I−R hybrid dysgenesis in Drosophila melanogaster: nature and site specificity of induced recessive lethals

This paper presents results of the genetic and cytological analysis of 144 sex-linked recessive lethals, plus 1 non-lethal. All of them were induced by I−R hybrid dysgenesis. This collection of mutants was pooled from experiments involving inducer chromosomes that differ in the chrosomal position of their I elements. Our results show that 30% of the recessive lethals are associated with chromosomal rearrangements which depend on the strength of the I−R interaction. These lethals are induced on both inducer- and reactive-origin chromosomes, and their frequency is dependent on the structure of the inducer chromosome used. The I−R-induced lethals occur along the entire length of the X chromosome. These sites probably correspond to specific loci which are more or less homologous with I. The complementation relationshups showed that some specific loci were more frequently involved in all the lethal mutations tested. The most sensitive loci are, in order of observation: l(1)J1, ct, f, ma¹ and m. Among induced recessive lethals considered to be point mutation, complementation tests showed that many of them are in fact multilocius deficiencies which can be detected only at the molecular level.

It seems that the production of I−R rearrangements (cytologically visible or not) may be the most important mechanism leading to lethal mutations. These mutations probably occur during the transposition of I elements, hence their importance from an evolutionary standpoint.     

The molecular basis of I-R hybrid dysgenesis in Drosophila melanogaster: identification, cloning, and properties of the I factor

We have analyzed two mutations of the white-eye gene, which arose in flies subject to I-R hybrid dysgenesis. These mutations are associated with insertions of apparently identical 5.4 kb sequences, which we have cloned. We believe that these insertions are copies of the I factor controlling I-R hybrid dysgenesis. The I factor is not a member of the copia-like or fold-back classes of transposable elements and has no sequence homology with the P factor that controls P-M dysgenesis. All strains of D. melanogaster contain I-factor sequences. Those present in reactive strains must represent inactive I elements. I elements have a remarkably similar sequence organization in all reactive strains and are located in peri-centromeric regions. Inducer strains appear to contain both I elements, located in peri-centromeric regions, and 10-15 copies of the complete I factor at sites on the chromosome arms.     

Genetic determinants of glutamine synthetase inDrosophila melanogaster: A gene for glutamine synthetase I resides in the 21B3-6 region

Recombinational and deletion mapping of electrophoretic variants of the glutamine synthetase I isozyme (GSI) in Drosophila melanogaster locates the gene in the 21B region on the second chromosome. We have conducted a genetic analysis of the region extending cytologically from 21A to 21B4-6. Recessive lethal mutations were generated by ethyl methanesulfonate (EMS) and ethyl nitrosourea (ENU) mutagenesis and by hybrid dysgenesis (HD). These lethals fall into seven functional groups, which were partially ordered by complementation with cytologically defined deficiencies of this region generated by hybrid dysgenesis. Two of the EMS- and two of the ENU-induced lethals fulfill biochemical criteria expected for null alleles of the GSI gene.     

Autosomal recessive lethal mutations in two mutator stocks of Drosophila ananassae.

The frequency of recessive lethals in the 2nd chromosome was examined in two mutator stocks of Drosophila ananassae, ca and ca; px. They are characterized respectively by possessing an extrachromosomal clastogenic mutator in males, and by the retrotransposon "tom", which induces Om mutability only in females. The frequencies of recessive lethal mutations in the 2nd chromosome among progenies from males and females of the ca; px stock are 0.35 and 0.34 percent, respectively. Similarity of these frequencies indicates that tom does not induce recessive lethals in females. In contrast to the ca; px stock, the frequency of recessive lethals in males of the ca mutator stock was estimated to be 1.54 percent for the 2nd chromosome. No visible mutants except Minutes were recovered. Some recessive lethals derived from ca stock males were associated with chromosomal rearrangements. Being consistent with its high rate of Minute mutation it was demonstrated that the ca clastogenic mutator also induced recessive lethals.     

