Average Mutation Rate Research Articles

Essential genes in the hDf6 region of chromosome I in Caenorhabditis elegans.

In this paper we describe the analysis of essential genes in the hDf6 region of chromosome I of Caenorhabditis elegans. Nineteen complementation groups have been identified which are required for the growth, survival or fertility of the organism (essential genes). Since ten of these genes were represented by more than one allele, a Poisson calculation predicts a minimum estimate of 25 essential genes in hDf6. The most mutable gene in this region was let-354 with seventeen alleles. An average mutation rate of 5 x 10(-5) mutations/gene/chromosome screened was calculated for an ethyl methanesulfonate dose of 15 mM. Mutations were recovered by screening for lethal mutations using the duplication sDp2 for recovery. Our analysis shows that duplications are very effective for maintenance and mapping of large numbers of lethal mutations. Approximately 600 lethal mutations were mapped in order to identify the 54 that are in the deficiency hDf6. The hDf6 region appears to have a lower proportion of early arresting mutations than other comparably sized regions of the genome.

Myosin heavy chain gene amplification as a suppressor mutation in Caenorhabditis elegans.

In the nematode, Caenorhabditis elegans, the body wall muscles contain paramyosin and two different types of myosin heavy chain, MHC A and MHC B. In mutants that do not express MHC B or that express defective paramyosin, muscle structure is disrupted and movement is impaired. Second site mutations in the sup-3 locus partially reverse these defects and are correlated with a 2- to 3-fold increase in the accumulation of the MHC A isoform. The sup-3 mutations occur at a high frequency (10(-4] after ethyl methanesulfonate (EMS) mutagenesis. This is comparable to the average EMS-induced mutation rate per gene in C. elegans. In this paper we show that the sup-3 mutation is an amplification of the structural gene for the MHC A protein, myo-3. We employed genomic Southern hybridization with MHC gene-specific probes in order to measure the copy number of the myo-3 gene relative to that of the MHC B gene, unc-54. We have identified the putative amplification junctions for these sup-3 alleles using a set of cosmid clones which encompass myo-3 region. Although it has been suggested that gene amplification plays an important role in evolution, there are few known cases of gene amplification in the germ line cells of multicellular organisms. The results shown here provide a clear example of a heritable gene amplification event that occurs at a high frequency in the germ line. Similar events may thus represent the initial event in the evolution of new function and in the formation of multigene families.

The average number of sites separating DNA sequences drawn from a subdivided population

The “infinite sites” model in the absence of recombination is examined in a subdivided population in which there is arbitrary migration among demes. It is shown that, if the migration matrix is symmetric and irreducible, the average number of sites that differ in two alleles chosen from the same deme depends only on an effective size of the whole population and not on either the elements of the migration matrix or the size of each deme separately. If there are n demes all of size N, the average number of sites that differ in two alleles chosen from the same deme is 4 nNμ, where μ is the average mutation rate per site. This is the same value as for two alleles drawn from a panmictic population of size nN. The average number of sites that differ in alleles drawn from the same and from different demes can provide some information about the degree of population subdivision, as is illustrated by using the data of Kreitman and Aquadé (1986, Proc. Nat. Acad. Sci. U.S.A., 83, 3562) on Drosophila melanogaster.

HEMOPHILIA A: RECURRENT MUTATIONS AND AN UNUSUAL DELETION

We have identified 14 mutations of the factor VIII (F8) gene from a panel of 107 patients with hemophilia A and have characterized these gene defects by restriction analysis, oligonucleotide hybridization, cloning and DNA sequencing. Recurrent point mutations that involve CG to TG transitions were identified in exon 18, exon 22 and exon 24, and a single CG to TG transition was identified in exon 23. In addition, a Taq I site alteration in intron 4 of the F8 gene was identified in a patient with mild hemophilia. Cloning and sequencing of this region suggests that a change in this Taq I site may create a new splice donor site. These data suggest that CG to TG transition is a prominent mechanism of mutation in hemophilia A. We estimate that the mutation rate of CG to TG in the F8 gene is at least 100 times greater than the average mutation rate per nucleotide. Six different deletions were also characterized. In one family, the deletion involved exon 26. However, the deletion endpoints in the male proband were different from those in his carrier mother, suggesting either gonadal mosaicism or a second deletion event in maternal meiosis.Of the 14 mutations, 6 occurred de novo within 2 generations; 4 in males and 2 in females. In these de novo mutations paternal age at conception was 35 (range = 32-38) and maternal age was 24 and 27. The ability to discover a sizable number of mutations in the F8 gene producing hemophilia A enables us to determine the frequency and nature of de novo mutations in man.

