Deleterious mutations in various <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> strains carrying meiotic mutation <Emphasis Type="Italic">c(3)G</Emphasis>

In the absence of meiotic recombination, deleterious mutations, decreasing the viability, are accumulated and fixed in small Drosophila populations. Study of the viability of hybrid progenies of three laboratory Drosophila melanogaster strains carrying meiotic mutation c(3)G ¹⁷ has suggested that the deleterious mutations are negatively synergistic in their interaction. The deleterious mutations localized to the pericentromeric region of chromosome 3 are threefold more efficient as compared with the mutations located in distal regions. Substitution of a new chromosome for the balancer chromosome in a strain with meiotic mutation c(3)G ¹⁷ partially restores (by ～20%) the viability of homozygotes c(3)G ¹⁷ /c(3)G ¹⁷ over the first 20–30 generations. Further cultivation for 30 generations with the same balancer again decreases the viability to the initial level. An epigenetic nature of deleterious mutations is discussed.     

Survival of Drosophila melanogaster progeny after prolonged suppression of pairing and recombination in autosome 3

Two laboratory strains of Drosophila melanogaster carrying autosome 3 with a meiotic mutation c(3)G, that is maintained since 1985 in various balancer chromosomes, were used to study progeny survival. The conditions of maintenance of these strains and the effect of c(3)G mutation completely suppress pairing and crossing over in autosome 3. In addition, selection pressure was reduced because of permanent heterozygosity, mediating mutation accumulation in the studied chromosome. In both strains, all homozygotes for autosome 3 (c(3)G/c(3)G) perished. The hybrid homozygotes carrying chromosomes with c(3)G mutation from different strains survived in 0.4 of the progeny. Higher viability was observed after normal pairing and meiotic recombination of the studied chromosome with the chromosome from the wild-type line. The possible nature of mutations accumulated after prolonged suppression of chromosome pairing and recombination is discussed.     

Survival ofDrosophila melanogaster Progeny after Prolonged Suppression of Pairing and Recombination in Autosome 3

Two laboratory strains of Drosophila melanogaster carrying autosome 3 with a meiotic mutation c(3)G, that is maintained since 1985 in various balancer chromosomes, were used to study progeny survival. The conditions of maintenance of these strains and the effect of c(3)G mutation completely suppress pairing and crossing over in autosome 3. In addition, selection pressure was reduced because of permanent heterozygosity, mediating mutation accumulation in the studied chromosome. In both strains, all homozygotes for autosome 3 (c(3)G/c(3)G) perished. The hybrid homozygotes carrying chromosomes with c(3)G mutation from different strains survived in 0.4 of the progeny. Higher viability was observed after normal pairing and meiotic recombination of the studied chromosome with the chromosome from the wild-type line. The possible nature of mutations accumulated after prolonged suppression of chromosome pairing and recombination is discussed.     

Nature of Deleterious Mutation Load in Drosophila

Much population genetics and evolution theory depends on knowledge of genomic mutation rates and distributions of mutation effects for fitness, but most information comes from a few mutation accumulation experiments in Drosophila in which replicated chromosomes are sheltered from natural selection by a balancer chromosome. I show here that data from these experiments imply the existence of a large class of minor viability mutations with approximately equivalent effects. However, analysis of the distribution of viabilities of chromosomes exposed to EMS mutagenesis reveals a qualitatively different distribution of effects lacking such a minor effects class. A possible explanation for this difference is that transposable element insertions, a common class of spontaneous mutation event in Drosophila, frequently generate minor viability effects. This explanation would imply that current estimates of deleterious mutation rates are not generally applicable in evolutionary models, as transposition rates vary widely. Alternatively, much of the apparent decline in viability under spontaneous mutation accumulation could have been nonmutational, perhaps due to selective improvement of balancer chromosomes. This explanation accords well with the data and implies a spontaneous mutation rate for viability two orders of magnitude lower than previously assumed, with most mutation load attributable to major effects.     