Characterization of 5' truncated transposed copies of the I factor in Drosophila melanogaster.

I factors in Drosophila melanogaster are transposable elements structurally related to Mammalian LINEs. Their transposition is activated at high frequencies during I-R hybrid dysgenesis and is associated with the production of mutations of various sorts. Very few of these mutations have been studied at the molecular level; those reported so far result either from chromosomal rearrangements or from insertions of complete I factors. We have analysed three I-R induced yellow mutations and have found that one of them is due to the insertion of an I element very similar to the complete I factor, whereas the other two are due to insertions of I elements that are truncated at their 5' ends; one of them exhibits an unusual 3' end. We discuss possible mechanisms of production of such modified I elements.     

Structure and genomic organization of I elements involved in I-R hybrid dysgenesis in Drosophila melanogaster.

I-R hybrid dysgenesis in D. melanogaster is controlled by transposable elements known as I factors which terminate at their 3' ends by an A-rich sequence. Inducer strains contain active I factors. Both reactive and inducer stocks possess defective I elements. We have cloned various I elements from both categories of strains. The I elements having recently transposed in inducer strains have a structure closely related to that of active I factors. However we have isolated one such I element that is truncated at its 5' end. The I elements common to reactive and inducer strains are affected by various rearrangements and many point mutations. They do not appear to be simple derivatives of complete I factors.     

Mutagenesis in CAENORHABDITIS ELEGANS. II. a Spectrum of Mutational Events Induced with 1500 R of γ-RADIATION

We previously established a gamma-ray dose-response curve for recessive lethal events (lethals) captured over the eT1 balancer. In this paper we analyze the nature of lethal events produced, with a frequency of 0.04 per eT1 region, at a dose of 1500 r. To do so, we developed a protocol that, in the absence of cytogenetics, allows balanced lethals to be analyzed for associated chromosomal rearrangements. A set of 35 lethal strains was chosen for the analysis. Although the dosage was relatively low, a large number of multiple-break events were observed. The fraction of lethals associated with rearrangements was found to be 0.76. Currently most X- and gamma-ray dosages used for mutagenesis in C. elegans are 6000-8000 r. From our data we conservatively estimated that 43% of rearrangements induced with 8000 r would be accompanied by additional chromosome breaks in the genome. With 1500 r the value was 5%. The 35 lethals studied were derived from 875 screened F1's. Among these lethals there were (1) at least two unc-36 duplications, (2) at least four translocations, (3) at least six deficiencies of chromosome V (these delete about 90% of the unc-60 to unc-42 region) and (4) several unanalyzed rearrangements. Thus, it is possible to recover desired rearrangements at reasonable rates with a dose of only 1500 r. We suggest that the levels of ionizing radiation employed in most published C. elegans studies are excessive and efforts should be made to use reduced levels in the future.     

Chromosomal Effects on Mutability in the P-M System of Hybrid Dysgenesis in DROSOPHILA MELANOGASTER

Two manifestations of hybrid dysgenesis were studied in flies with chromosomes derived from two different P strains. In one set of experiments, the occurrence of recessive X-linked lethal mutations in the germ cells of dysgenic males was monitored. In the other, the behavior of an X-linked P-element insertion mutation, snw, was studied in dysgenic males and also in dysgenic females. The chromosomes of one P strain were more proficient at causing dysgenesis in both sets of experiments. However, there was variation among the chromosomes of each strain in regard to the ability to induce lethals or to destabilize snw. The X chromosome, especially when it came from the stronger P strain, had a pronounced effect on both measures of dysgenesis, but in combination with the major autosomes, these effects were reduced. For the stronger P strain, the autosomes by themselves contributed significantly to the production of X-linked lethals and also had large effects on the behavior of snw, but they did not act additively on these two characters. For this strain, the effects of the autosomes on the X-linked lethal mutation rate suggest that only 1/100 P element transpositions causes a recessive lethal mutation. For the weaker P strain, the autosomes had only slight effects on the behavior of snw and appeared to have negligible effects on the X-linked lethal mutation rate. Combinations of chromosomes from either the strong or the weak P strain affected both aspects of dysgenesis in a nonadditive fashion, suggesting that the P elements on these chromosomes competed with each other for transposase, the P-encoded function that triggers P element activity. Age and sex also influenced the ability of chromosomes and combinations of chromosomes to cause dysgenesis.     