Modifiers of mutation rate: A general reduction principle

A deterministic two-locus population genetic model with random mating is studied. The first locus, with two alleles, is subject to mutation and arbitrary viability selection. The second locus, with an arbitrary number of alleles, controls the mutation at the first locus. A class of viability-analogous Hardy-Weinberg equilibria is analyzed in which the selected gene and the modifier locus are in linkage equilibrium. It is shown that at these equilibria a reduction principle for the success of new mutation-modifying alleles is valid. A new allele at the modifier locus succeeds if its marginal average mutation rate is less than the mean mutation rate of the resident modifier allele evaluated at the equilibrium. Internal stability properties of these equilibria are also described.

An evaluation of tubulin as a molecular clock

The available sequence data for tubulin indicates that it cannot be used as a molecular clock. Apparent α-tubulin mutation rates, for example, vary from 0.16 to 3.8 PAMs per 100 million years depending on which two α-tubulins are compared. All animal α-tubulin mutation rates seem to be quite low, whereas those of non-animals are relatively high. A similar division is not present amongst the β-tubulins; their apparent mutation rates, however, vary just as much. For any given tubulin, the largest number of amino acid sequence differences are obtained when comparing it to the tubulins of yeasts. Sequence comparisons with the tubulins of unicellular algae and ciliates show far fewer differences. Cytochrome c data, however, show that the ciliates diverged from animals well before the yeasts. This means, therefore, that the average tubulin mutation rates in yeasts and ciliates since the time they shared a common ancestor must be quite different. The high mutation rate of yeast tubulins may possibly reflect the absence of cilia. Structural constraints imposed on tubulin by the large number of interactions with other components of the complex ciliary axoneme probably have a significant effect on its rate of mutation.

COMPETITION BETWEEN HIGH AND LOW MUTATING STRAINS OF ESCHERICHIA COLI.

The dynamics of bacterial populations are often characterized by several distinctive features: under optimal growth conditions they double every few hours; they usually contain in excess of 106 individuals; higher fitness mutants have a good chance of arising in a population since average mutation rates are approximately 10-6 to 10-7 per gene replication; new favorable mutations, in the absence of genetic recombination, always increase to fixation in linkage with the genome of the parent clone in which they originally occurred; and higher fitness mutants often exhibit 10% to 20% higher growth rates than their parental clones. Consequently, when populations of such organisms are exposed to a new environment, a series of replacement cycles rapidly ensues, each cycle corresponding to the fixation of a higher fitness mutation in linkage with the genome of its parent clone. The linkage between the new mutation and the genome of the parent clone, and the rapidity of these clonal replacements, are two features that distinguish such asexual populations from ones that reproduce sexually. The existence of such cycles in asexual populations was first studied systematically by Atwood et al. (1951) in a study with laboratory populations of Escherichia coli in long-term cultures. By comparing the relative fitness of a series of bacterial clones isolated from these cultures, Atwood et al. were able to show that populations underwent a succession of clonal changes, each clonal replacement

Mutation in Honeybees 2. Average Rate of Mutation Based on Seven Genes for Eye Colour

SummaryThe average numbers of ommatidia in the eyes of workers was 3826 and in the eyes of males 7082. Somatic mutations from recessive to dominant were detected in 7 genes, some reported in this paper, some in an earlier publication. The rates of reverse mutation estimated for males were: for chrr, μ=3·38 × 10−6; for oc, μ=1·96 × 10−6; for pe, μ=0·17 × 10−6, for s−la, μ=1·10 × 10−6 and for sp, mu=2·17 × 10−6. The average was μ= 1·75 × 10−6 or, without pe, μ=2·15 × 10−6. Estimates for workers were: for by, μ=0·67 × 10−6; for chr, μ=1·45 × 10−6; for chli, μ=1·39 × 10−6; for oc, μ=1·56 × 10−6; for sla, μ=0·25 × 10−6; for sp, μ=1·53 × 10−6. The average for workers was μ=1·14 × 10−6 or, without sla, μ= 1·32 × 10−6. The balanced average somatic mutation rate for drones and workers was μ= 1·6 × 10−6. So far, cr has not shown a single case of reverse mutation, which indicates that either its rate of mutation is less than 2·0 × 10−8 or that it results from a chromosomal deletion. In all cases the mutation rate for a particular gene was greater in males than in workers.

