Analysis of chromosome 4 inDrosophila melanogaster. I. Spontaneous and X-ray-induced lethals

An analysis of 17 spontaneous and 37 X-ray-induced lethal mutations on the fourth chromosome ofDrosophila melanogaster has revealed a minimum of 22 loci on this microchromosome capable of mutating to lethality. A few of these loci had been identified earlier by their visible alleles but 16 are new discoveries. Seven of the 22 lethal loci are situated within that proximal section of the right arm of chromosome 4 delimited by theMinute-4 deficiency.Genetic tests indicate that two translocations and five deletions are included among the lethals of X-ray origin. No chromosomal aberrations were found among the spontaneous mutants. Allelism was encountered both within and between lethals from the two groups.Three independent estimates of the total number of lethal loci to be expected on this small autosome are presented. These appraisals are based on (1) the size of theMinute-4 deficiency, (2) the number of bands in salivary chromosome 4, and (3) the frequency of recurrence among the lethals. Considering the uncertainties inherent in each determination, the three estimates (34, 35 and 38) show remarkably good agreement.This investigation was supported in part by U.S. Public Health Service Research Grant GM 11627-01, from the Division of General Medical Sciences.     

Genetic Damage at Specific Gene Loci in Drosophila with Betatron X-Rays

The mutation rate was determined for mature sperm at eight specific gene loci on the third chromosome of Drosophila melanogaster using the low ion density radiations of 22 Mev betatron X-rays. A dose of 3000 rads of betatron X-rays produced a mutation rate of 4.36 x 10^-8 per rad/locus. Among the mutations observed, 66% were recessive lethals and 34% viable when homozygous. Only one of the 24 viable mutations was associated with a chromosome aberration. Among the 47 recessive lethals, no two-break aberrations were detected in 48.9% of the lethals, deletions were associated with 42.2%, inversions with 6.7% and translocations with 2.2%.—When these genetic results are compared to those for 250 KV X-rays, the mutation rate for betatron treatments was slightly lower (.76), the recessive lethal rate among induced mutations was higher, and the chromosome aberrations among lethal mutations were slightly lower than with 250 KV X-rays. Although the two types of irradiations differ by an ion density of approximately ten, the amount and types of inheritable genetic damage induced by the two radiations in mature sperm were not significantly different.     

Analyses of genetic change-over in Drosophila populations

Genetic change-overs in Drosophila populations similar to those frequently observed in microbial ones maintained in chemostats are described. In two cases, the change-overs were revealed by an abrupt and sustained increase in the probability that two independently sampled lethal chromosomes proved to be allelic. These favored lethals (or closely linked non-lethal elements) were estimated to have fitnesses 1.30–1.35 in combination with “ordinary” lethals or 1.05–1.10 in combination with non-lethal chromosomes (fitness 1.00 is assigned to carriers of non-lethal chromosomes only). A decrease in the frequency of lethal chromosomes in a third population was adequately explained by postulating the origin of a non-lethal “favored” element whose relative fitness was 1.25, a value similar to those estimated above. Genetic change-over in populations is discussed in relation to biochemical and morphological conservatism.     

Lethal genes on the sex chromosomes concealed in a population of the mosquito Aedes aegypti L.

A population has been examined in which an overall parity between the sexes hides considerable between-family variation in sex ratio. A proportion of families show highly distorted sex ratios, with either an excess of females or an excess of males. Distorted sex ratios are invariably associated with mortality in the immature stages at a level appropriate to the action of recessive lethal genes. It has been shown that 26% of M-bearing (Y) chromosomes and at least 24% of m-bearing (X) chromosomes carry a recessive lethal gene.Two such genes have been investigated. l kills males and, in a cross between two heterozygotes, gives rise to a sex ratio close to 2:1 (excess families). k kills females and, in a cross between two heterozygotes, gives rise to a sex ratio close to 1:2 (excess families). Selection for excess or excess did not increase the level of sex ratio distortion.No crossing over occurs between k and the M/m locus whereas l shows 5–10% recombination with M/m. A test for allelism confirmed that l and k are not allelic. The penetrance of k is complete whereas l shows somewhat less than full penetrance. The penetrance of l has been improved by selection.The high frequency of lethals remained in the population during the two year period of study. There was evidence for heterosis preserving this frequency, the heterozygotes living longer and producing more progeny. However lethals were no longer to be found after four further years of laboratory culture.     