The rate of mutation and the homozygous and heterozygous mutational effects for competitive viability: a long-term experiment with Drosophila melanogaster

The effect of 250 generations of mutation accumulation (MA) on the second chromosome competitive viability of Drosophila melanogaster was analyzed both in homozygous and heterozygous conditions. We used full-sib MA lines, where selection hampers the accumulation of severely deleterious mutations but is ineffective against mildly deleterious ones. A large control population was simultaneously evaluated. Competitive viability scores, unaffected by the expression of mutations in heterozygosis, were obtained relative to a Cy/L(2) genotype. The rate of decline in mean DeltaM approximately 0.1% was small. However, that of increase in variance DeltaV approximately 0.08 x 10(-3) was similar to the values obtained in previous experiments when severely deleterious mutations were excluded. The corresponding estimates of the mutation rate lambda > or = 0.01 and the average effect of mutations E(s) < or = 0.08 are in good agreement with Bateman-Mukai and minimum distance estimates for noncompetitive viability obtained from the same MA lines after 105 generations. Thus, competitive and noncompetitive viability show similar mutational properties. The regression estimate of the degree of dominance for mild-to-moderate deleterious mutations was approximately 0.3, suggesting that the pertinent value for new unselected mutations should be somewhat smaller.     

Spontaneous and Ethyl Methanesulfonate-Induced Mutations Controlling Viability in DROSOPHILA MELANOGASTER. II. Homozygous Effect of Polygenic Mutations

Polygenic mutations affecting viability were accumulated on the second chromosome of Drosophila melanogaster by treating flies with EMS in successive generations. The treated chromosomes were later made homozygous and tested for their effects on viability by comparison of the frequency of such homozygotes with that of other genotypes in the same culture. The treated wild-type chromosomes were kept heterozygous in Pm/+ males by mating individual males in successive generations to Cy/Pm females. The number of generations of accumulation was 1 to 30 generations, depending on the concentration of EMS. A similar experiment for spontaneous polygenic mutations was also conducted by accumulating mutations for 40 generations. The lower limit of the spontaneous mutation rate of viability polygenes is estimated to be 0.06 per second chromosome per generation, which is about 12 times as high as the spontaneous recessive lethal mutation rate, 0.005. EMS-induced polygenic mutations increase linearly with the number of treated generations and with the concentration of EMS. The minimum mutation rate of viability polygenes is about 0.017 per 10(-4)m, which is only slightly larger than the lethal rate of 0.013 per 10(-4) m. The maximum estimate of the viability reduction of a single mutant is about 6 to 10 percent of the normal viability. The data are consistent with a constant average effect per mutant at all concentrations, but this is about three times as high as that for spontaneous mutants. It is obvious that one can obtain only a lower limit for the mutation rate, since some mutants may have effects so near to zero that they cannot be detected. The possibility of measuring something other than the lower limit is discussed. The ratio of the load due to detrimental mutants to that caused by lethals, the D/L ratio, is about 0.2 to 0.3 for EMS-induced mutants, as compared to about 0.5 for spontaneous mutants. This is to be expected if EMS treatment produces a large fraction of small deletions and other chromosome rearrangements which are more likely to be lethal.     

Features of the chromocenter structure in Drosophila melanogaster larva cells, mutant for genes C(3)G and NOD

Homolog pairing, chromosome morphology, and chromosome disjunction in the first meiotic division were studied in the oocytes of c(3)G/c(3)G female Drosophila melanogaster at developmental stages 3-4 and 14. It was found that homologs were completely or partly paired in some cells (about 20% in either case). The lengths of chromosomes in +/+, +/c(3)G, and c(3)G/c(3)G cells were at a ratio of 1.0:1.6:2.2. The chromocenters of homozygous cells had an abnormal structure. There was no meiotic block in metaphase 1, and chromosomes only segregated equally in about 80% of anaphases of the first meiotic division. The data obtained correspond to the abnormal variants of the formation of the chromocenter in c(3)G/c(3)G females that could be predicted based on the two-ring structure of the chromocenter. The mechanism of the effect of the homo- and heterozygosity for the hypomorphic mutation c(3)G on the formation of the synaptonemal complex (SC) and crossing over frequency was suggested. In nod/nod homozygous females, asynapsis of pericentromeric regions of homologs was observed in the chromocenter. It was assumed that NOD kinezin is necessary at the last stages of pairing of the pericentromeric regions of homologs and formation of the coordinating bonds between them.     