SPONTANEOUS ALLOZYME MUTATIONS IN DROSOPHILA MELANOGASTER: RATE OF OCCURRENCE AND NATURE OF THE MUTANTS

After additional generations of accumulation of allozyme mutants, the 1,000 lines of Mukai and Cockerham (1977) were again screened for the same five loci (alpha-Gpdh, cMdh, Adh, Hex-C and alpha-Amy), as well as for two new loci (Got-2 and Dip-A). Based on 3,111,598 allele generations: (1) the average mutation rate to new mobility variants with normal function was estimated to be 1.28 x 10(-6), and (2) the average mutation rate to null alleles was estimated to be 3.86 x 10(-6). A qualitative analysis of the nulls provided evidence that most of the mutants recovered are due to base substitutions. No apparent correlation was observed between structural gene size and mutation rate.

Is there a proportionality between the spontaneous and the X-ray induction rates of mutations?: Experiments with mutations at 13 X-chromosome loci in Drosophila melanogaster

The X-ray induction of recessive visible specific locus mutations at 14 X-chromosome loci was studied in Drosophila melanogaster using the “Maxy” technique. The X-ray exposure was 3000 R to 5-day-old males and the sampling of germ cells was restricted to mature spermatozoa. Presumptive mutant females recovered in the F 1 generation were tested for transmission, allelism, fertility and viability in males. A total of 128 mutations (115 completes and 13 mosaics including those that were male-viable as well as male-lethal) recovered among 38 898 female progeny were found to be transmitted. On the basis of the above frequency, the average mutation rate can be estimated as 7.8 × 10 −8/locus/R; for mutations that were viable and fertile in males, the rate is 3.0 × 10 −8/locus/R (49 mutations among 38 898 progeny). The frequency of mutations at the different loci encompassed a wide range: while no mutations were recovered at the raspberry and carnation loci, at others, the numbers ranged from 1 at echinus to 31 at garnet; in addition, the proportion of mutations that was male-viable was also different, depending on the locus. Schalet's extensive data on spontaneous mutations at 13 (of the 14 loci employed in the present study) loci permit an estimate of the spontaneous rate which is 6.1 × 10 −6/locus (a total of 39 mutations among 490 000 progeny); for mutations that were viable and fertile in males, the rate is 3.0 × 10 −6/locus (19 mutations among 490 000 progeny). The mutability of the different loci varied over a 9-fold range. When the different loci are ranked depending on their relative mutability (for spontaneous and induced mutations) it is found that in general, loci that mutate spontaneously relatively more frequently are also those at which more mutations have been recovered in the radiation experiments and likewise, those that are less mutable spontaneously are also those that mutate less after irradiation. Since the data are limited, it is concluded that the above finding is not inconsistent with the assumption of proportionality between spontaneous and induction rates of mutations. On the basis of the above results, a doubling dose of 100 R can be calculated for the X-ray induction of specific-locus mutations in Drosophila spermatozoa.

THE DYNAMICS OF FINITE HAPLOID POPULATIONS WITH OVERLAPPING GENERATIONS. II. THE DIFFUSION APPROXIMATION

The dynamics of a gene in a haploid population can be explained approximately by considering the average reproductive value of the gene. The dynamics of the average reproductive value are similar to those of a gene in a population with nonoverlapping generations with the following modifications: The effective population size, N(e), replaces N; the average mutation rates micro* and nu* replace micro and nu; the average overall selection r*+(T-1)s** replaces s; and time is measured in terms of generations, T. The implications of the average selection coefficient to adaptive life histories are discussed.

The estimation of mutation rates when premeiotic events are involved.

When mutation or recombination events occur premeiotically, the distribution of exceptional individuals among the offspring will be "clustered" as opposed to binomial. Even though the exact nature of the clustering is usually unknown, unbiased methods for measuring mutation rate and determining the precision of these measurements are given to replace a biased method now frequently used. When clustering is pronounced, the unweighted average mutation rate is found to be a more efficient estimator than the usual average weighted by family size. Methods of statistical inference and optimal experimental design in the absence of specific knowledge of the mechanism of clustering are also discussed.