The Frequency and Allelism of Lethal Chromosomes in Isolated Desert Populations of DROSOPHILA PSEUDOOBSCURA

Second-chromosome lethals were extracted from four populations of Drosophila pseudoobscura in Southern California. Two of the populations were from desert oases and two from the classic habitat on Mt. San Jacinto, previously studied by Dobzhansky. Allelism tests were made on the lethals within and between all locations. The frequency of lethal second-chromosomes in each location was 0.18, and this was not different from the results of other workers for samples throughout the species range. Interpopulational allelism rates were about 0.005, and not different from earlier results of Dobzhansky. Intrapopulational rates in this study were, with one exception, the same as the interpopulational rates, and significantly lower than Dobzhansky found using the third chromosome. This may be due to lethals being linked with heterotic third-chromosome inversions. The allelism rate of the exceptional population (about 0.03 and equal to Dobzhansky''s intrapopulational results) may be due to heterotic lethals, or a founder effect. Two lethals were found in three populations each, possibly due to migration among these populations, which are up to 334 km apart.     

The effect of linkage on sample size determination for multiple trait selection

Sufficient sample sizes are needed in breeding programs to be confident, with a specified probability , of obtaining a specified number of plants of a desired genotype in segregating populations. We develop a method of determining the minimum sample size needed to produce, with specified probability , at least m individuals of a desired genotype. This method takes into consideration factors affecting differential selection of gametes, segregation at a single locus, and linkage among the loci of interest. We first consider the effects in the gametophyte (haploid level) of fitness and linkage on the frequencies of alleles at two linked loci, then at three or more linked loci. The probability of obtaining at least m successes, or occurrences of the desired allele, among n gametes is given by a formula based on the binomial distribution. This probability is affected by fitness and linkage through their impact on the probability that a single randomly chosen gamete is of the desired type. Using an extension of this approach, we examine the effects of the altered allelic frequencies on the likelihood of obtaining the desired genotype from a randomly chosen pair of gametes in the sporophyte (diploid level). A table and a figure show the sample size required to produce, with probability 0.95, m individuals of the desired g enotype or phenotype, as a function of m and the probability that a randomly selected individual is of the desired type.BU-1031-MC in the Technical Report Series of the Biometrics Unit, Cornell University, Ithaca, New York 14853     

HIGH INBREEDING DEPRESSION,SELECTIVE INTERFERENCE AMONG LOCI,AND THE THRESHOLD SELFING RATE FOR PURGING RECESSIVE LETHAL MUTATIONS

The evolutionary dynamics of recessive or slightly dominant lethal mutations in partially self-fertilizing plants are analyzed using two models. In the identity-equilibrium model, lethals occur at a finite number of unlinked loci among which genotype frequencies are independent in mature plants. In the Kondrashov model, lethals occur at an infinite number of unlinked loci with identity disequilibrium produced by partial selfing. If the genomic mutation rate to (nearly) recessive lethal alleles is sufficiently high, such that the mean number of lethals (or lethal equivalents) per mature plant maintained at equilibrium under complete outcrossing exceeds 10, selective interference among loci creates a sharp discontinuity in the mean number of lethals maintained as a function of the selfing rate. Virtually no purging of the lethals occurs unless the selfing rate closely approaches or exceeds a threshold selfing rate, at which there is a precipitous drop in the mean number of lethals maintained. Identity disequilibrium lowers the threshold selfing rate by increasing the ratio of variance to mean number of lethals per plant, increasing the opportunity for selection. This theory helps to explain observations on plant species that display very high inbreeding depression despite intermediate selfing rates.     

Purging deleterious mutations under self fertilization: paradoxical recovery in fitness with increasing mutation rate in Caenorhabditis elegans

Background

The accumulation of deleterious mutations can drastically reduce population mean fitness. Self-fertilization is thought to be an effective means of purging deleterious mutations. However, widespread linkage disequilibrium generated and maintained by self-fertilization is predicted to reduce the efficacy of purging when mutations are present at multiple loci.

Methodology/Principal Findings

We tested the ability of self-fertilizing populations to purge deleterious mutations at multiple loci by exposing obligately self-fertilizing populations of Caenorhabditis elegans to a range of elevated mutation rates and found that mutations accumulated, as evidenced by a reduction in mean fitness, in each population. Therefore, purging in obligate selfing populations is overwhelmed by an increase in mutation rate. Surprisingly, we also found that obligate and predominantly self-fertilizing populations exposed to very high mutation rates exhibited consistently greater fitness than those subject to lesser increases in mutation rate, which contradicts the assumption that increases in mutation rate are negatively correlated with fitness. The high levels of genetic linkage inherent in self-fertilization could drive this fitness increase.

