High recombinagenic activities of three antiviral agents, adenine derivatives, in the Drosophila wing spot test

Three adenine derivatives (R,S)-9-(2,3-dihydroxypropyl)adenin (DHPA), D-eritadine (EA), and 9-(2-phosphonylmethoxyethyl)adenine (PMEA), prospective antiviral drugs, were subjected to genotoxicity analysis using the somatic mutation and recombinatino test in Drosophila melanogaster. All three compounds were found to be very potent inducers of mosaic spots on Drosophila wings in a dose-related fashion. Data obtained in inversion-free flies revealed that the compounds, in particualr DHPA and EA (nucleoside analogues), are highly effective in the induction of mitotic recombination. PMEA, a nucleotide, exhibited a rather different genotoxic profile from those of DHPA and EA, indicating a different mechanism of genetic action of this compound. Of somatic mutations, chromosome aberrations, rather than point mutations seem to play a major role in the genotoxicity of PMEA. In flies carrying an inversion chromosome, which eliminates most products of mitotic recombination, reduced spot frequencies were obtained, which, however, were still unexpectedly high for compounds with strong recombinagenic activities. Most probably, in additino to structural mutations of chromosomes, double mitotic crossing-over and non-reciprocal recombinatino events similar to unequal sister-strand recombination of gene conversion significantly contributed to spot induction in the inversion heterozygous flies. Concerning the mechanism of genotoxic action, we suggest that these adenine derivatives can be incorporated into DNa chains during replication. This would result, via breaks and DNa repair mechanisms, either in various recombination events or in chromosome aberrations.     

Investigation of the potential for mitotic recombination in the mouse

A variation of the mouse spot test is described that is designed to distinguish between spots of altered coat colour that arise by reciprocal mitotic recombination and those caused by somatic mutation or non-disjunction. Mouse fetuses that were heterozygous for two, linked coat colour genes were irradiated (1.5 Gy X-rays) in utero at 10.25-10.50 days post coitum (p.c.) or left untreated. Subsequently, the coats were classified for the presence of spots of altered colour. The irradiated embryos were heterozygous for the linked genes pink-eyed dilution (p) and albino (c) and were produced by both the repulsion and coupling crosses. Half of the reciprocal recombination events between the centromere and the proximal marker (p), in heterozygotes with p and c in repulsion, should produce twin spots. No such twin spots would be expected from a similar event in the coupling heterozygotes. The coats of 238 irradiated and 208 untreated repulsion heterozygotes plus 107 irradiated and 314 untreated coupling heterozygotes were classified for spots. One irradiated, repulsion heterozygote had a diffuse twin spot that was only recognisable by microscopic examination of the hairs. We conclude that if the treatment described induces mitotic recombination in the mouse, it does so with low efficiency.     

Resistance and mitotic instability to chloroneb and 1,4-oxathiin in Aspergillus nidulans.

Mutants resistant to two fungicides, chloroneb (1,4-dichloro-2,5-dimethoxybenzene) and vitavax (2,3-dihydro-5-carboxanilido-6-methyl-1,4-oxathiin) were spontaneously obtained from a strain of Aspergillus nidulans with frequencies of 12.5 and 1.1 respectively, in 10(8) conidia. One chloroneb-resistant mutant (Chl 1) segregated as a single gene and was mapped in linkage group IV. It also caused a partial dependence of the strain on the fungicide and was semi-dominant. The mutant resistant to vitavax (Vit 1) also segregated as a single gene and was dominant. Both fungicides altered the instability of diploid and duplication strains. Chloroneb mainly increased haploidization, and vitavax reduced the mitotic recombination in diploids. Chloroneb increased the instability of duplication strains, and vitavax reduced such instability. The possible mode of action of such fungicides affecting stability is discussed.     

Segregation of recessive phenotypes in somatic cell hybrids: role of mitotic recombination, gene inactivation, and chromosome nondisjunction.