Estimation of expected number of rare alleles of a locus and calculation of mutation rate

An approach is described for the estimation of the number of rare variants in the population from the number in a sample drawn at random from the population. This quantity is used to derive an estimate of the mutation rate. The data required are the number of rare variants in the sample and the distribution of offspring within a population of well-defined size with little or no immigration. Application of this approach to data on 28 loci assayed in the Yanamamo, a tribe of South American Indians, yields an average mutation rate of 0.1 approximately 0.2 x 10(-5). Determination of this figure is subject to several assumptions concerning the nature of the rare variants and the structure of the population. Violation of these assumptions will generally result in the underestimate of the true mutation rate.

Isolation and characterization of 5-fluorotryptophan-resistant mutants with altered L-tryptophan transport.

Mutants of A9 mouse fibroblast, resistant to the killing effect of 0.4 mM 5-flurotryptophan (5-FT), have altered L-tryptophan transport properties. The resistant phenotype is stable for at least 90 generations of growth in MEM. A fluctuation test indicated that clones resistant to 0.4 mM 5-FT occurred spontaneously. An average mutation rate was estimated at 1.6 X 10(-6). Treatment with N-methyl-N'-nitro-N-nitrosoguanidine increased the frequency of these clones by at least 100-fold. These results indicate that the resistant clones arose as a result of a mutation. All the resistant mutant tested accumulate less 5-FT at near steady-state conditions than the wild type. Lineweaver-Burk plots of initial rates of tryptophan uptake yield a biphasic curve suggesting that tryptophan is transported by two transport systems. Kinetic constants determined by a computer program indicate that both proposed transport systems were modified in each of two 5-FT resistant mutants.

The impact of induced mutations on human populations

The major impact of an increase in genetic damage will be expressed as an increase in autosomal dominant and X-linked traits as well as chromosomal disorders. The present incidence of dominant traits has been estimated as 1% of live births, but recent data from British Columbia indicate the true value in that population may be an order of magnitude lower. These estimates are important if one measures the damage in terms of doubling dose. Neither the average mutation rate nor the number of loci capable of mutating to dominant detrimental form is known. Mutations that cause sterility or early embryonic loss are determental in the Darwinian sense but have little impact on society. Mutations that are more fit biologically may be a heavy burden to society if the affected persons require medical or institutional care. Since exposure to some mutagens is unavoidable, these factors must ultimately be included in a cost-benefit analysis.

Somatic cell mutation detection and quantification of X-ray-induced mutation in cultured, diploid human fibroblasts

We describe a system for detecting somatic cell mutation to 8-azaguanine (8AG) resistance in cultured, diploid human fibroblasts. Hypoxanthine-guanine phosphoribosyltransferase (HG-PRT)-deficient, AG-resistant fibroblasts from boys with the X-chromosomal, Lesch-Nyhan (L-N) mutation served as one type of prototype mutant cells. Both spontaneous and X-ray-induced mutation were studied. Recovery of L-N cells was a function both of density of normal cells and of the AG concentration used for selection. Optimum recovery was achieved at an initial inoculum of 2·10 4 normal cells per 60 mm diameter culture dish and an AG concentration of 8·10 −6 M. Efficiency of recovery was between 39 and 90% and controls to determine this efficiency were included in mutagenesis experiments. Attempts to free normal cell populations of pre-existing AG-resistant mutant cells by pregrowth in HAT medium failed because, unlike L-N mutants, most spontaneous AG-resistant mutants can grow in HAT medium. Although pre-existing mutants probably caused overestimation, the average spontaneous mutation rate derived from our experiments was 4.5·10 −6 per cell generation. Eliminating one large-yieldv experiment reduced this estimate to 1.9·10 −6. Clonal survival of cultured human fibroblasts as a function of X-ray dose was studied. X-Irradiation increased the mutation rate above spontaneous background. Minimum estimates of the increases were 1.13·10 −9 per R per cell at 75 R, 7.49·10 −8 per R per cell at 125 R, 6.87·10 −8 per R per cell at 150 R and 2.16·10 −7 per R per cell at 250 R. The total mutagenic effect and the induced mutation rate appeared to be dose-dependent. Normal parental cell strains and their derived AG-resistant mutants had similar X-ray sensitivities indicating that X-rays induced mutations rather than selected for pre-existing mutants. Because of the realism of the cultured diploid, human fibroblast model vis-a-vis in vivohuman cellular events, the mutation detection system described herein is proposed as being potentially useful for environmental monitoring.

