Studies on the Sex-Specific Lethals of DROSOPHILA MELANOGASTER. V. Sex Transformation Caused by Interactions between a Female-Specific Lethal, Sxlf#1, and the Male-Specific Lethals mle(3)132, msl-227, and mle

Interactions between a female-specific lethal mutant, Sxl^f#1, and each of three male-specific lethal mutants, mle(3)132, msl-2²⁷ and mle, of Drosophila melanogaster were observed to produce morphological changes in various sexually dimorphic external characters. Chromosomal females heterozygous for Sxl^f#1 and homozygous for any one of the male-specific lethals (and to a lesser degree heterozygous for male-specific lethals) sometimes had sex combs, male-type tergites, male-type sternites, male-type anal plates or male-type external genitalia. Penetrance was not high and expression was often incomplete; single individuals never had all the sexually dimorphic structures transformed. When mothers were homozygous for male-specific lethals, higher proportions of female progeny were affected than when mothers were heterozygous, suggesting a maternal effect.     

Analysis of lethals in selected lines of Drosophila melanogaster

Summary Five lines of Drosophila melanogaster that reached an extreme phenotype after long-term selection for increased dorsocentral bristle number, were analysed for the presence of lethals. Seven chromosome II and three chromosome III lethal types were detected in four of the lines, at frequencies ranging from between 6% and 36%. No lethal had any demonstrable effect over the selected trait. In one line, where almost every chromosome II was a lethal carrier, it was shown that the main lethal (at a frequency of 36%) was associated with the transmission ratio distortion in males. The processes which could lead to the accumulation of this lethal and others linked in disequilibrium to it is discussed. Some results suggest similar mechanisms for the accumulation of lethals in the other lines. These findings show that causes other than the direct effect of artificial selection must be taken into account when trying to explain the accumulation of lethals in selected lines.     

Heterozygous effects on viability of Drosophila supergenes that are lethal when homozygous

Eight fourth chromosomes which were homozygous lethal and 170 which were homozygous nonlethal were extracted from the same Drosophila melanogaster cage. The lethals were complementary, i.e., they were viable in all 28 nonreflexive pairwise combinations. Three different lethals produced sterile homozygotes; these are called leaky lethals. Different lethal heterozygotes' viabilities were compared by means of paired-t tests. The difference in mean relative viabilities between a pair of genotypes containing different lethals but exactly the same nonlethal was treated as one observation. The mean difference for any pair of lethals was based on only part of the full array of nonlethals. Of 17 possible paired comparisons, nine were statistically significant. In eight out of ten possible pairs and in six out of seven significant pairs, the heterozygous viability of leaky lethals was less than that of absolute (nonleaky) lethals. There was no association between stage of homozygous lethal action and heterozygous viability effect. In general, different lethals had different heterozygous effects on viability. The results are summarized in Table 5. In memoriam David Walter Kenyon (1939–1972)Research supported by The National Science Foundation of the United States (GB-3759).     

CONTINUING STUDIES ON THE SOUTH AMHERST DROSOPHILA MELANOGASTER NATURAL POPULATION DURING THE 1970'S AND 1980'S

Continuing investigations on the South Amherst Drosophila melanogaster natural population following the significant decline and recovery of lethal (le) and semilethal (sle) frequencies in the late 1960's (Ives, 1970) show that the population has been remarkably stable although it contains MR (male recombination) and/or P DNA elements (Kidwell et al., 1977a; Green, 1980). A 13-year study affirms that the lethals present are nonrandomly distributed along the second chromosome and deficient on the right; they differ significantly in distribution from spontaneous (Ives, 1973) and δ-induced lethals (Minamori and Ito, 1971). Between 1970 and 1977, a total of 4,083 second chromosomes from the Markert subpopulation were analyzed; 28.9% of the chromosomes were lethal and 7.25% were semilethal in homozygous condition. Frequencies are similar for early summer and late fall collections although the rate of allelism among lethals is significantly higher in early summer than in late fall. For the large fall (1970–1979) Porch site population, 2,519 second chromosomes were analyzed; 29.5% were lethal and 8.0% were sublethal as homozygotes; the rate of allelism among lethals was 1.50%. At Hockanum, 1977–1983, lethal and semilethal frequencies were lower; the rate of allelism among lethals was 1.43%. The chromosome map distribution of lethals does not change between summer and late fall at Markert. The overall distributions of lethals at the Markert and Hockanum sites are similar. In tests for male recombination (MR) activity in the population over a 6-year period, a total of 0.47% recombinants were observed; these were uniformly distributed along the second chromosome. Comparisons are made with other long studied D. melanogaster populations.     

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.     