Somatic cell hybrids heterozygous at the emetine resistance locus (emtr/emt+) or the chromate resistance locus (chrr/chr+) are known to segregate the recessive drug resistance phenotype at high frequency. We have examined mechanisms of segregation in Chinese hamster cell hybrids heterozygous at these two loci, both of which map to the long arm of Chinese hamster chromosome 2. To follow the fate of chromosomal arms through the segregation process, our hybrids were also heterozygous at the mtx (methotrexate resistance) locus on the short arm of chromosome 2 and carried cytogenetically marked chromosomes with either a short-arm deletion (2p-) or a long-arm addition (2q+). Karyotype and phenotype analysis of emetine- or chromate-resistant segregants from such hybrids allowed us to distinguish four potential segregation mechanisms: (i) loss of the emt+- or chr+-bearing chromosome; (ii) mitotic recombination between the centromere and the emt or chr loci, giving rise to homozygous resistant segregants; (iii) inactivation of the emt+ or chr+ alleles; and (iv) loss of the emt+- or chr+-bearing chromosome with duplication of the homologous chromosome carrying the emtr or chrr allele. Of 48 independent segregants examined, only 9 (20%) arose by simple chromosome loss. Two segregants (4%) were consistent with a gene inactivation mechanism, but because of their rarity, other mechanisms such as mutation or submicroscopic deletion could not be excluded. Twenty-one segregants (44%) arose by either mitotic recombination or chromosome loss and duplication; the two mechanisms were not distinguishable in that experiment. Finally, in hybrids allowing these two mechanisms to be distinguished, 15 segregants (31%) arose by chromosome loss and duplication, and none arose by mitotic recombination.     

Recombination Suppression by Heterozygous Robertsonian Chromosomes in the Mouse

Robertsonian chromosomes are metacentric chromosomes formed by the joining of two telocentric chromosomes at their centromere ends. Many Robertsonian chromosomes of the mouse suppress genetic recombination near the centromere when heterozygous. We have analyzed genetic recombination and meiotic pairing in mice heterozygous for Robertsonian chromosomes and genetic markers to determine (1) the reason for this recombination suppression and (2) whether there are any consistent rules to predict which Robertsonian chromosomes will suppress recombination. Meiotic pairing was analyzed using synaptonemal complex preparations. Our data provide evidence that the underlying mechanism of recombination suppression is mechanical interference in meiotic pairing between Robertsonian chromosomes and their telocentric partners. The fact that recombination suppression is specific to individual Robertsonian chromosomes suggests that the pairing delay is caused by minor structural differences between the Robertsonian chromosomes and their telocentric homologs and that these differences arise during Robertsonian formation. Further understanding of this pairing delay is important for mouse mapping studies. In 10 mouse chromosomes (3, 4, 5, 6, 8, 9, 10, 11, 15 and 19) the distances from the centromeres to first markers may still be underestimated because they have been determined using only Robertsonian chromosomes. Our control linkage studies using C-band (heterochromatin) markers for the centromeric region provide improved estimates for the centromere-to-first-locus distance in mouse chromosomes 1, 2 and 16.     

Cure of ADPKD by selection for spontaneous genetic repair events in Pkd1-mutated iPS cells

Induced pluripotent stem cells (iPSCs) generated by epigenetic reprogramming of personal somatic cells have limited therapeutic capacity for patients suffering from genetic disorders. Here we demonstrate restoration of a genomic mutation heterozygous for Pkd1 (polycystic kidney disease 1) deletion (Pkd1(+/-) to Pkd1(+/R+)) by spontaneous mitotic recombination. Notably, recombination between homologous chromosomes occurred at a frequency of 1~2 per 10,000 iPSCs. Southern blot hybridization and genomic PCR analyses demonstrated that the genotype of the mutation-restored iPSCs was indistinguishable from that of the wild-type cells. Importantly, the frequency of cyst generation in kidneys of adult chimeric mice containing Pkd1(+/R+) iPSCs was significantly lower than that of adult chimeric mice with parental Pkd1(+/-) iPSCs, and indistinguishable from that of wild-type mice. This repair step could be directly incorporated into iPSC development programmes prior to cell transplantation, offering an invaluable step forward for patients carrying a wide range of genetic disorders.     

Mitotic recombination and genetic changes in Saccharomyces cerevisiae during wine fermentation

Natural strains of Saccharomyces cerevisiae are prototrophic homothallic yeasts that sporulate poorly, are often heterozygous, and may be aneuploid. This genomic constitution may confer selective advantages in some environments. Different mechanisms of recombination, such as meiosis or mitotic rearrangement of chromosomes, have been proposed for wine strains. We studied the stability of the URA3 locus of a URA3/ura3 wine yeast in consecutive grape must fermentations. ura3/ura3 homozygotes were detected at a rate of 1 x 10(-5) to 3 x 10(-5) per generation, and mitotic rearrangements for chromosomes VIII and XII appeared after 30 mitotic divisions. We used the karyotype as a meiotic marker and determined that sporulation was not involved in this process. Thus, we propose a hypothesis for the genome changes in wine yeasts during vinification. This putative mechanism involves mitotic recombination between homologous sequences and does not necessarily imply meiosis.     

