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.     

Meiotic transmission of Drosophila pseudoobscura chromosomal arrangements

Drosophila pseudoobscura harbors a rich gene arrangement polymorphism on the third chromosome generated by a series of overlapping paracentric inversions. The arrangements suppress recombination in heterokaryotypic individuals, which allows for the selective maintenance of coadapted gene complexes. Previous mapping experiments used to determine the degree to which recombination is suppressed in gene arrangement heterozygotes produced non-recombinant progeny in non-Mendelian ratios. The deviations from Mendelian expectations could be the result of viability differences between wild and mutant chromosomes, meiotic drive because of achiasmate pairing of homologues in heterokaryotypic females during meiosis, or a combination of both mechanisms. The possibility that the frequencies of the chromosomal arrangements in natural populations are affected by mechanisms other than adaptive selection led us to consider these hypotheses. We performed reciprocal crosses involving both heterozygous males and females to determine if the frequency of the non-recombinant progeny deviates significantly from Mendelian expectations and if the frequencies deviate between reciprocal crosses. We failed to observe non-Mendelian ratios in multiple crosses, and the frequency of the non-recombinant classes differed in only one of five pairs of reciprocal crosses despite sufficient power to detect these differences in all crosses. Our results indicate that deviations from Mendelian expectations in recombination experiments involving the D. pseudoobscura inversion system are most likely due to fitness differences of gene arrangement karyotypes in different environments.     

Cytoplasmic Incompatibility in Drosophila Simulans: Dynamics and Parameter Estimates from Natural Populations

In Drosophila simulans, cytoplasmically transmitted Wolbachia microbes cause reduced egg hatch when infected males mate with uninfected females. A Wolbachia infection and an associated mtDNA variant have spread northward through California since 1986. PCR assays show that Wolbachia infection is prevalent throughout the continental US and Central and South America, but some lines from Florida and Ecuador that are PCR-positive for Wolbachia do not cause incompatibility. We estimate from natural populations infection frequencies and the transmission and incompatibility parameter values that affect the spread of the infection. On average, infected females from nature produce 3-4% uninfected ova. Infected females with relatively low fidelity of maternal transmission show partial incompatibility with very young infected laboratory males. Nevertheless, crosses between infected flies in nature produce egg-hatch rates indistinguishable from those produced by crosses between uninfected individuals. Incompatible crosses in nature produce hatch rates 30-70% as high as those from compatible crosses. Wild-caught infected and uninfected females are equally fecund in the laboratory. Incompatibility decreases with male age, and age-specific incompatibility levels suggest that males mating in nature may often be 2 or 3 weeks old. Our parameter estimates accurately predict the frequency of Wolbachia infection in California populations.     

Hybrid dysgenesis in Drosophila melanogaster: a new type of gonadal dysgenic sterility expressed at the pre-pupal stage

Two sublines, B-202 and B-207, which were derived from crosses between Or-R (M) females and Cy/Pm (P) males were found to cause a new type of gonadal dysgenic sterility, designated as the GD-3. GD-3 sterility showed a typical reciprocal cross effect under the P-M system. It was caused at the frequency close to 100% in dysgenic offsprings reared above 25 degrees C, of which gonads were morphologically clearly different from those of usual GD sterility, whereas there was no indication of GD-3 sterility at temperatures below 24 degrees C. Temperature sensitive period of GD-3 sterility was estimated to the prepupal stage by shift-down experiment. In the B-202 subline, the 2nd chromosomes marked with Pm alone carried GD-3 elements. Those of the B-207 subline, however, were estimated to locate both on the 2nd and 3rd chromosomes, acting synergistically with each other.     

Inviability of hybrids between D. melanogaster and D. simulans results from the absence of simulans X not the presence of simulans Y chromosome.