Site-Specific Instability in DROSOPHILA MELANOGASTER: The Origin of the Mutation and Cytogenetic Evidence for Site Specificity

During a study of delayed mutations, an unstable X chromosome (Uc) was detected. Spontaneous X-linked recessive lethal mutations were detected in 34 of 993 sperm sampled from 50 males carrying this chromosome. All but three of the 34 lethals originated as clusters in three of the 50 males Cytogenetic and complementation analyses revealed 14 intrachromosomal rearrangements: ten inversions, two reverse repeats, one deficiency and one transposition. Eight of the 14 rearrangements have one break in the 6F1-2 doublet and two rearrangements have a break in 6F1-5 of the X chromosome. The remaining four rearrangements have in addition to the aberrations a lethal point mutation between 6F1 and 6F5. Though each of the lethal lines was established from a single lethal-bearing female, chromosome polymorphism is evident in 17 of the 18 lines having rearrangements, with certain aberrations recurring in several lines. The lethal mutations revert frequently to the nonlethal state, and cytological evidence indicates that more than one mutational event may occur at the unstable locus of the chromosome during one generation. Two lethal lines had more than one type of chromosome rearrangement sharing a common breakpoint. These observations are consistent with the view that the instability of the Uc lines is caused by a transposable element capable of site-specific chromosome breaks and perpetual generation of mutations. The mutagenic and genetic properties of transposable elements can be related to the two-mutation theory of KNUDSON (1971) for cancer initiation.     

I element distribution in mitotic heterochromatin of Drosophila melanogaster reactive strains: identification of a specific site which is correlated with the reactivity levels

The I factor is a Drosophila melanogaster LINE-like element that efficiently transposes in the genetic system of I-R hybrid dysgenesis. It has been suggested that some of the I-related sequences located in the heterochromatin of D. melanogaster are involved in the regulation of I factor activity. In this work we have performed fluorescent in situ hybridization (FISH) mapping of I element sequences in mitotic heterochromatin of nine differentially reactive D. melanogaster strains. The results of our analysis showed that a single hybridization site mapping to region h28 of the distal heterochromatin of the X chromosome is present in three strains with low or intermediate levels of reactivity, while it is undetectable in six highly reactive strains. Together, these observations suggest a negative correlation between I sequences located at h28 and the level of reactivity. To this regard, it is intriguing that flamenco and COM, two loci that regulate the activity of D. melanogaster endogenous retroviruses also map to the distal heterochromatin of the X chromosome. Our data represent the first experimental evidence in favour of a silencing effect exerted by naturally occurring I element sequences located in pericentromeric heterochromatin.     

Analysis of Dysgenesis-Induced Lethal Mutations on the X Chromosome of a Q Strain of DROSOPHILA MELANOGASTER