Increase of the spontaneous mutation rate in a long-term experiment with Drosophila melanogaster

In a previous experiment, the effect of 255 generations of mutation accumulation (MA) on the second chromosome viability of Drosophila melanogaster was studied using 200 full-sib MA1 lines and a large C1 control, both derived from a genetically homogeneous base population. At generation 265, one of those MA1 lines was expanded to start 150 new full-sib MA2 lines and a new C2 large control. After 46 generations, the rate of decline in mean viability in MA2 was approximately 2.5 times that estimated in MA1, while the average degree of dominance of mutations was small and nonsignificant by generation 40 and moderate by generation 80. In parallel, the inbreeding depression rate for viability and the amount of additive variance for two bristle traits in C2 were 2-3 times larger than those in C1. The results are consistent with a mutation rate in the line from which MA2 and C2 were derived about 2.5 times larger than that in MA1. The mean viability of C2 remained roughly similar to that of C1, but the rate of MA2 line extinction increased progressively, leading to mutational collapse, which can be ascribed to accelerated mutation and/or synergy after important deleterious accumulation.     

Increase in viability due to the accumulation of X chromosome mutations in <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> males

To increase our understanding of the role of new X-chromosome mutations in adaptive evolution, single-X Drosophila melanogaster males were mated with attached-X chromosome females, allowing the male X chromosome to accumulate mutations over 28 generations. Contrary to our hypothesis that male viability would decrease over time, due to the accumulation and expression of X-linked recessive deleterious mutations in hemizygous males, viability significantly increased. This increase may be attributed to germinal selection and to new X-linked beneficial or compensatory mutations, possibly supporting the faster-X hypothesis.     

The build up of mutation-selection- drift balance in laboratory Drosophila populations

The build up of an equilibrium between mutation, selection, and drift in populations of moderate size is an important evolutionary issue, and can be critical in the conservation of endangered populations. We studied this process in two Drosophila melanogaster populations initially lacking genetic variability (C1 and C2) that were subsequently maintained during 431 or 165 generations with effective population size N(e) approximately 500 (estimated by lethal complementation analysis). Each population originated synchronously to a companion set of full-sib mutation accumulation (MA) lines, C1 and MA1 were derived from an isogenic origin and C2 and MA2 from a single MA1 line at generation 265. The results suggest that both C1 and C2 populations were close to the mutation-selection-drift balance for viability and bristle traits, and are consistent with a 2.5-fold increase of the mutation rate in C2 and MA2. Despite this increase, the average panmictic viability in C2 was only slightly below that of C1, indicating that the expressed loads due to segregating deleterious mutation were small, in agreement with the low deleterious mutation rate (0.015-0.045) previously reported for the MA1 lines. In C1, the nonlethal inbreeding depression rate for viability was 30% of that usually estimated in segregating populations. The genetic variance for bristles regenerated in C1 and C2 was moderately smaller than the average value reported for natural populations, implying that they have accumulated a substantial adaptive potential. In light of neutral and selective predictions, these results suggest that bristle additive variance was predominantly due to segregation of mutations with deleterious effects of the order of 10(-3), and is consistent with relatively weak causal stabilizing selection (V(s) approximately 30).     

Deleterious genomic mutation rate for viability in Drosophila melanogaster using concomitant sibling controls

New deleterious mutations may reduce health and fitness and are involved in the evolution and maintenance of numerous biological processes. Hence, it is important to estimate the deleterious genomic mutation rate (U) in representative higher organisms. However, these estimated rates vary widely, mainly because of inadequate experimental controls. Here we describe an experimental design (the Binscy assay) with concomitant sibling controls and estimate U for viability in Drosophila melanogaster to be 0.31. This estimate, like most published studies, focuses on viability mutations and the overall deleterious genomic mutation rate would therefore be higher.     