Interspecific crosses between D. melanogaster and D. simulans or its sibling species result in unisexual inviability of the hybrids. Mostly, crosses of D. melanogaster females x D. simulans males produce hybrid females. On the other hand, only hybrid males are viable in the reciprocal crosses. A classical question is the cause of the unisexual hybrid inviability on the chromosomal level. Is it due to the absence of a D. simulans X chromosome or is it due to the presence of a D. simulans Y chromosome? A lack of adequate chromosomal rearrangements available in D. simulans has made it difficult to answer this question. However, it has been assumed that the lethality results from the absence of the D. simulans X rather than the presence of the D. simulans Y. Recently I synthesized the first D. simulans compound-XY chromosome that consists of almost the entire X and Y chromosomes. Males carrying the compound-XY and no free Y chromosome are fertile. By utilizing the compound-XY chromosome, the viability of hybrids with various constitutions of cytoplasm and sex chromosomes has been examined. The results consistently demonstrate that the absence of a D. simulans X chromosome in hybrid genome, and not the presence of the Y chromosome, is a determinant of the hybrid inviability.     

Reproductive barriers between populations of the cereal rust mite <Emphasis Type="Italic">Abacarus hystrix</Emphasis> confirm their host specialization

Recent studies have shown that certain host populations of the cereal rust mite Abacarus hystrix are highly specialized in their host use and it is likely that reproductive isolation exists between them. Here I verified this expectation by testing for reproductive barriers between ryegrass and quackgrass populations of A. hystrix. I performed reciprocal crosses between individuals from both populations and observed results of crosses. Leaves of the three grass species, ryegrass, quackgrass and wheat, were used as mating arenas. I used two criteria to determine reproductive barriers between strains: the proportion of female progeny and viability of progeny. If studied populations of this haplodiploid species are reproductively isolated a male-biased sex ratio and/or hybrid progeny of reduced viability would be expected. I found that in the presence of quackgrass and ryegrass pre-zygotic barriers between studied populations exists. On wheat I observed asymmetry in reproductive barriers. Between females from quackgrass and males from ryegrass a pre-zygotic barrier existed (only males obtained). However, the opposite reciprocal cross (females from ryegrass and males from quackgrass) produced progeny of both sexes. A male-biased sex ratio and low adult emergence suggests that post-zygotic mechanisms acted here. Low viability of progeny obtained from crosses in which females from ryegrass were engaged suggests that the origin of the female nymph acted as a predictor of hybrid inviability. The pattern of sterility observed in the cereal rust mite indicates that in the presence of three hosts (ryegrass, quackgrass and wheat) pre- and post-zygotic reproductive barriers between quackgrass and ryegrass populations of this mite exist. In addition to host fidelity (which acts as pre-zygotic barrier) there are post-zygotic barriers to gene flow.     

Influence of the MR (mutator) factor on X-ray-induced genetic damage

The genetical effects induced by MR, in the progeny of outcrossed MR-males, include very high frequencies of visible and lethal mutations and chromosome aberrations. The hypothesis is that MR causes breaks at specific sites in the DNA where, subsequently, insertion sequences become integrated. To examine whether there exists an interaction between breaks and radiation induced lesions, MRh12/Cy males were crossed to Berlin K females and the male progeny from this cross carrying the MR or Cy chromosome were irradiated. The frequencies of X-linked recessive lethals and II-III translocations were determined. Non-irradiated MR and non-MR (Cy) male progeny were used in concurrent controls. The results show that the frequencies of II-III translocations in the MR-containing males is not significantly higher than in the controls. However, with regard to the production of recessive lethal mutations a clear synergism between MRh12 and X-irradiation was observed.     

Meiotic drive of t haplotypes: chromosome segregation in mice with tertiary trisomy

The properties of the t haplotypes, specific mutant states of the proximal region of chromosomes 17 in the house mouse, are of continuing interest. One such property is increased transmission of the t haplotype by heterozygous t/+ males to offspring. Using the reciprocal translocation T(16;17)43H we have constructed males with tertiary trisomy of chromosome 17 (+T43/+ +/Rb7+) carrying the Robertsonian translocation Rb(16.17)7Bnr. Only the progeny of these males which had inherited either T43/+ or Rb7 from their male parent were viable. The segregation patterns in the offspring of t-bearing trisomics were analysed on days 16-18 of embryonic development. It was found that, when the t12 haplotype is in the normal acrocentric (males+ +T43/+ t12 + /Rb7+ +), its presence in the gamete +t12+/+ + T43 does not produce meiotic drive. However, when t6 is in Rb7, meiotic drive was observed: 80% of offspring carried the t haplotype. It is concluded that the meiotic drive is probably inhibited by the presence of a normal homologue of chromosome 17 in the same sperm. Possible mechanisms for the t haplotype effect are discussed.     