The Q strain known as v6 was tested for its ability to induce X-linked lethal mutations in male and female hybrids from crosses with M strains in the P-M system of hybrid dysgenesis. All measurements of the mutation rate were made on the X chromosome derived from the v6 strain. The lethal rate for young hybrid males from the cross M female X v6 male was 1.11% per chromosome. For older males, it was only 0.44%, suggesting that there is less mutational or more repair activity in the germ cells of the older males or that mutant cells are selectively eliminated as the hybrid males age. The lethal rate for hybrid females from comparable crosses was approximately the same for both ages that were tested. However, it was substantially less than the rate for the hybrid males--only 0.26% per chromosome. Genetically identical hybrid females from reciprocal crosses also showed a low mutation rate, 0.13% per chromosome. Again, there was no difference between young and old flies. Mapping experiments established that most of the lethal mutations that were recovered from the male and female hybrids were located in two regions on the X chromosome, one between bands 14B13 and 15A9 , the other between bands 19A1 and 20A , which encompasses the maroonlike locus. More refined mapping of the lethals in the maroonlike region demonstrated that the vast majority of these affected a single gene located in band 19C4 . Cytological analysis of the lethal chromosomes revealed that several carried rearrangements, including inversions, duplications and deficiencies. Chromosome breakage occurred primarily in bands 14D1 -3 and 18F- 20A , and most of the breaks in the latter segment were located in 19C . However, rearrangements involving 19C and mutations of the gene in 19C4 were mutually exclusive events. In situ hybridization of a P element probe to the chromosomes of v6 demonstrated that P elements reside at a minimum of five sites on the X chromosome. These P element sites correspond to the mutational and breakage hot spots on that chromosome. The combined genetic and cytological data imply that most of the X-linked lethal mutations that occur in M X v6 hybrids are due to local P element action. Consideration of these and other data suggest that v6 is a weak P strain in the P-M system of hybrid dysgenesis and that other Q strains might also be regarded in this way.     

The relationship between radiation-induced and transposon-induced genetic damage during Drosophila oogenesis

The combined effect of X-irradiation and transposon mobility on the frequencies of X-linked recessive lethals and dominant lethals was investigated in female hybrids in the P-M system of hybrid dysgenesis. X-linked lethals were measured in G2 hybrid dysgenic females whose X chromosome was derived from the M X P cross. To test for additivity or synergism, the mutation rate in irradiated dysgenic females was compared to that of unirradiated females as well as to irradiated nondysgenic hybrid females derived from M X M crosses. Eggs collected for 2 days after irradiation, were represented by the more radiation-sensitive A and B oocytes (about 75%) and the least sensitive C oocytes (about 25%). The production of X-linked lethal events in X-irradiated dysgenic females was 8.1%, as compared to 4.5% in dysgenic controls and 3.4% in irradiated, nondysgenic controls, demonstrating an additive effect of radiation and dysgenesis-induced genetic damage. The effect of irradiation on sterility of dysgenic hybrid females was a negative one, resulting in 20% less sterility than expected from an additive effect. The combined effect of radiation and dysgenesis on dominant lethality tested in A, B and C oocytes of the same hybrid females was synergistic. Egg broods collected for 3.5 days after irradiation showed that significantly more damage was induced in the presence of ionizing radiation in dysgenic females than in their nondysgenic counterparts. This effect was most obvious in B and C oocytes. The synergism observed may be related to the inability of cells to repair the increased number of chromosome breaks induced both by radiation and transposon mobility.     

Some aspects of the detection of potential mutagenic agents in Drosophila.

The Drosophila system is a valuable test for detecting and characterizing mutagenic agents. Tester strains are available or can be synthesized for determining almost all types of genetical change ranging from gene mutations to chromosome rearrangements in a great variety of cell types of both sexes. Metabolic activation of all groups of indirect mutagens tested so far (aryldialkyltriazenes, cyclophosphamides, nitrosamines, azo-, hydrazo- and azoxyalkanes, aflatoxins, and polycyclic hydrocarbons; about 35 representatives in all), gives strong although indirect support for the considerable metabolizing ability of Drosophila. This capability would be expected from comprehensive biochemical data on bioactivation of foreign compounds in other insects. From a comparison of which types of genetical change are induced at high, low and threshold concentrations, it appears that lethal tests remain the most reliable method for any screening program. Mutagenic agents such as diethylnitrosamine, hycanthone and certain triazenes, which are highly efficient in the induction of recessive lethals (gene mutations and/or deficiencies), would not have been detected in Drosophila if chromosome breakage were the only indicator for mutagenic activity. Moreover, for several mono- and polyfunctional agents, the lowest dose which is still genetically active was definitely lowest for recessive lethals when compared with dominant lethals, chromosome rearrangements or loss. If a new mutagen is discovered by a screening procedure using Drosophila, an accurate picture of its ability to cause either or both gene mutations and chromosome aberrations can be drawn. Such work will be valuable in helping to clarify similar problems in mammalian systems. For instance, it was important to learn that mutagens of the nitrosamine type apparently fail to produce breakage events in Drosophila. Similarly, three cyclophosphamides appeared not to have chromosome breaking ability. However, from a more detailed study, in which a series of concentrations was used, it became obvious that a penetration effect or, more likely, a rate-limiting factor in bioactivation, was the cause of the negative results obtained with these agents.     

