crossovers. Interestingly, heterozygous chromosome rearrangements cause Pch2-dependent pachytene delays and as little as two breaks in the continuity of the paired chromosome axes are sufficient to evoke checkpoint activity. These chromosome rearrangements also cause an interchromosomal effect on recombination whereby crossing over is suppressed between the affected chromosomes but is increased between the normal chromosome pairs. We have shown that this phenomenon is also due to pachytene checkpoint activity.     

Age-related changes in crossing over in Drosophila resemble the picture of interchromosomal effect of chromosome rearrangement on crossing over

Crossing over in the left arm of chromosome 2 (2L) was studied in successive broods of Drosophila melanogaster females carrying intact chromosomes (+/+), inversion Muller-5 in the X chromosome (M-5/+), and insertion of the Y-chromosome material into region 34A (Is(2L)/+). The regions net-dp, dp-b, b-pr and pr-cn were examined in 14 two-day-old broods of females +/+ and M-5/+ and in 10 broods of females Is(2L)/+. In all lines, the highest level of crossing over was in the first three broods (eggs laid during the first 6 days of oviposition) and the lowest level in the broods 7-8 (eggs laid at days 14-16). A high rate of crossing over in the first broods of females +/+ and M-5/+ was due to an increment of exchanges in the proximal euchromatin regions (b-pr and pr-cn) and to an increase in the number of tetrads with double exchanges. These changes are similar to a pattern of the interchromosomal effect on crossing over (IEC) in structurally normal chromosomes. In Is(2L)/+ females, a high level of crossing over was due to extensive exchanges in the interstitial regions net-dp and dp and an increase in the number of tetrads with single exchanges. These changes resembled the IEC in rearranged chromosomes (in this case, in chromosomes bearing an insertion). Thus, the age changes of crossing over are similar to the consequences of the presence or absence of IEC. Age changes in crossing over in a chromosome depended both on the local rearrangements in this chromosome (the local effect on crossing over, LEC) and on rearrangements in nonhomologous chromosomes (IEC). In the first broods, both LEC and IEC decreased with an increase in the level of crossing over. In subsequent broods, the reduced level of crossing over was accompanied by an increase in both LEC and IEC. This suggests that the mechanisms responsible for the age changes in crossing over and IEC may have common steps. The contact model of crossing over may explain the similarity between the age changes in crossing-over and IEC. It is suggested that both phenomena result from delayed determination of crossing over in a meiotic cell. This may occur due to the retarded formation of the local contacts in one of the homologous chromosome pairs or because a higher number of local contacts is required to trigger crossing over in a meiotic cell (of early age).     

Studies on the expression of somatic crossing over in Glycine max L.

Summary Variety T219 of Glycine max L. has spontaneous yellow, dark green and double (yellow-dark green) spots on the leaves of plants of genotype Y ₁₁ y ₁₁ but no such spots are found on leaves of Y ₁₁ Y ₁₁ or y₁₁y₁₁ plants. It was suggested (Vig and Paddock, 1968) that the double spots result from somatic crossing over whereas the two types of single spots primarily originate from chromosomal disturbances.Cold shocks disproportionately increased the frequency of double spots, but ethyl methanesulfonate (EMS) did not do so. However, in most cases of each treatment, the frequency of single spots increased. It is suggested that EMS is not very potent in bringing about somatic recombination whereas cold shocks are. Plants from a sample of seeds of variety L65-1237 that had been harvested in 1968 at Urbana, Illinois, did not express the spotting phenomenon, but plants from seeds harvested in 1969 at Reno did have spots. Application of mitomycin C(MC) to the seeds of this variety as well as of T219 increased the incidence of double spots manyfold indicating that MC can reveal the potential for somatic crossing over in a variety which might not otherwise express it. Soaking dry seeds of L65-1237 in aqueous solutions of MC for intervals as short as 2 hours was found effective in increasing the frequency of double spots. The role of MC in relation to DNA synthesis and somatic crossing over is discussed. Application of the chromosome-breaking agent, claunomycin (DM) was ineffective in causing double spots.     