Estimation of Fitness Components in DROSOPHILA MELANOGASTER . I. Heterozygote Viability Indices

We examine the assumption of "dominance" with regard to viability of the Cy and Pm marker chromosomes in D. melanogaster . This assumption is often invoked for the extraction of wild-type second chromosomes from natural populations and for the calculation of relative viability indices. Significant genotypic variances for viability are found among both Cy/+_j and Pm/+_i heterozygotes in California and Japanese populations. The magnitude of the Pm/+ _i genotypic variance is substantially less than that of the Cy/+_j heterozygotes (less than one half). Significant reciprocal effects are also found to influence Cy/+_j, Pm/+_i and +_i/+_j viabilities. We conclude that viability indices of heterozygotes based on the Curly method are biased. We suggest that viability indices in the future be expressed relative to the viability of the Cy/Pm genotype (Curly-Plum method) or possibly that of the Pm/+_i genotype (Plum method).     

Selective elimination of chromosomally unbalanced zygotes at the two-cell stage in the Chinese hamster

The gametic and zygotic selection of genome imbalance was investigated in the Chinese hamster by direct chromosome analyses of spermatocytes and preimplantation embryos from crosses between chromosomally normal females and males heterozygous for a reciprocal translocation, T(2;10)3Idr, abbreviated here as T3. The karyotypes and the frequencies of embryos observed at the first cleavage in the cross +/+female X T3/+male were consistent with those expected from MII scoring in male T3 heterozygotes. Therefore, it was concluded that there was neither gametic selection against genome imbalance nor zygotic selection from fertilization until the first cleavage metaphase. However, 9.1-10.8% of embryos were arrested at the two-cell stage, and karyotypes of these embryos were confirmed as 22(2,10,10,10(2)), 21(2,10,10), and 21(2,10,10(2)). The common abnormality of these embryos was partial monosomy of chromosome 2. Among day 4 embryos, some chromosomally unbalanced embryos, mainly with a deficiency of other segments of chromosomes 2 and 10, had fewer blastomeres than chromosomally balanced embryos. This finding suggests that cleavage of these embryos had been retarded by day 4 of gestation.     

Extrachromosomal Element Delta in DROSOPHILA MELANOGASTER. VII. Relation to Fertility in a Second-Chromosome Line

The ID(b)-45 chromosome line usually carries an appreciable amount of delta b, but it is not susceptible to the killing action of this delta. The fertility of this line was examined when it carried various amounts of delta b. More than one third of the Cy/ID(b)-45 males and females tested became sterile when they carried cytoplasm of the Cy/Pm stock which is assumed to carry no delta b. The number of progeny is smaller when flies of this line are raised at 25 degrees C than at 28 degrees C at which temperature the multiplication of delta is accelerated. The progeny number was appreciably reduced when the flies were raised at 18 degrees C at which temperature the multiplication of delta is suppressed. This line could not be maintained at that temperature, since both males and females became sterile. Thus, the conclusion may be drawn that the presence of an appreciable amount of delta b is necessary for the gametogenesis of the Cy/ID(b)-45 flies.     

Genetics of speciation in the Aedes (Stegomyia) scutellaris subgroup (Diptera:Culicidae). 4. Chromosomal relationships of Aedes cooki with four sibling species

Morphology and behavior of chromosomes and development of testes and sperm were examined in hybrids from interspecific crosses involving Aedes cooki and four sibling species of the Aedes (Stegomyia) scutellaris subgroup of mosquitoes. The degree of abnormality in hybrid spermatogenesis in interspecific crosses involving Aedes cooki males and females of four sibling species paralleled the geographic distributions of these species and the genetic divergence indicated by other genetic studies. Hybrids from crosses involving Aedes malayensis females and Aedes cooki males were characterized by atrophied testes and extensive chromosome breakage. Hybrids from crosses involving Aedes alcasidi females and Aedes cooki males suggested a possible pericentric inversion distinguishing the largest autosome of Aedes alcasidi from that of Aedes cooki. Hybrids from interspecific crosses involving females of Aedes polynesiensis and Aedes pseudoscutellaris and males of Aedes cooki showed high percentages of univalents of the smallest chromosome pair. Hybrid spermatogenesis in two interspecific crosses involving Aedes cooki females differed from results of reciprocal crosses. Data were scant, however, and interpretation was difficult in view of negligible hatch in all interspecific crosses involving Aedes cooki females.     