黑腹果蝇3号染色体裂翅平衡致死位点的发现及2号、3号染色体裂卷翅双平衡染色体的建立

果蝇的平衡染色体在遗传研究中被广泛应用.文章通过分析黑腹果蝇裂翅新突变体与野生型、982紫眼及黑檀体杂交后代裂翅性状情况,首次将裂翅基因定位于3号染色体上,并阐明了裂翅平衡致死、杂合子纯繁的遗传机制,获得了以裂翅为显性标记的3号平衡染色体品系.探索了双平衡染色体显性标记基因聚合的杂交模式,成功建立了以裂翅和卷翅为标记的2号、3号双平衡染色体.裂翅的发现为3号染色体平衡子提供了更加方便识另0的显性翅型标记,同时裂卷翅双平衡体的建立丰富了果蝇常用工具平衡子,可以广泛用于基因定位及突变筛选过程.     

The Baker's Yeast Diploid Genome Is Remarkably Stable in Vegetative Growth and Meiosis

Accurate estimates of mutation rates provide critical information to analyze genome evolution and organism fitness. We used whole-genome DNA sequencing, pulse-field gel electrophoresis, and comparative genome hybridization to determine mutation rates in diploid vegetative and meiotic mutation accumulation lines of Saccharomyces cerevisiae. The vegetative lines underwent only mitotic divisions while the meiotic lines underwent a meiotic cycle every ∼20 vegetative divisions. Similar base substitution rates were estimated for both lines. Given our experimental design, these measures indicated that the meiotic mutation rate is within the range of being equal to zero to being 55-fold higher than the vegetative rate. Mutations detected in vegetative lines were all heterozygous while those in meiotic lines were homozygous. A quantitative analysis of intra-tetrad mating events in the meiotic lines showed that inter-spore mating is primarily responsible for rapidly fixing mutations to homozygosity as well as for removing mutations. We did not observe 1–2 nt insertion/deletion (in-del) mutations in any of the sequenced lines and only one structural variant in a non-telomeric location was found. However, a large number of structural variations in subtelomeric sequences were seen in both vegetative and meiotic lines that did not affect viability. Our results indicate that the diploid yeast nuclear genome is remarkably stable during the vegetative and meiotic cell cycles and support the hypothesis that peripheral regions of chromosomes are more dynamic than gene-rich central sections where structural rearrangements could be deleterious. This work also provides an improved estimate for the mutational load carried by diploid organisms.     

Age-specific Effects of Novel Mutations in Drosophila Melanogaster I. Mortality

Theories for the evolution of aging rest on the assumption that at least some deleterious mutations have effects that are limited to certain ages. Many mutation accumulation studies have tried to measure the number and magnitude of deleterious mutations, but few studies have tried to determine the extent to which the effects of mutations are limited to particular ages. Here we estimate the age-specific effect of deleterious mutations on mortality rate in an outbred population of the fruit fly, Drosophila melanogaster. We used the ‘middle class neighborhood’ approach to accumulation mutations in populations of flies that had recently been obtained from the wild. There are mutations that increase mortality rates, but whose effects are limited to specific ages. The age-specificity of mutational effects differs between the sexes, between virgin and mated flies, and over time. After 10 and 20 generations of mutation accumulation, there were clear age-specific effects of mutations. After 30 generations, however, the degree of age-specificity decreased. In addition, mutation accumulation led to a steady increase in larval mortality and a small but significant increase in the sex ratio of eclosing flies. We discuss the implications of these results for models of aging, and suggest approaches that future studies should take to obtain accurate information on the age-specificity of novel mutations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sustained and rapid chromosome movements are critical for chromosome pairing and meiotic progression in budding yeast