Further studies on the interchromosomal effect on crossing over in Drosophila melanogaster affecting the preconditions of exchange

The second chromosome inversion In (2L+2R) Cy in a heterozygous condition was studied for its effect on frequency and interference of crossing over in three different regions of the X chromosome of Drosophila melanogaster. A significant increase in crossing over frequency was observed in the proximal and distal regioins of the X chromosome while in the middle of the chromosome crossing over frequency remained unaltered. The effect on interference remained unaltered at both ends of the X chromosome while a significant decrease was observed in the middle of the chromosome. These results suggest that the interchromosomal effect on crossing over affects the preconditions of exchange differently in different regions of the X chromosome, and possibly the duration of chromosome pairing.     

Chromosome rearrangement: two ways of influencing crossing over in Drosophila]

Insertion of the Y-material into the 34A Is(Y;2L)419 region diminished recombinational length of the left arm of chromosome 2 (2L) from 49.1 to 15.0 cM. This decrease was compensated by the increase of recombinational length in the other chromosomal arms due to interchromosomal effect. The increase in the X chromosome was 11.4 cM; it was 2.0 cM in chromosome 2R; and 17.3 cM in chromosome 3. The insertion-induced decrease of the 2L recombinational length could be eliminated by evoking interchromosomal effects from other chromosomes. The presence of the inversion in the X chromosome increased the 2L recombinational length from 15.0 to 30.2 cM, while its association with the In(3LR)D inversion increased this length to 45.6 cM. The interchromosomal effects in the inductor chromosome were induced by distortion of pairing rather than by the low recombinational length of this chromosome. For example, the interchromosomal effect of the insertion on the X chromosome was higher in the Is(Y;2L)/+; In(3LR)/+ females than in the Is(Y;2L)/+; +/+ females (15.4 versus 11.5 cM), though the 2L recombinational length in the females with the former genotype (30.2 cM) was twofold higher than in females with the latter genotype (15.0 cM). It is suggested that chromosomal rearrangement hampers the development of local contacts in the homologues. This delay affects crossing over in the given pair of homologues in two ways: directly via diminishing the number of exchange sites, and indirectly through regulatory delay of crossing over determination in the meiocyte. The effects of the insertion on crossing over in nonhomologous chromosomes are implemented by through the second way.     

Chiasmata and chromosome breakages are related to crossing over in Drosophila ananassae males.

A cytogenetic analysis of male crossing over in Drosophila ananassae revealed that cytological exchanges resulted in genetic crossing over, and that chiasma frequency and the genetic recombination correlated positively in chromosomes 2 and 3. Furthermore, the frequency of chromosome breakages correlated positively with chiasma frequency. Paracentric inversion heterozygosity had no detectable influence on the chromosome pairing or exchange events within the inversion loop at meiosis. Scoring of the chiasma demonstrated that males homozygous for the previously mapped enhancers of male crossing over had low frequencies of chiasmata, whereas higher frequencies of chiasmata were observed in males heterozygous for enhancers. The results presented here indicate that the genetic factors controlling male crossing over are involved in the origin of chromosome breakages and in exchange events.     

Elevated levels of expression associated with regions of the Drosophila genome that lack crossing over

The recombinational environment influences patterns of molecular evolution through the effects of Hill-Robertson interference. Here, we examine genome-wide patterns of gene expression with respect to recombinational environment in Drosophila melanogaster. We find that regions of the genome lacking crossing over exhibit elevated levels of expression, and this is most pronounced for genes on the entirely non-crossing over fourth chromosome. We find no evidence for differences in the patterns of gene expression between regions of high, intermediate and low crossover frequencies. These results suggest that, in the absence of crossing over, selection to maintain control of expression may be compromised, perhaps due to the accumulation of deleterious mutations in regulatory regions. Alternatively, higher gene expression may be evolving to compensate for defective protein products or reduced translational efficiency.     