Strong biases in the transmission of sex chromosomes in the aphid Rhopalosiphum padi

The typical life cycle of aphids involves several parthenogenetic generations followed by a single sexual one in autumn, i.e. cyclical parthenogenesis. Sexual females are genetically identical to their parthenogenetic mothers and carry two sex chromosomes (XX). Male production involves the elimination of one sex chromosome (to produce X0) that could give rise to genetic conflicts between X-chromosomes. In addition, deleterious recessive mutations could accumulate on sex chromosomes during the parthenogenetic phase and affect males differentially depending on the X-chromosome they inherit. Genetic conflicts and deleterious mutations thus may induce transmission bias that could be exaggerated in males. Here, the transmission of X-chromosomes has been studied in the laboratory in two cyclically parthenogenetic lineages of the bird cherry-oat aphid Rhopalosiphum padi . X-chromosome transmission was followed, using X-linked microsatellite loci, at male production in the two lineages and in their hybrids deriving from reciprocal crosses. Genetic analyses revealed non-Mendelian inheritance of X-chromosomes in both parental and hybrid lineages at different steps of male function. Putative mechanisms and evolutionary consequences of non-Mendelian transmission of X-chromosomes to males are discussed.     

Meiotic drive of t-haplotypes: segregation of chromosomes in mice with partial trisomy

The properties of the t haplotypes, specific mutant states of the proximal region of chromosome 17 in the house mouse keep renewing interest. One such property is increased transmission of the t haplotype from heterozygous t/+ males to their offspring. By means of reciprocal translocation T (16; 17)43H, we have constructed males with tertiary trisomy 17 (+T43/++/RB7+) carrying Robertsonian translocation Rb(16.17)7Bnr. The offspring of these males was viable when sperm of +T43/++ and Rb7+ was used. The segregation patterns in the offspring of t-bearing trisomics were analysed on days 16-18 of embryonic development. It was found that in the case when the t haplotype is on the normal acrocentric (male male ++T43/+t12+/Rb7++), its presence in the gamete +t12+/++T43 does not produce meiotic drive. However, when t6 is on Rb7, meiotic drive was equal to 80%. It is concluded that the presence of a normal homolog and a t-bearing chromosome in sperm does not result in meiotic drive. Possible mechanisms of meiotic drive of the t haplotypes are discussed.     

Multigeneration exposure test of Drosophila melanogaster to ELF magnetic fields

Mutations, other than dominant lethals, were accumulated on wild type second chromosomes (+) of Drosophila melanogaster during exposure to 50 Hz sinusoidal alternating magnetic fields of 0.5 or 5 mT (rms) for 40 generations by the Curly/Plum(Cy/Pm) accumulation method. We maintained, for 40 generations under continuous exposure, each (+) chromosome as a heterozygote with (Cy) chromosome. Viability of the (+) chromosome was tested by sib-mating of (Cy/+) male and (Cy/+) female in a culture every 10th generation to obtain the homozygote. Viability indices, defined as twice the ratio of number of (+/+) flies to that of (Cy/+) flies plus 1 in the progeny of the test mating, also were calculated, which equaled 1.00 at the starting point. For the control and 0.5 and 5 mT exposed groups, percent frequencies of recessive lethal lines, defined as a line with (+/+) flies less than 0.3% in the test mating, were, respectively, 1.9, 0.9, and 2.9% (10th), 9.0, 4.9, and 9.5% (20th), 30.3, 22.9, and 30.4% (30th), and 39.9, 32.4, and 43.3% (40th generation). For the control and 0.5 and 5 mT groups, average viability indices, excluding lethals and markedly deleterious, were, respectively, 0.778, 0.796, and 0.752 (20th), 0.704, 0.698, and 0.694 (30th), and 0.669, 0.678, and 0.595 (40th generation). Their decreasing rates were 0.0054, 0.0059, and 0.0078 per generation. No significant difference was detected among the exposure levels in either the recessive lethal mutation frequency or the viability index. Bioelectromagnetics 19:335–340, 1998. © 1998 Wiley-Liss, Inc.     