Telomere-led chromosome movements are a conserved feature of meiosis I (MI) prophase. Several roles have been proposed for such chromosome motion, including promoting homolog pairing and removing inappropriate chromosomal interactions. Here, we provide evidence in budding yeast that rapid chromosome movements affect homolog pairing and recombination. We found that csm4Δ strains, which are defective for telomere-led chromosome movements, show defects in homolog pairing as measured in a "one-dot/two-dot tetR-GFP" assay; however, pairing in csm4Δ eventually reaches near wild-type (WT) levels. Charged-to-alanine scanning mutagenesis of CSM4 yielded one allele, csm4-3, that confers a csm4Δ-like delay in meiotic prophase but promotes high spore viability. The meiotic delay in csm4-3 strains is essential for spore viability because a null mutation (rad17Δ) in the Rad17 checkpoint protein suppresses the delay but confers a severe spore viability defect. csm4-3 mutants show a general defect in chromosome motion but an intermediate defect in chromosome pairing. Chromosome velocity analysis in live cells showed that while average chromosome velocity was strongly reduced in csm4-3, chromosomes in this mutant displayed occasional rapid movements. Lastly, we observed that spo11 mutants displaying lower levels of meiosis-induced double-strand breaks showed higher spore viability in the presence of the csm4-3 mutation compared to csm4Δ. On the basis of these observations, we propose that during meiotic prophase the presence of occasional fast moving chromosomes over an extended period of time is sufficient to promote WT levels of recombination and high spore viability; however, sustained and rapid chromosome movements are required to prevent a checkpoint response and promote efficient meiotic progression.     

The Distribution of Mutation Effects on Viability in Drosophila Melanogaster

Parameters of continuous distributions of effects and rates of spontaneous mutation for relative viability in Drosophila are estimated by maximum likelihood from data of two published experiments on accumulation of mutations on protected second chromosomes. A model of equal mutant effects gives a poor fit to the data of the two experiments; higher likelihoods are obtained with leptokurtic distributions or for models in which there is more than one class of mutation effect. Minimum estimates of mutation rates (events per generation) at polygenes affecting viability on chromosome 2 are 0.14 and 0.068, but estimates are strongly confounded with other parameters in the model. Separate information on rates of molecular divergence between Drosophila species and from rates of movement of transposable elements is used to infer the overall genomic mutation rate in Drosophila, and the viability data are analyzed with mutation rate as a known parameter. If, for example, a mutation rate for chromosome 2 of 0.4 is assumed, maximum likelihood estimates of mean mutant effect on relative viability are 0.4% and 1%, but the majority of mutations have very much smaller effects than these values as distributions are highly leptokurtic. The methodology is applied to estimate viability effects of single P element insertional mutations. The mean effect per insertion is found to be higher, and their distribution is found to be less leptokurtic than for spontaneous mutations. The equilibrium genetic variance of viability predicted by a mutation-selection balance model with parameters estimated from the mutation accumulation experiments is similar to laboratory estimates of genetic variance of viability from natural populations of Drosophila.     

A carbohydrate epitope expressed uniquely on the cell surface of Drosophila neurons is altered in the mutant nac (neurally altered carbohydrate).

Antibodies against horseradish peroxidase (anti-HRP) recognize neural specific cell surface antigens in Drosophila and other insects. The nature of these antigens was investigated in Drosophila and found to include a complex set of developmentally regulated proteins. Their common epitope appears to be a carbohydrate that shares features with the sugar moiety of pineapple stem bromelain, a plant glycoprotein whose carbohydrate structure has been determined. A mutation was identified that eliminates staining by the antibody in imaginal and adult neural tissue. Tissue specific glycoconjugates, although widespread in the animal kingdom, are little understood. This mutation provides a unique opportunity to address the consequences of altering a neural specific carbohydrate moiety in an otherwise intact and behaving animal. The mutation maps to 84F. A second mutation, contained on the third chromosome balancer, TM3, eliminates anti-HRP staining in embryos. These mutations appear to be separate genes.     

mei-P22 encodes a chromosome-associated protein required for the initiation of meiotic recombination in Drosophila melanogaster