A comparison of mutation induction by 14 MeV fast neutrons and 137Cs γ-rays in meiotic spermatocytes in the silkworm

This paper presents the results of experiments on the relative biological effectiveness (RBE) of 14 MeV fast neutrons as compared with 137Cs γ-rays for recessive visible mutation induction on the primary meiotic spermatocytes in the silkworm. 1-day-old fifth instar silkworm larvae were treated with five different doses (167, 250, 500, 684 and 1000 R) of γ-rays, or three different doses (160, 250 and 540 rad) of 14 MeV neutrons. Mutations at the pe and re loci were detected by the egg-colour specific locus method. Primary meiotic spermatocytes showed an extremely high radiosensitivity of cell killing which has not been previously found in postmeiotic cells. With 1000 R of γ-rays and 540 rad of neutrons, more than 90% of the cells in meiotic divisions were killed. However, these effects were negligible with lower doses either for neutrons (160 and 250 rad) or γ-rays (167, 250 and 500 R). Hence the results presented are from the remaining low doses for both radiations. The average mutation rate obtained on the two loci was 0.9·10 −6 per rad for γ-rays and 1.9·10 −6 per rad for neutrons. The mutation rate for γ-rays was somewhat lower than that of spermatids which have the highest rate throughout spermatogenesis. The RBE of 14 MeV neutrons was determined only for whole-body mutants because the frequency of fractional-body mutants was nearly the same as the control. The initial dose mutation frequency for both loci and both radiations were almost linear at lower doses, though at higher doses they tended to reach a maximum followed by a decline in mutation frequency. The RBE was calculated on the assumption that the dose-response curve was linear in lower dose regions. Thus, RBE values, as the ratio of the slope of the weighted linear regression line for neutrons to γ-rays, were estimated to be 1.7 at the pe locus and 2.2 at the re locus. These values are about one-third of those for mature sperm and are much the same as those in primordial germ cells and in spermatogonia.

A study of spontaneous mutation rates at ten loci detectable by starch gel electrophoresis in Drosophila melanogaster.

Spontaneous mutation rates at ten allozyme loci on chromosomes II and III of Drosophila melanogaster were studied. Over the three and a half years study, one alpha-GPD mutation and two different IDH mutations were obtained. The alpha-GPD mutation was inherited in the Mendelian fashion, as expected. The two IDH mutations were peculiar in that the band of new types appeared only in females. In males, only the original bands were stained, and the positions where mutant alleles' bands should be present were blank. Both IDH mutant homozygotes appeared as null allele homozygotes, while in females clear-cut single bands were present.-The rates of spontaneous mutation varied greatly. Eight loci studied (MDH, ADH, EST-6, APH, EST-C, ODH, XDH, AO) did not give any germ-line mutation. The average germ-line mutation rate over all ten loci was estimated at 4.5 x 10(-6). This rate is considerably smaller than that for sex-linked recessive visible mutations (Muller, Valencia and Valencia 1950), but it is somewhat less than autosomal recessive visible mutations (Glass and Ritterhoff 1956).

Extrachromosomal element delta in Drosophila melanogaster: VI. Induction of recurrent lethal mutations in definite regions of second chromosomes

The extrachromosomal element denoted by delta in Drosophila melanogaster was found to induce frequent recessive lethal mutations on the second chromosome. The average mutation rate of lethals induced was 1.47% in the presence of delta b, being about 3–4 times the spontaneous mutation rate (0.41%). A number of instances of mutation clustering were observed. Hence, delta b appears to induce the mutation at the premeiotic cell stage. A total of 113 lethal genes originating independently were found to locate at 27 different sites on the chromosome. The locations of these sites were determined and it was found that the distribution of lethals along the chromosome was markedly nonrandom; lethals were strongly concentrated upon units 0–10 (13.3%), 55–65 (30.1%) and 70–85 (50.4%) of the genetic map, and there was a notable absence of lethals in the 35 map-units immediately to the left of the contromere.

Probability distribution of drug-resistant mutants in unselected populations of Mycobacterium tuberculosis.

The fluctuation test shows that Mycobacterium tuberculosis mutates to resistance to isoniazid, streptomycin, ethambutol and rifampin spontaneously and at random. The average mutation rates for the drugs, in the same order, were calculated to be 2.56 x 10(-8), 2.95 x 10(-8), 10(-7), and 2.25 x 10(-10) mutation per bacterium per generation. The relatively high mutation rate to ethambutol resistance and the low mutation rate to rifampin resistance were confirmed by analyzing the increase in the proportion of mutants with time in a growing population of the tubercle bacilli. The highest proportions of mutants to be expected in unselected populations of the tubercle bacilli were calculated from the results of fluctuation tests.