Unstable B chromosomes in Silene maritima With. (Caryophyllaceae)

Unstable B chromosomes in Silene maritima With. (Carophyllaceae). B chromosomes have been found in 7 out of 39 populations of Silene maritima With. (Carophyllaceae) studied. These 7 populations are all from the Norfolk coast. The B chromosome is small, 0.8 μm long, telocentric and appears euchromatic. Within plants the B chromosome is highly unstable with different numbers in cells of the same root or pollen mother cells (pmcs) of an anther. The transmission of the Bs is efficient and in controlled crosses progeny plants usually have higher mean B frequencies than the B-containing parents. At metaphase I the pairing behaviour of the Bs is variable and they often appear as univalents. In general the presence of Bs in pmcs results in an increase in chiasma frequency/cell and populations with Bs have higher mean chiasma frequencies than those without Bs. Cloned plants grown under stress conditions induced by the addition of NaCl to a nutrient solution showed no difference in mean B number/cell or distribution when compared with controls grown in nutrient solution only.     

Fitness Effects of Ems-Induced Mutations on the X Chromosome of DROSOPHILA MELANOGASTER. II. Hemizygous Fitness Effects

X chromosomes mutagenized with EMS were tested for their effects on the fitness of hemizygous carriers. The tests were carried out in populations in which treated and untreated X chromosomes segregated from matings between males and attached-X females; the populations were maintained for several generations, during which time changes in the frequencies of the treated and untreated chromosomes were observed. From the rates at which the frequencies changed, the fitness effects of the treated chromosomes were determined. It was found that flies hemizygous for a mutagenized chromosome were 1.7% less fit per mM EMS treatment than those hemizygous for an untreated chromosome. Since the same flies were only 0.5% per mM less viable than their untreated counterparts, the total fitness effect of an X chromosome carrying EMS-induced mutants is three to four times greater than its viability effect. By comparing the heterozygous effect of a mutagenized X chromosome on fitness with the corresponding hemizygous effect, the dominance value for the chromosome is estimated to be about 0.25.     

Inviability of hybrids between D. melanogaster and D. simulans results from the absence of simulans X not the presence of simulans Y chromosome

Interspecific crosses between D. melanogaster and D. simulans or its sibling species result in unisexual inviability of the hybrids. Mostly, crosses of D. melanogaster females X D. simulans males produce hybrid females. On the other hand, only hybrid males are viable in the reciprocal crosses. A classical question is the cause of the unisexual hybrid inviability on the chromosomal level. Is it due to the absence of a D. simulans X chromosome or is it due to the presence of a D. simulans Y chromosome? A lack of adequate chromosomal rearrangements available in D. simulans has made it difficult to answer this question. However, it has been assumed that the lethality results from the absence of the D. simulans X rather than the presence of the D. simulans Y. Recently I synthesized the first D. simulans compound-XY chromosome that consists of almost the entire X and Y chromosomes. Males carrying the compound-XY and no free Y chromosome are fertile. By utilizing the compound-XY chromosome, the viability of hybrids with various constitutions of cytoplasm and sex chromosomes has been examined. The results consistently demonstrate that the absence of a D. simulans X chromosome in hybrid genome, and not the presence of the Y chromosome, is a determinant of the hybrid inviability.     

Genetic mapping of three alleles at the Pm3 locus conferring powdery mildew resistance in common wheat (Triticum aestivum L.).

A set of differential isolates of Blumeria graminis f.sp. tritici was used to identify 10 alleles at the Pm3 locus on the short arm of chromosome 1A. Three F3 populations were used to map Pm3h in Abessi, Pm3i in line N324, and Pm3j alleles in GUS 122 relative to microsatellite markers. In total, 13 marker loci were mapped on chromosome 1AS and 1 marker on 1AL. The order of marker loci in the 3 mapping populations is consistent with previously published maps. All 3 alleles were mapped in the distal region of chromosome 1AS. The present study indicated that microsatellite markers are an ideal marker system for comparative mapping of alleles at the same gene locus in different mapping populations. The linkage distances of the closest microsatellite marker, Xgwm905-1A, to Pm3h, Pm3i, and Pm3j were 3.7 cM, 7.2 cM, and 1.2 cM, respectively. The microsatellite marker Xgwm905-1A cannot be used to distinguish between Pm3 alleles. The development of specific markers for individual Pm3 alleles is discussed on the basis of the recently cloned Pm3b allele.