Metronidazole (Flagyl): lack of induction of sister-chromatid exchanges in CHO-K1 cells

328 X-linked recessive lethal mutations induced in late spermatids by hycanthone methanesulfonate were tested for coverage by duplications that comprised, in total, about 24% of the euchromatic X chromosome; 78 lethals appeared to be covered. Crossover localization tests of a random sample of 38 non-covered lethals revealed 4 chromosomes carrying a lethal within a duplicated segment. Lethals localized to a particular region were crossed to reference deficiencies and single-locus mutations, and inter se, to ascertain their genetic extent. The proportion of multi-locus deletions among these 78 covered and 4 non-covered lethals was 3/48, 1/10 and 13/24 for the distal, medial and proximal regions, respectively. A storage period of 9 days did not noticeably influence these proportions. In the sample of 38 non-covered lethals, and among 17 of the covered single-site lethals, 4 cases of strong crossover suppression were detected. Comparison of these results with data obtained with other mutagens suggests that induction of multi-locus deletions, and possibly of other types of chromosome rearrangement, could in part depend on other mechanisms than those acting in the formation of translocations and chromosome loss. For the purpose of mutagen testing, these findings imply that, in Drosophila, results in the regular genetic tests for chromosome breakage events do not always accurately predict the capacity of a mutagen to induce multi-locus deletions. This is of importance since transmissible multi-locus deletions have been considered a significant source of genetic damage in man.     

The main effect of chromosomal rearrangement is changing the action of regulatory genes

Effect of chromosomal rearrangements on the expression of mutations was studied in Drosophila melanogaster regulatory genes. These were facultative dominant lethals and recessive lethals on the X chromosome obtained by the classical Muller-5 method. Chromosomal rearrangements drastically changed the expression of regulatory gene mutations. Rearrangements either caused the lethal effect of mutations or suppressed the already present lethality. The action of rearrangements exhibited the maternal or paternal effect. Irrespective of the presence in the genome of mutations at regulatory genes, a rearrangement acted as a factor decreasing fertility of the organism. The rearrangement effect is identical to the expression of regulatory genes per se. It is concluded that the chromosomal rearrangement affects the examined regulatory genes indirectly through a change in the operation of regulatory genes located within the rearrangement. Thus, rearrangements gain great importance for the definition of the pattern of genome functional activity. Widespread distribution of rearrangements in individual genotypes and their effectivity in the process of speciation are thus explained.     

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.     

The Main Effect of Chromosomal Rearrangement Is Changing the Action of Regulatory Genes

Effect of chromosomal rearrangements on the expression of mutations was studied in Drosophila melanogaster regulatory genes. These were facultative dominant lethals and recessive lethals on the X chromosome obtained by the classical Muller-5 method. Chromosomal rearrangements drastically changed the expression of regulatory gene mutations. Rearrangements either caused the lethal effect of mutations or suppressed the already present lethality. The action of rearrangements exhibited the maternal or paternal effect. Irrespective of the presence in the genome of mutations of regulatory genes, a rearrangement acted as a factor decreasing fertility of the organism. The rearrangement effect is identical to the expression of regulatory genes per se. It is concluded that the chromosomal rearrangement affects the examined regulatory genes indirectly through a change in the operation of regulatory genes located within the rearrangement. Thus, rearrangements gain great importance for the definition of the pattern of genome functional activity. Widespread distribution of rearrangements in individual genotypes and their effectivity in the process of speciation are thus explained.     