X-Y crossing over in the chimpanzee

Summary Single-copy DNA sequences defining several pseudoautosomal loci on the human sex chromosomes are shown to be highly conserved in the genome of the chimpanzee. Segregation analysis of polymorphic pseudoautosomal probes in a chimpanzee pedigree revealed that the transmission of the paternal alleles was not strictly sex-linked. In situ hybridization localized the pseudoautosomal probe 29C1 specifically to Xp22-Xpter and to Yq12.2-Yqter on the chimpanzee sex chromosomes. Thus, our results demonstrate the existence of homologous segments on the chimpanzee X and Y chromosomes, which regularly undergo recombinatory exchange in male meiosis. The chimpanzee is now the third mammalian species, besides man and mouse, in which there is genetic evidence for a pseudoautosomal segment on the sex chromosomes.     

Regions of lower crossing over harbor more rare variants in African populations of Drosophila melanogaster

A correlation between diversity levels and rates of recombination is predicted both by models of positive selection, such as hitchhiking associated with the rapid fixation of advantageous mutations, and by models of purifying selection against strongly deleterious mutations (commonly referred to as "background selection"). With parameter values appropriate for Drosophila populations, only the first class of models predicts a marked skew in the frequency spectrum of linked neutral variants, relative to a neutral model. Here, we consider 29 loci scattered throughout the Drosophila melanogaster genome. We show that, in African populations, a summary of the frequency spectrum of polymorphic mutations is positively correlated with the meiotic rate of crossing over. This pattern is demonstrated to be unlikely under a model of background selection. Models of weakly deleterious selection are not expected to produce both the observed correlation and the extent to which nucleotide diversity is reduced in regions of low (but nonzero) recombination. Thus, of existing models, hitchhiking due to the recurrent fixation of advantageous variants is the most plausible explanation for the data.     

Control of crossing over

The Holliday junction is a central intermediate in homologous recombination. It consists of a four-way structure that can be resolved by cleavage to give either the crossover or noncrossover products observed. We show here that the formation of these products is controlled by the E. coli resolvasome (RuvABC) in such way that double-strand break repair (DSBR) leads to crossing over and single-strand gap repair (SSGR) does not lead to crossing over. We argue that the positioning of the RuvABC complex and its consequent direction of junction-cleavage is not random. In fact, the action of the RuvABC complex avoids crossing over in the most commonly predicted situations where Holliday junctions are encountered in DNA replication and repair. Our observations suggest that the positioning of the resolvasome may provide a general biochemical mechanism by which cells can control crossing over in recombination.     

The Synaptonemal complex component C(2)M regulates meiotic crossing over in Drosophila

BACKGROUND: The synaptonemal complex (SC) is a proteinaceous structure that forms between homologously paired meiotic chromosomes. Previous studies have suggested that the SC is required for meiotic crossing over in Drosophila. However, only one component of this structure, C(3)G, has been identified in Drosophila. RESULTS: Mutations in c(2)M cause a reduced frequency of meiotic crossing over due, in part, to how recombination events are resolved. Cytological evidence suggests that C(2)M is a component of the SC and is required for the assembly of C(3)G (a putative transverse filament of the SC) along the chromosomes. Additionally, C(2)M localizes along the chromosomes in the absence of C(3)G. Despite having a defect in C(3)G localization, c(2)M mutants unexpectedly affect crossing over less severely than a c(3)G mutant. There is virtually no crossing over in a c(3)G mutant, but c(2)M or c(2)M; c(3)G double mutants produce a substantial number of crossovers. The appearance of C(3)G-independent crossovers in c(2)M mutants suggests that C(2)M prevents recombination in the absence of complete SC formation. CONCLUSIONS: We have identified a new Drosophila SC component, C(2)M, that promotes the formation of crossovers. Furthermore, the appearance of C(3)G-independent crossovers in c(2)M mutants suggests a novel role in preventing recombination in the absence of complete SC.     

Interaction of the mutants sx,tra and dsx in Drosophila melanogaster. Chaetotaxy of the basitarsus of the forelegs in males and females

The basitarsal bristle pattern of the mutants sx (sexcombless), tra (transformer), and dsx (doublesex), and of the combinations sx-dsx and tra-dsx is described. Epistasis of dsx over both sx and tra for many of the chaetotaxal characteristics was found. The various effects of interaction observed, in individuals of male as well as female chromosomal constitution, are discussed in the light of the levels of action of the mutant genes in modifying the development of sex. It is suggested that intersexes induced by dsx are a class by itself, and that the action of dsx might be at a primary level of sex determination.