Linkage and the elimination of deleterious mutant genes from experimental populations

The behavior of a balanced lethal system and its two component loci was examined in laboratory populations of the flour beetle,Tribelium castaneum. The two lethal genes wereShort antenna (Sa), a dominant morphological mutant which is a recessive lethal when homozygous, and a lethal (l) located 1 to 2 map units fromSa in linkage group VII.Frequencies of both lethals declined rapidly when present individually in populations. The elimination curve forl closely paralleled the theoretical curve for a recessive lethal;Sa was eliminated more rapidly, indicating that heterozygotes also suffer a reduced fitness relative to wild-type homozygotes. These observations were corroborated by measurements of several components of fitness.When populations were initiated with the two lethals in repulsion phase, marked differences were observed in the rate of elimination ofSa in six replicate populations. Since the initial phase of elimination depends on the occurrence of an effective crossover between the two loci, it was possible to apply a correction factor so that the observed elimination curve closely approximated a theoretical curve generated using parameters obtained from the assays of fitness components.It is argued that many cases of apparently intermediate gene frequency equilibrium reported in studies withTribolium and also withDrosophila result from linkage disequilibrium rather than overdominance. The data presented provide a striking example of the effects of linkage on the elimination of mutant genes from experimental populations.     

The nature of reverse mutations in Drosophila melanogaster

A selection system for the screening of reversions has been constructed and used to test reversions of lethals located in the proximal region of the X chromosome of Drosophila and of Kpn mutations.Spontaneous and induced reversions have been screened, X-rays and ethyl methanesulphonate (EMS) being the mutagens used in the induction experiments.No genuine back-mutation was found in 6·10⁵ gametes scored. Sterile reversions of all four lethals tested were obtained. Their frequency suggested that at least in three of the lethals the sterile reversions represented “escapers” of the lethal effect rather than true revertants.Three fertile reversions of l^x4 were found and analyzed. All three were autosomal suppressors, located on the second chromosome, allelic to each other, dominant in males and recessive in females.One fertile reversion of l^3DES was found to be an X-linked suppressor. It is suggested that this suppressor is a Y-suppressed lethal, showing a V-type position effect, resulting from an aberration included in the proximal heterochromatin of the X chromosome.Reversions of Kpn were obtained at a similar rate to that found in previous reports²².The absence of true back-mutants in our experiments, in contrast to findings in previous reports, is discussed. From the existing literature on spontaneous and induced back-mutations in Drosophila melanogaster it appears that for several mutations the rates of forward and back-mutation are of the same order of magnitude. It is suggested that reported cases of back-mutations represent mainly inter- and intrachromosomal recombination in duplicated regions rather than mutational events and that the frequency of true back-mutation in Drosophila is usually of an order of magnitude, similar to that known for microorganisms and fungi.     

An Increase in the X-Linked Lethal Mutation Rate Associated with an Unstable Locus in DROSOPHILA MELANOGASTER

The behavior of an unstable allele of the singed-bristle locus on the X chromosome was studied in connection with the occurrence of lethal mutations on that same chromosome. The unstable allele, weak singed (sn^w), is under the control of the P-M system of hybrid dysgenesis and, in the M cytotype, mutates secondarily to extreme singed (sn^e) and to wild type (sn⁺) at high rates. Chromosomes whose sn^w allele had mutated in this fashion sustained lethal mutations at a rate of 3%; whereas, those whose sn^w allele had apparently remained unchanged, acquired lethals at a lower rate, 1.3%. The significant difference between these values indicates a statistical coincidence between the phenomena of sn^w instability and X-linked lethal mutation induction. This coincidence can be explained by postulating that mutations at the singed locus sometimes release a genetic element capable of reinserting elsewhere in the chromosome. Alternately, sn^w instability and lethal induction might be associated because they are the effects of a common cause, perhaps some mutation-inducing substance present in various amounts in the germ cells of dysgenic flies.—The lethals that occurred on chromosomes whose sn^w allele had mutated to sn^e mapped preferentially close to singed. The lethals on the sn^w and sn⁺ chromosomes did not show this concentration on the map. Cytological analysis of samples of all three types of lethal chromosomes indicated that, with one exception, there was no detectable breakage at the singed locus itself. The single instance of breakage at singed was not associated with any change in the singed phenotype. Thus, the instability of sn^w apparently does not involve detectable breakage of the singed locus, or if it does, this breakage is not a common event.     

Genetic instability at the cut locus of Drosophila melanogaster induced by the MR-h12 chromosome

Summary Unstable mutations were generated at the cut locus by the MR-h12 factor which induces male recombination. The unstable allele ct ^MR2, containing the MR-transposon in the cut locus is a very powerful mutator producing a number of different viable and lethal mutations both in the cut locus and outside it.I describe several types of mutations: stable reversion to wild type, which were sometimes associated with the appearance of unstable mutations in other loci; of stable deficiencies at the cut locus (lethals); new unstable mutations at different loci with the ct ^MR2 allele conserved; new unstable cut alleles with a phenotype other than that of ct ^MR2. The possible mechanisms of these mutational events are discussed. The genetic system constructed in the present work affords an opportunity for molecular studies of the cut locus and the MR-transposon, as a sequence from the cut locus has recently been cloned (Tchurikov et al. 1981).     