Recombination in Drosophila Melanogaster Male

T-007 strain of Drosophila melanogaster is known to show recombination in males. The present study established the following points: (1) Clustering occurrence of recombinant, unequal recovery of complementary products of recombination, relatively high frequency of recombination around centromeric region, and relatively frequent occurrence of mosaic phenontype flies-all of these seem to indicate that a considerable fraction of male recombination in the T-007 strain is of premeiotic, or somatic origin, although a fraction still could be of meiotic origin; (2) Male recombination occurs in the third as well as in the second chromosomes, and the frequencies of recombinations are comparable between these two chromosome pairs.     

Genotoxicity testing of five herbicides in the Drosophila wing spot test

Four triazine herbicides: amitrole, metribuzin, prometryn and terbutryn, and the bipyridal compound diquat dibromide have been evaluated for genotoxicity in the wing somatic mutation and recombination test of Drosophila melanogaster, following standard procedures. Third-instar larvae trans-heterozygous for the third chromosome recessive markers multiple wing hairs (mwh) and flare-3 (flr(3)) were chronically fed with different concentrations of the test compounds. Feeding ended with pupation of the surviving larvae. Genetic changes induced in somatic cells of the wing's imaginal discs lead to the formation of mutant clones on the wing blade. Point mutation, chromosome breakage and mitotic recombination produce single spots; while twin spots are produced only by mitotic recombination. Exposure to 0.5 mM and 1 mM of amitrole clearly increased the frequency of small single, large single and total spots. Terbutryn, at the concentration of 5 mM, induced a slight increase in the frequency of small single and total spots, but this result could be false positive. The other three herbicides tested did not show any genotoxic effect. When heterozygous larvae for mwh and the multiple inverted TM3 balancer chromosomes were treated, significant increases in the frequency of mutant spots were only detected for amitrole. The observed spot frequencies were lower than those found in mwh/flr(3)50%) of the total spot induction was due to mitotic recombination.     

Genetic mechanisms of tumor-specific loss of 11p DNA sequences in Wilms tumor.

Wilms tumor, a common childhood renal tumor, occurs in both a heritable and a nonheritable form. The heritable form may occasionally be attributed to a chromosome deletion at 11p13, and tumors from patients with normal constitutional chromosomes often show deletion or rearrangement of 11p13. It has been suggested that a germinal or somatic mutation may occur on one chromosome 11 and predispose to Wilms tumor and that a subsequent somatic genetic event on the normal homologue at 11p13 may permit tumor development. To study the frequency and mechanism of such tumor-specific genetic events, we have examined the karyotype and chromosome 11 genotype of normal and tumor tissues from 13 childhood renal tumor patients with different histologic tumor types and associated clinical conditions. Tumors of eight of the 12 Wilms tumor patients, including all viable tumors examined directly, show molecular evidence of loss of 11p DNA sequences by somatic recombination (four cases), chromosome loss (two cases), and recombination (two cases) or chromosome loss and duplication. One malignant rhabdoid tumor in a patient heterozygous for multiple 11p markers did not show any tumor-specific 11p alteration. These findings confirm the critical role of 11p sequences in Wilms tumor development and reveal that mitotic recombination may be the most frequent mechanism by which tumors develop.     

Genomic structure of and genome-wide recombination in the Saccharomyces cerevisiae S288C progenitor isolate EM93