Conclusions

Compensatory mutations can be more frequent under high mutation rates and may alleviate a portion of the fitness lost due to the accumulation of deleterious mutations through epistatic interactions with deleterious mutations. The prolonged maintenance of tightly linked compensatory and deleterious mutations facilitated by self-fertilization may be responsible for the fitness increase as linkage disequilibrium between the compensatory and deleterious mutations preserves their epistatic interaction.     

Lethals, Steriles and Deficiencies in a Region of the X Chromosome of CAENORHABDITIS ELEGANS

Twenty-one X-linked recessive lethal and sterile mutations balanced by an unlinked X-chromosome duplication have been identified following EMS treatment of the small nematode, Caenorhabditis elegans. The mutations have been assigned by complementation analysis to 14 genes, four of which have more than one mutant allele. Four mutants, all alleles, are temperature-sensitive embryonic lethals. Twelve mutants, in ten genes, are early larval lethals. Two mutants are late larval lethals, and the expression of one of these is influenced by the number of X chromosomes in the genotype. Two mutants are maternal-effect lethals; for both, oocytes made by mutant hermaphrodites are rescuable by wild-type sperm. One of the maternal-effect lethals and two larval lethals are allelic. One mutant makes defective sperm. The lethals and steriles have been mapped by recombination and by complementation testing against 19 deficiencies identified after X-ray treatment. The deficiencies divide the region, about 15% of the X-chromosome linkage map, into at least nine segments. The deficiencies have also been used to check the phenotypes of hemizygous lethal and sterile hermaphrodites.     

Developmental Genetics of the 2e-F Region of the Drosophila X Chromosome: A Region Rich in "Developmentally Important" Genes

We have analyzed the 2E1-3A1 area of the X chromosome with special attention to loci related to embryogenesis. Published maps indicate that this chromosomal segment contains ten bands. Our genetic analysis has identified 11 complementation groups: one recessive visible (prune), two female steriles and eight lethals. One of the female sterile loci is fs(1)k10 for which homozygous females produce both egg chambers and embryos with a dorsalized morphology. The second female sterile is the paternally rescuable fs(1)pecanex in which unrescued embryos have a hypertrophic nervous system. Of the eight lethal complementation groups two are recessive embryonic lethals: hemizygous giant (gt) embryos possess segmental defects, and hemizygous crooked neck (crn) embryos exhibit a twisted phenotype. Analysis of these mutations in the female germ line indicates that gt does not show a maternal effect, whereas normal activity of crn is required for germ cell viability. Analysis of the maternal effect in germ line clones of the remaining six recessive lethal complementation groups indicates that four are required for germ cell viability and one produces ambiguous results for survival of the germ cells. The remaining, l(1)pole hole, is a recessive early pupal lethal in which embryos derived from germ line clones and lacking wild-type gene activity exhibit the "torso" or "pole hole" phenotype.     

Accumulation of lethals in highly selected lines of Drosophila melanogaster

Summary Four synthetic lines of D. melanogaster selected for low sternopleural bristle number for 50 generations were screened for lethals on chromosome III when their mean score equalled 2.5. Each line originated from a cross between line M (previously selected for the same trait during 130 generations) and a different unselected cage population. Line M was already known to carry a recessive lethal on chromosome III affecting the selected trait, such that the bristle score of the lethal heterozygote was lower than that of the viable homozygote. Tests revealed 18 lethals, 15 of these present in at least two lines. Each line carried from 10 to 16 lethals. All lines carried groups of lethals present on the same chromosome, and at least six lethals in each line were included in such an association with a frequency of 0.18 or higher. It appears that the lethal affecting bristle score in line M has protected a segment of chromosome III from natural selection and that the remaining 14 lethals have accumulated later in that line.     

The mutability of the minute loci of Drosophila melanogaster with ethyl methanesulfonate.

It has been suggested that the Minute loci of Drosophila melanogaster are the redundant structural loci for the transfer RNA's [31]. To inquire whether the Minute loci differed from other loci in their genetic organization we have determined the dose response curves for the induction of Minutes and sex-linked recessive lethals with ethyl methanesulfonate (EMS). There are approx. 67.75 +/- 9.35 Minute mutants induced for every 5000 recessive lethals induced in the genome and this relationship is independent of EMS dosage. This is in good agreement with the relative numbers of Minute and lethal loci in the genome. Because the target size of the average Minute locus is the same as that of the average locus capable of mutating to a lethal, these data do not support the view that the Minute loci are special in their genetic organization. Since Minute mutants can be scored in the F1 of mutagenized flies it is suggested that the induction of Minute mutants may provide a more rapid and economical means of assessing mutagenicity than do traditional screens for the induction of recessive lethals.     