Double-strand breaks (DSB) initiate meiotic recombination in a variety of organisms. Here we present genetic evidence that the mei-P22 gene is required for the induction of DSBs during meiotic prophase in Drosophila females. Strong mei-P22 mutations eliminate meiotic crossing over and suppress the sterility of DSB repair-defective mutants. Interestingly, crossing over in mei-P22 mutants can be restored to almost 50% of wild-type by X irradiation. In addition, an antibody-based assay was used to demonstrate that DSBs are not formed in mei-P22 mutants. This array of phenotypes is identical to that of mei-W68 mutants; mei-W68 encodes the Drosophila Spo11 homolog that is proposed to be an enzyme required for DSB formation. Consistent with a direct role in DSB formation, mei-P22 encodes a basic 35.7-kD protein, which, when examined by immunofluorescence, localizes to foci on meiotic chromosomes. MEI-P22 foci appear transiently in early meiotic prophase, which is when meiotic recombination is believed to initiate. By using an antibody to C(3)G as a marker for synaptonemal complex (SC) formation, we observed that SC is present before MEI-P22 associates with the chromosomes, thus providing direct evidence that the development of SC precedes the initiation of meiotic recombination. Similarly, we found that MEI-P22 foci did not appear in a c(3)G mutant in which SC does not form, suggesting that DSB formation is dependent on SC formation in Drosophila. We propose that MEI-P22 interacts with meiosis-specific chromosome proteins to facilitate DSB creation by MEI-W68.     

Two new mouse chromosome 11 balancers

Segmental inversions causing recombination suppression are an essential feature of balancer chromosomes. Meiotic crossing over between homologous chromosomes within an inversion interval will lead to nonviable gametes, while gametes generated from recombination events elsewhere on the chromosome will be unaffected. This apparent recombination suppression has been widely exploited in genetic studies in Drosophila to maintain and analyze stocks carrying recessive lethal mutations. Balancers are particularly useful in mutagenesis screens since they help to establish the approximate genomic location of alleles of genes causing phenotypes. Using the Cre-loxP recombination system, we have constructed two mouse balancer chromosomes carrying 8- and 30-cM inversions between Wnt3 and D11Mit69 and between Trp53 and EgfR loci, respectively. The Wnt3-D11Mit69 inversion mutates the Wnt3 locus and is therefore homozygous lethal. The Trp53-EgfR inversion is homozygous viable, since the EgfR locus is intact and mutations in p53 are homozygous viable. A dominantly acting K14-agouti minigene tags both rearrangements, which enables these balancer chromosomes to be visibly tracked in mouse stocks. With the addition of these balancers to the previously reported Trp53-Wnt3 balancer, most of mouse chromosome 11 is now available in balancer stocks.     

Retention of induced mutations in a Drosophila reverse-genetic resource

Targeting induced local lesions in genomes (TILLING) is a reverse-genetic method for identifying point mutations in chemically mutagenized populations. For functional genomics, it is ideal to have a stable collection of heavily mutagenized lines that can be screened over an extended period of time. However, long-term storage is impractical for Drosophila, so mutant strains must be maintained by continual propagation of live cultures. Here we evaluate a strategy in which ethylmethane sulfonate (EMS) mutagenized chromosomes were maintained as heterozygotes with balancer chromosomes for >100 generations before screening. The strategy yielded a spectrum of point mutations similar to those found in previous studies of EMS-induced mutations, as well as 2.4% indels (insertions and deletions). Our analysis of 1887 point mutations in 148 targets showed evidence for selection against deleterious lesions and differential retention of lesions among targets on the basis of their position relative to balancer breakpoints, leading to a broad distribution of mutational densities. Despite selection and differential retention, the success of a user-funded service based on screening a large collection several years after mutagenesis indicates sufficient stability for use as a long-term reverse-genetic resource. Our study has implications for the use of balancer chromosomes to maintain mutant lines and provides the first large-scale quantitative assessment of the limitations of using breeding populations for repositories of genetic variability.