The diploid isolate EM93 is the main ancestor to the widely used Saccharomyces cerevisiae haploid laboratory strain, S288C. In this study, we generate a high-resolution overview of the genetic differences between EM93 and S288C. We show that EM93 is heterozygous for >45,000 polymorphisms, including large sequence polymorphisms, such as deletions and a Saccharomyces paradoxus introgression. We also find that many large sequence polymorphisms (LSPs) are associated with Ty-elements and sub-telomeric regions. We identified 2,965 genetic markers, which we then used to genotype 120 EM93 tetrads. In addition to deducing the structures of all EM93 chromosomes, we estimate that the average EM93 meiosis produces 144 detectable recombination events, consisting of 87 crossover and 31 non-crossover gene conversion events. Of the 50 polymorphisms showing the highest levels of non-crossover gene conversions, only three deviated from parity, all of which were near heterozygous LSPs. We find that non-telomeric heterozygous LSPs significantly reduce meiotic recombination in adjacent intervals, while sub-telomeric LSPs have no discernable effect on recombination. We identified 203 recombination hotspots, relatively few of which are hot for both non-crossover gene conversions and crossovers. Strikingly, we find that recombination hotspots show limited conservation. Some novel hotspots are found adjacent to heterozygous LSPs that eliminate other hotspots, suggesting that hotspots may appear and disappear relatively rapidly.     

Non-Mendelian Female Sterility in DROSOPHILA MELANOGASTER: Characterization of the Noninducer Chromosomes of Inducer Strains

In relation to non-Mendelian female sterility, Drosophila melanogaster strains can be divided into two main classes, inducer and reactive. The genetic element responsible for the inducer condition (I factor) is chromosomal and may be linked to any inducer-strain chromosome. Each chromosome carrying the I factor (i(+) chromosome) can, when introduced by the paternal gamete into a reactive oocyte, give rise to females (denoted SF) showing more-or-less reduced fertility. As long as i(+) chromosomes are transmitted through heterozygous males with reactive originating chromosomes (r chromosomes), I factor follows Mendelian segregation patterns. In contrast, in heterozygous i(+)/r females, a varying proportion of r chromosomes may irreversibly acquire I factor, independently of classical genetic recombination, by a process called chromosomal contamination. The contaminated reactive chromosomes behave as i(+) chromosomes.-In the present paper, evidence is given that the Luminy inducer strain displays a polymorphism for two kinds of second chromosomes. Some of them are i(+), while others, denoted i(o), are unable to induce any SF sterility when introduced by paternal gametes into reactive oocytes. They are also unable to induce contamination of r chromosomes, but, like r chromosomes, they may be contaminated by i(+) chromosomes in SF or RSF females. The study of the segregation of i(+) and i(o) second chromosomes in the progeny of heterozygous Luminy males and females leads to the conclusion that on chromosome 2 of the Luminy stock the I factor is at a single locus. -X, second and third i(o) chromosomes have been found in several inducer strains. Since these chromosomes can be maintained with i(+) chromosomes in inducer strains in spite of their ability to be contaminated in RSF females, it can be concluded that chromosomal contamination does not take place in females of inducer strains. This implies that contamination occurs only in cells having cytoplasm in a reactive state.     

Unequal genetic exchange in Escherichia coli tandem duplications may represent a special pathway of homologous recombination

Heterozygous tandem duplications that appear in Escherichia coli conjugation matings segregate different types of haploid and diploid recombinants because of unequal crossing over between sister chromosomes. As shown previously, the frequency of segregants in the extended duplication D104 (approximately 150 kb or more than 3 min of the genetic map) heterozygous for E. coli deo-operon genes (deoA deoB::Tn5/deoC deoD) is not decreased in strains with defective RecBCD and RecF recombination pathways. Analysis of a shorter duplication of this type (approximately 46 kb) showed that the frequency of segregants in the strain recBC sbcBC recF was similar to that in a strain with undamaged system of recombination. Thus, genetic exchange between direct DNA repeats in tandem duplications may follow a special pathway of homologous recombination, which is independent of the recBC and recF genes.     

Allelic and Ectopic Interactions in Recombination-Defective Yeast Strains

Ectopic recombination in the yeast Saccharomyces cerevisiae has been investigated by examining the effects of mutations known to alter allelic recombination frequencies. A haploid yeast strain disomic for chromosome III was constructed in which allelic recombination can be monitored using leu2 heteroalleles on chromosome III and ectopic recombination can be monitored using ura3 heteroalleles on chromosomes V and II. This strain contains the spo13-1 mutation which permits haploid strains to successfully complete meiosis and which rescues many recombination-defective mutants from the associated meiotic lethality. Mutations in the genes RAD50, SPO11 and HOP1 were introduced individually into this disomic strain using transformation procedures. Mitotic and meiotic comparisons of each mutant strain with the wild-type parental strain has shown that the mutation in question has comparable effects on ectopic and allelic recombination. Similar results have been obtained using diploid strains constructed by mating MATa and MAT alpha haploid derivatives of each of the disomic strains. These data demonstrate that ectopic and allelic recombination are affected by the same gene products and suggest that the two types of recombination are mechanistically similar. In addition, the comparison of disomic and diploid strains indicates that the presence of a chromosome pairing partner during meiosis does not affect the frequency of ectopic recombination events involving nonhomologous chromosomes.     