Inbreeding depression maintained by recessive lethal mutations interacting with stabilizing selection on quantitative characters in a partially self‐fertilizing population

The bimodal distribution of fitness effects of new mutations and standing genetic variation, due to early‐acting strongly deleterious recessive mutations and late‐acting mildly deleterious mutations, is analyzed using the Kondrashov model for lethals (K), with either the infinitesimal model for selfing (IMS) or the Gaussian allele model (GAM) for quantitative genetic variance under stabilizing selection. In the combined models (KIMS and KGAM) high genomic mutation rates to lethals and weak stabilizing selection on many characters create strong interactions between early and late inbreeding depression, by changing the distribution of lineages selfed consecutively for different numbers of generations. Alternative stable equilibria can exist at intermediate selfing rates for a given set of parameters. Evolution of quantitative genetic variance under multivariate stabilizing selection can strongly influence the purging of nearly recessive lethals, and sometimes vice versa. If the selfing rate at the purging threshold for quantitative genetic variance in IMS or GAM alone exceeds that for nearly recessive lethals in K alone, then in KIMS and KGAM stabilizing selection causes selective interference with purging of lethals, increasing the mean number of lethals compared to K; otherwise, stabilizing selection causes selective facilitation in purging of lethals, decreasing the mean number of lethals.     

Genetic and molecular variation in the RpII215 region of Drosophila melanogaster

Summary The RpII215 region of the X chromosome of Drosophila melanogaster was investigated to identify genetic functions and correlate these with the known molecular organization of the region. Five genetic loci were identified in a subregion that is reported to transcribe nine or more messages. One locus is nod, which causes meiotic abnormalities, and three other loci are recessive lethal mutations whose developmental lesions are unknown. The fifth and most mutable of the loci is RpII215, which encodes the 215,000 dalton subunit of RNA polymerase II. Mutant effects of RpII215 alleles include: temperature-dependent (heat and cold) survival, altered sensitivity to -amanitin, male sterility, maternal effects and epistatic enhancement of mutant effects of other loci.     

Frequency of gamma-Ray Induced Specific Locus and Recessive Lethal Mutations in Mature Germ Cells of the Zebrafish, BRACHYDANIO RERIO

Specific locus and recessive lethal mutations are induced by γ-rays with approximately first order kinetics in the zebrafish (Brachydanio rerio) with frequencies of 4 x 10^-5 r^-1 and 4 x 10^-3 r^-1, respectively. The surprisingly low ratio (100:1) of recessive lethals to specific locus mutations may be due to the induction of large deficiencies by γ-rays.     

The genetic conditions in heterozygous and homozygous populations ofDrosophila I. The fate of alien chromosomes

In experimental populations ofDrosophila melanogaster lethal chromosomes with dominant markers and inversions were introduced and the frequency changes of the markers studied during a period of several generations. The base populations of the various experiments differed from each other with respect to their degree of heterozygosity. Monochromosomal populations were isogenic for a quasinormal + chromosome, dichromosomal populations contained the genetic material of two different + chromosomes, trichromosomal of three, tetrachromosomal of four, hexachromosomal of six and polychromosomal populations of many normal chromosomes. Marker chromosomes with the dominant genesLCy, Cy, Pm orD respectively were added to the populations with an initial frequency of 16,6 per cent. The fate of the dominant markers was different in different populations. In some the marker chromosome reached equilibrium frequencies, in others they were eliminated with variable speed. As a rule the lethal marker chromosomes were accepted by monochromosomal populations; however, they were eliminated from populations with a higher degree of heterozygosity. Since in all populations one genotype, namely the homozygote for the marker chromosome, was lethal, the adaptive values c of the +/LCy, +/Cy, +/Pm or +/D heterozygotes could easily be calculated from the experimental data. This c value can be used as a measure for the combining ability of the marker chromosomes. It could be shown that c depends on the degree of heterozygosity of a population or in other words that the average degree of heterozygosity of the marker free individuals determines the selection processes. An equation can be arrived at which fits the experimental results very well if superiority of heterozygous +/+ individuals over +/+ homozygotes is assumed. From that it was concluded that heterosis is the determining variable in our experiments. An attempt was undertaken in order to decide if in our case the observed heterosis was due to dominance or to overdominance. It was postulated that in di-, tri-, tetra- or hexachromosomal populations the adaptive values of the marker free normals should progressively increase if recessive detrimental genes are the cause of heterosis but not if heterozygosity on many loci leads to overdominance. The a values of the +/+ individuals were ealeulated from the frequency changes of the marker chromosomes for each subsequent two-generation period. Unfortunately only two different dichromosomal populations were available. These showed increasing adaptive values for the normals. The tri-, tetra-, and hexachromosomal populations, however, gave different results, some with increasing, some with fluctuating adaptive values. From that it was concluded that heterosis can be due in one case to dominance and in the other to overdominance. In either case, the recessive genetic load may be rather important as a determinating factor in the dynamics of populations.Dedicated to Prof. Th. Dobzhansky on the occasion of his seventieth birthday in deepest gratitude.     