Characteristics of inserted mutations obtained in the P-M hybrid dysgenesis system in the 9F12-10A7 region of the Drosophila melanogaster X-chromosome

19 new mutations in the 9F12-10A7 region of Drosophila melanogaster X chromosome was obtained in the system of P-M hybrid dysgenesis. They appeared to be lethals, as judged from viability of homo- or hemizygous females. In situ hybridization of P DNA with polytene chromosomes revealed P-element insertion in the 10A1-2 band in the majority of the mutants. As a result of complementation analysis, all these mutations were localized at previously known loci: l(1)BP1, l(1)BP5, l(1)BP8, l(1)BP7. No insertion mutations were found at the vermilion locus. This can imply for non-random distribution of insertion mutations in the region studied. Further comparison of these mutations with previously EMS-induced ones revealed that insertion mutations are predominantly hypomorph lethals which do not influence the viability, morphology and fertility of homozygous males and females, but drastically reduce viability of hemizygous females.     

Dose-response relationships and R.B.E. values of dominant lethals induced by X-rays and 1.5 MeV neutrons in prophase-1 oocytes and in mature sperm of the two-spotted spider mite Tetranychus urticae Koch (acari, tetranychidae)

Mature sperm and prophase-1 oocytes of Tetranychus urticae Koch were irradiated with 250-kVp X-rays or 1.5 MeV fast neutrons. The X-ray doses ranged from 0.5 to 24.0 krad, and those of the fast neutrons from 0.1 to 16.0 krad. The genetic endpoint measured was lethality, expressed in the stages from egg to adulthood in the F1 progeny. The frequency of recessive lethals in female germ cells was estimated by comparing survival of fertilized versus unfertilized F1 eggs, after irradiation with the same dosage. X-Rays induce dominant lethals in prophase-1 oocytes by the action of both single hits on single targets and multiple hits on multiple targets. 1.5-MeV neutrons induce these effects predominantly by the action of multiple tracks on multiple targets. Dominant lethals were induced in mature sperm by X-rays and by fast neutrons by the action of both single hits on single targets and multiple hits on multiple targets. Both for prophase-1 oocytes and for mature sperm the low R.B.E. value corresponded with the relatively large multiple-target component of induction of dominant lethals by fast neutrons. The nature of dominant lethality in relation to the kinetochore organization of the chromosome is discussed. A non-linear trend in the dose--effect relationship was observed for both X-rays and fast neutrons for the estimated frequency of recessive lethals induced in prophase-1 oocytes. X-Rays were more effective than neutrons in inducing recessive lethals in prophase-1 oocytes at doses lower than 3 krad.     

Polymorphism of the Mdg-1 and I mobile elements in Drosophila melanogaster

The polymorphism of the mobile elements Mdg-1 (a copia-like element) and I (an element involved in I-R hybrid dysgenesis) was analysed in a mass-mated population of Drosophila melanogaster by in situ hybridization, using biotinylated DNA probes, on polytene chromosomes. The Mdg-1 and I elements were inserted independently but were within the same bulk of DNA insertion points of the Drosophila genome, which contained on average about 30 insertion sites for each element. The X chromosome contained the lowest copy number of elements while 2R and 3R had the highest number: 3R had the highest variability. There was no correlation between the copy numbers of elements among the chromosome arms. The average expected per locus heterozygosity was equal to 0.17 for both the Mdg-1 and the I elements. Although these two elements differ in sequence, they appeared to behave similarly in the Drosophila melanogaster genome. This suggests that they may compete for target insertion sites and may be under the same control mechanisms.