Genotoxic evaluation of selective serotonin-reuptake inhibitors by use of the somatic mutation and recombination test in Drosophila melanogaster

This study evaluated different concentrations of selective serotonin-reuptake inhibitors (citalopram and sertraline) for genotoxicity by use of the somatic mutation and recombination test (SMART) in Drosophila melanogaster. Three-day-old larvae, trans-heterozygous for the multiple wing hairs (mwh) and flare (flr3) genes were treated with these two compounds. Two recessive markers were located on the left arm of chromosome 3, i.e. 'multiple wing hairs' (mwh) in map position 0.3 and 'flare-3' (flr3) at 38.8, while the centromere was located in position 47.7. SMART is based on the loss of heterozygosity, which may occur through various mechanisms, such as mitotic recombination, mutation, deletion, half-translocation, chromosome loss, and non-disjunction. Genetic changes occurring in somatic cells of the wing's imaginal discs, cause the formation of mutant clones on the wing blade. The results of this study show that citalopram had a genotoxic effect in the Drosophila SMART. Sertraline, however, did not show any genotoxic effect in balancer heterozygous wings. This study concluded that more information is needed to be certain regarding the mutagenic effects of sertraline.     

Cytology, RAPD, and seed colour of progeny plants from Brassica rapa-alboglabra aneuploids and development of monosomic addition lines.

Progeny plants from Brassica rapa-alboglabra aneuploids were characterized genetically by scoring random amplified polymorphic DNA (RAPD) markers and seed colour and cytologically as to chromosome number and pairing. Sets of RAPD markers specific for each of the encountered eight alien Brassica alboglabra chromosomes were defined. The finding of subsets of markers associated with the presence or absence of alien chromosomes inferred the frequent occurrence of intergenomic genetic recombination and introgression. The chromosome numbers were in the range 2n = 20-28, with a maximum of seven alien B. alboglabra chromosomes and one trisomic B. rapa chromosome. Five types of monosomic addition lines were obtained, two of which have not been developed before. Differences in chromatin condensation patterns made it possible to differentiate between the B. rapa and B. alboglabra chromosomes at diakinesis, and to detect intergenomic homoeological pairing. In addition to the frequent formation of trivalents by homoeologous pairing of an alien B. alboglabra chromosome and a background B. rapa pair, occasional heteromorphic intergenomic bivalents and B. rapa univalents were encountered. Homoeological intergenomic pairing occurred between chromosomes with similar centromeric and karyotypic positions. Plants with structurally changed alien chromosomes were found. The RAPD and cytological data substantiated each other. Observations of the colour of sown and harvested seeds indicated that B. alboglabra chromosome 4 carries a gene for brown seed colour. It exerts its control embryonically, and thus it differs from chromosome 1 which controls seed colour maternally.     

Analysis of male recombination in third chromosomes of Drosophila melanogaster

Data on male recombination in twenty third-chromosomal lines of Drosophila melanogaster are presented. Frequencies of female and male recombination have been calculated in seven intervals along the third chromosome. The influence on male recombination (M.R.) exercised by different factors such as population origin (cellar, vineyard), the presence of heterozygous inversions and recessive lethal chromosomes, is analyzed. The results obtained lead to the main conclusion that M.R. is not affected by the presence of heterozygous inversions which reduce female recombination in the same lines. In the light of this effect, the possible mechanism operating on male recombination is discussed. Lethal chromosomes reduce significantly the number of male recombination events as compared with wild chromosomes. We have not obtained significant differences in male recombination frequencies between the cellar and the vineyard lines.     

Requirements for ectopic homologous recombination in mammalian somatic cells.