Multiple vital functions of the daughterless (da) gene in Drosophila melanogaster and factors influencing its expression

Multiple sub-vital effects, here designated as reductions in fitness S₁–S₅, for the second chromosome gene daughterless (da) of Drosophila melanogaster were described as (S₁) a recessive maternal lethality for daughters, (S₂) a reduced fertility of da/da females, (S₃) a recessive sub-vital zygotic effect, (S₄) a recessive female-specific zygotic effect and (S₅) a recessive maternal interaction effect on sons. (For S_1–3 see also: Bell, 1954; Sandler, 1972; Cline, 1976). These five distinct effects were initially quantified from estimates of viability in single generation crossing experiments. Dynamic estimates of these fitness parameters were obtained by fitting the elimination rate of da from a series of large random mating cage populations to a recurrence equation by the method of minimum chi-square. The stability of these estimates discerns those effects which are truly pleiotropic versus those due to linked genes.The dynamic estimates supported only S₁ and S₄ effects. Evidence for S₂ and S₅ was indeterminate, but the S₃ effect was rejected (P<0.01). The observed reduction in fitness, supposedly due to this recessive zygotic effect for da, was most likely the result of linked deleterious genes. These results indicate that pleiotropic vital effects observed in single generation test-cross matings may be caused by linked genes rather than the specific mutant per se. This problem is of particular importance when the mutant allele has been maintained with a balancer chromosome.Experiments on the rescue of daughters from da/da mothers with low temperatures during embryogenesis and with dechorionation of eggs were described in which the findings failed to confirm previously reported actions of the da gene. Modifying genes rather than environmental variables were cited as the probable cause for these conflicting results.Journal paper number 8000 of the Purdue University Agricultural Experiment Station. This study was supported by a research grant from the Comite Conjunto Hispano-Norteamericano para Asuntas Educativos y Culturales, Madrid.     

The action of the Notch locus in Drosophila melanogaster III. Biochemical effects of recessive visible mutations on mitochondrial enzymes

The effects of six recessive visible Notch mutations on the activities of four enzymes of the mitochondrial respiratory chain are described. Their effects in hemizygous condition in males are similar to those of the recessive lethal Notch mutations in heterozygous condition. This explains their viability. The characteristic morphological Notch expression cannot be related to the different activity patterns of four enzymes caused by the recessive visible mutations. Whereas there is a correspondence between the location of the recessive lethals and the recessive visibles in relation to their enzyme activity patterns, this is not so in relation to their morphological effects. In contrast to the enzyme activity determinations in heterozygotes for recessive lethals, the effects of recessive visibles are determined in the hemizygous condition, thereby excluding the influence of wildtype Notch alleles. Such an influence is found in heterozygotes for the exceptional fa ^no mutant, in which morphological expression is dependent on the wildtype X chromosome.     

Evolution du polymorphisme enzymatique dans des populations expérimentales de Drosophila melanogaster III. Déséquilibre de linkage entre les locus Adh et α-Gpdh

Evolution of enzyme polymorphism in experimental populations of Drosophila melanogaster III. Linkage disequilibrium between alleles of the Adh and -Gpdh loci — The evolution of gametic frequencies and linkage disequilibria between alleles of the Adh and -Gpdh-loci was followed in two experimental populations of Drosophila melanogaster maintained at different temperatures. The results observed were compared with those expected on the basis of theoretical models of gametic selection. Gametic fitness values were estimated from the analysis of the productivity of the different homozygous genotypes.Our experimental results indicate that the selection favours an association between the alleles Adh-F and -Gpdh-S, but it is impossible to generate and/or maintain a stable linkage disequilibrium between the two alleles.