INTERSPECIFIC INCOMPATIBILITY IN GOSSYPIUM. II. LIGHT AND ELECTRON MICROSCOPE STUDIES OF CELL NECROSIS AND TUMORIGENESIS IN HYBRIDS OF G. KLOTZSCHIANUM

Many Gossypium interspecific hybrids that involve G. klotzschianum result in either embryo or seedling lethals. Histologically, lethal symptoms are characterized by necrotic cells and tumors that appear 10–15 days after fertilization in the embryo lethals and at the first true-leaf stage in the seedling lethals. Ultrastructural studies of cell necrosis in seedling lethals show that the first subcellular abnormality is the degeneration of the inner membrane and cristae of the mitochondria. Mitochondrial degeneration is essentially complete before other organelles show evidence of structural aberrations.     

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 Analysis of Recessive Lethal Mutations Induced by the Influenza Virus in the X Chromosome of Drosophila melanogaster

Mutagenic potential of the influenza virus was evaluated. Based on its capacity of inducing recessive lethal mutations in the X chromosome of Drosophila melanogaster, the influenza virus can be classified as a moderate-activity mutagen. Its mutagenicity does not depend on ability to reproduce in the cell system. This virus was shown to disrupt formation of the wing, particularly wing vein M_{1 + 2}. Cytogenetic examination of polytene X chromosomes bearing recessive lethal mutations in Drosophilasalivary glands did not reveal chromosome rearrangements. These lethals are assumed to be small deletions or point mutations. The determination of the lethal activity stage of these mutations showed that they disrupt the expression of genes functioning at various developmental stages of Drosophila.Two of them were conditionally lethal (temperature-sensitive). Two of 15 mutations analyzed were mapped to region 2B9-10–3C10-11.     

Effects of a small X-ray exposure to Drosophila melanogaster females on the recovery of genetic damage from X-irradiated males.

Wild-type (Oregon-K) Drosophila melanogaster males were X-irradiated and mated to Oster females (y sc^s1 In49sc⁸; bw; st p^p) that had received a 20 R X-ray exposure (Group MF) or no irradiation (group M). Mature spermatozoa of the irradiated males were sampled and the frequencies of dominant lethals, sex-linked recessive lethals and 2–3 translocations were measured. In the group in which the irradiated males were mated to irradiated females, the survival of eggs was significantly higher than in the group in which only the males were irradiated. However, there was no consistent and detectable difference between the two groups with respect to the frequencies of recessive lethals and translocations.The relatively higher egg survival in the MF group is amenable to an interpretation based on an inducible repair process in females that acts on radiation damage induced in spermatozoa but, such an explanation is inadequate to explain the other results. It is concluded that the observations considered together preclude a general and unifying interpretation based on a low-dose-X-ray-inducible genetic repair process in females (acting on damage in spermatozoa). Possible reasons for the discrepancy between the expectation of differences in response between the MF and M groups (in sex-linked lethal and translocation frequencies) and the observation of no consistent differences between them are discussed.     

Genetic characterization of the 44D-45B region of the Drosophila melanogaster genome based on an F2 lethal screen

We have performed an F₂ genetic screen to identify lethal mutations that map to the 44D-45B region of the Drosophila melanogaster genome. By screening 8500 mutagenized chromosomes for lethality over Df(2R)Np3, a deficiency which encompasses nearly 1% of the D. melanogaster euchromatic genome, we recovered 125 lines with lethal mutations that represent 38 complementation groups. The lethal mutations have been mapped to deficiencies that span the 44D-45B region, producing an approximate map position for each complementation group. Lethal mutations were analyzed to determine the phase of development at which lethality occurred. In addition, we have linked some of the complementation groups to P element-induced lethals that map to 44D-45B, thus possibly providing new alleles of a previously tagged gene. Some of the complementation groups represent potentially novel alleles of previously identified genes that map to the region. Several genes have been mapped by molecular means to the 44D-45B region, but do not have any reported mutant alleles. This screen may have uncovered mutant alleles of these genes. The results of complementation tests with previously identified genes in 44D-45B suggests that over half of the complementation groups identified in this screen may be novel. Received: 13 July 1999 / Accepted: 4 November 1999     

Test of recessive lethals in the mouse

The presence of spontaneous lethal mutations in inbred strains is discussed with special reference to their variation and influence on estimates to induced mutations.A model is presented that will facilitate classification of lethal-free and lethal heterozygotes.The model is used in classification of sons to lethal heterozygous males carrying a spontaneous mutation.The observed results are in good agreement with the model.From experience it is concluded that the most efficient way to use the facilities in lethal tests is to examine 10 or more full brothers to the P parents. By doing so pre-existing spontaneous lethals can be excludde by eliminating families in which any of the P parents were lethal heterozygous. The observed total rate of recessive lethals gives slight over-estimation of the induced rate of mutations, as spontaneous mutations in the gametes forming the F₁ cannot be excluded.     