Ectopic recombination occurs between DNA sequences that are not in equivalent positions on homologous chromosomes and has beneficial as well as potentially deleterious consequences for the eukaryotic genome. In the present study, we have examined ectopic recombination in mammalian somatic (murine hybridoma) cells in which a deletion in the mu gene constant (Cmu) region of the endogenous chromosomal immunoglobulin mu gene is corrected by using as a donor an ectopic wild-type Cmu region. Ectopic recombination restores normal immunoglobulin M production in hybridomas. We show that (i) chromosomal mu gene deletions of 600 bp and 4 kb are corrected less efficiently than a deletion of only 2 bp, (ii) the minimum amount of homology required to mediate ectopic recombination is between 1.9 and 4.3 kb, (iii) the frequency of ectopic recombination does not depend on donor copy number, and (iv) the frequency of ectopic recombination in hybridoma lines in which the donor and recipient Cmu regions are physically connected to each other on the same chromosome can be as much as 4 orders of magnitude higher than it is for the same sequences located on homologous or nonhomologous chromosomes. The results are discussed in terms of a model for ectopic recombination in mammalian somatic cells in which the scanning mechanism that is used to locate a homologous partner operates preferentially in cis.     

Extensive Recombination of a Yeast Diploid Hybrid through Meiotic Reversion

In somatic cells, recombination between the homologous chromosomes followed by equational segregation leads to loss of heterozygosity events (LOH), allowing the expression of recessive alleles and the production of novel allele combinations that are potentially beneficial upon Darwinian selection. However, inter-homolog recombination in somatic cells is rare, thus reducing potential genetic variation. Here, we explored the property of S. cerevisiae to enter the meiotic developmental program, induce meiotic Spo11-dependent double-strand breaks genome-wide and return to mitotic growth, a process known as Return To Growth (RTG). Whole genome sequencing of 36 RTG strains derived from the hybrid S288c/SK1 diploid strain demonstrates that the RTGs are bona fide diploids with mosaic recombined genome, derived from either parental origin. Individual RTG genome-wide genotypes are comprised of 5 to 87 homozygous regions due to the loss of heterozygous (LOH) events of various lengths, varying between a few nucleotides up to several hundred kilobases. Furthermore, we show that reiteration of the RTG process shows incremental increases of homozygosity. Phenotype/genotype analysis of the RTG strains for the auxotrophic and arsenate resistance traits validates the potential of this procedure of genome diversification to rapidly map complex traits loci (QTLs) in diploid strains without undergoing sexual reproduction.     

Variation of spontaneous and induced mitotic recombination in different Drosophila populations: a pilot study on the effects of polyaromatic hydrocarbons in six newly constructed tester strains

A set of six Drosophila strains was developed, by inducing by chemical treatment with N-ethyl-N-nitrosourea (ENU) new white and, in some strains, yellow mutations in 3 wild-type (WT) and 3 insecticide-resistant (IR) populations. These strains were previously shown to vary with regard to contents and inducibility of microsomal oxidative enzymes (Zijlstra et al., 1984). In this pilot study results from a first evaluation of these strains in somatic mutation experiments are reported, using as genotoxins an aromatic amine (2-naphthylamine, 2-NA), one substituted (9,10-dimethylanthracene, DA) and one non-substituted (benzo[a]pyrene, BP) polycyclic aromatic hydrocarbon. Developing larvae heterozygous for white were chronically exposed to three different exposure doses of each carcinogen. Adult females were inspected for the occurrence of mosaic light clones in their eyes, using the somatic mutation and recombination test (SMART).

Evidence is presented indicating strong genotype-dependent variation in both spontaneous and chemically induced mutational and recombinational events in somatic cells of Drosophila. The spontaneous frequencies varied from 3.5% (Hikone-R), 4.3% (Berlin-K), 6.3% (Oregon-K), 9.1% (91-C), 20.5% (Haag-79) to 49.1% (91-R), corresponding to a 14-fold difference in spot frequencies between the two extremes. BP, DA and 2-NA were readily detectable in both Hikone-R (IR) and Oregon-K (WT), less so in 91-C (WT) and Haag-79 (IR), whereas the performance of strain Berlin-K (WT) was rather poor. The special problem with strain 91-R was the high frequency with which mosaic light spots occur not only in female genotypes heterozygous for white, but also in homozygous condition in the original stock.

The up to 20-fold variation in induced spot frequencies between different genotypes poses questions for further investigations with respect to the genetic constitution of the various strains and the role of enzyme induction on somatic cell mutagenicity, which in this system is predominantly the result of mitotic recombination.