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.     

Mapping genes essential for embryo development in Arabidopsis thaliana.

Summary We have previously isolated and characterized over 90 recessive mutants of Arabidopsis thaliana defective in embryo development. These emb mutants have been shown to differ in lethal phase, extent of abnormal development, and response in culture. We demonstrate in this report the value and efficiency of mapping emb genes relative to visible and molecular markers. Sixteen genes essential for embryo development were mapped relative to visible markers by analyzing progeny of selfed F₁ plants. Embryonic lethals are now the most common type of visible marker included on the linkage map of Arabidopsis. Backcrosses were used in several cases to orient genes relative to adjacent markers. Three genes were located to chromosome arms with telotrisomics by screening for a reduction in the percentage of aborted seeds produced by F₁ plants. A restriction fragment length polymorphism (RFLP) mapping strategy that utilizes pooled EMB/EMB F₂ plants was devised to increase the efficiency of mapping embryonic lethals relative to molecular markers. This strategy was tested by demonstrating that the biol locus of Arabidopsis is within 0.5 cM of an existing RFLP marker. Mapping embryonic lethals with both visible and molecular markers may therefore help to identify large numbers of genes with essential functions in Arabidopsis.     

The genetic conditions in heterozygous and homozygous populations of Drosophila. II. X-ray induced lethals in a homozygous and a heterozygous population

Strains of Drosophila melanogaster were made isogenic for their second chromosomes by means of the marker strain LCy/Pm. One of these strains was used as a founder for a homozygous experimental population (W). All other strains were mixed and established a heterozygous population (LKW). Both populations were free of lethals in the beginning with respect to their second chromosomes. After they had been exposed to an X-ray irradiation of 7000 r they contained about 26 per cent newly induced lethal chromosomes. Whereas in the heterozygous population the lethal frequency decreased rather fast to 10 per cent, that of the homozygous population remained rather constant at 25 per cent during a period of 135 days. After a year of continuation, however, both populations reached the same lethal frequency of about 10 per cent. Allelism tests carried out after 10 generations revealed that there was a highly heterotic lethal factor in the homozygous population. After excluding this heterotic lethal from the calculations, the lethal frequencies of the two populations remained significantly different. It was assumed that the relative mean fitness of lethal heterozygotes was generally higher in the homo-than in the heterozygous populations. The results indicate that homozygous populations are much more capable of incorporation new mutations than heterozygous.     

Genetic analysis of viable and lethalfused mutants ofDrosophila melanogaster

Summary Fused is a segmentation gene belonging to the segment-polarity class. Mutations at thefused locus are known to display pleiotropic effects, causing zygotically determined anomalies of ovaries and of some adult cuticular structures, and maternally determined embryonic segmentation defects. In order to determine the amorphic phenotype offused and to study the genetical basis of its pleiotropy, newfused alleles (18 viable and 11 lethal) were isolated. The phenotype of these mutants and of others already known are described, taking into account zygotic and maternal effects. The main results provided by this analysis are as follows. Firstly, allfused alleles show the whole complex fused phenotype, and a good correlation is observed between the strength of the wing and segmentation defects, suggesting that a single function is involved in both processes. Secondly, all embryonic and larval lethals carry deficiencies which allow us to localizefused between the 17C4 and 17D2 bands of the X-chromosome. Thirdly, the 24 viable and 2 pupal lethals examined behave as point mutants, as shown cytologically or by Southern blot analysis. However, only one of them, the pupal lethalfu ^mH63 was proven to carry a nullfused allele, since it displays in germ-line clones a strong maternal phenotype and a very low zygotic rescue, similar to those of the small deficiencyDf(1)fu ^z4. The phenotype of the amorphic mutant indicates that zygotic ezpression offused is required for normal metamorphosis, while maternal expression is necessary for a normal segmentation pattern, since a complete loss offused expression during oogenesis cannot be compensated zygotically.     

Mutagenicity of hycanthone in Drosophila melanogaster

The schistosomicidal agent hycanthone was tested for mutagenicity in Drosophila melanogaster. The compound was administered either by injection into adult males or by larval feeding. The following types of genetic damage were measured:(1) complete and mosaic sex-linked recessive lethal mutations; (2) II–III translocations; and (3) dominant lethals.In postmeiotic germ cells, especially in late spermatids, a pronounced increase was found in the frequency of sex-linked recessive lethals, both completes and mosaics. By contrast, translocations and dominant lethals were not induced.