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Characterization of the progeny of X-ray irradiated males from two Drosophila virilis strains differing in genetic instability
Mutation Research, 282 (1992) 197-202
197
© 1992 Elsevier Science Publishers B.V. All rights reserved 0165-7992/92/$05.00
MUTLET 0679
Characterization of the progeny of X-ray irradiated males from two Drosophila virilis strains differing in genetic instability I.S. Gubenko and R.P. Subbota Department of Molecular Genetics, Institute of Molecular Biology and Genetics, Academy of Sciences of the Ukraine, 252627 Kiev, Ukraine (Received 19 August 1991) (Revision received 17 March 1992) (Accepted 27 March 1992)
Keywords: Genome instability; X-ray mutagenesis; Chromosome radiosensitivity
Summary Significant effects of X-ray treatment on the increase in the number of phenotypic variations, two visible mutations, and chromosome aberrations were found in the progeny of irradiated males from the D. virilis laboratory stock that is capable of hybrid dysgenesis syndrome induction. This effect is much more pronounced than in the progeny of irradiated males from strong wild-type strains studied. A correlation between genetic instability and chromosome radiosensitivity was outlined. The mechanism of this phenomenon and the possibilities of using the property of genome instability for the productive induction of gene and chromosome damage in radiation mutagenesis experiments are discussed.
The noticeable phenotypical variation depending on the hypermutability and instability of the mutants is considered to be one of several characteristic hybrid dysgenesis syndrome traits such as male recombination, sterility, gonadal dysgenesis, transmission ration distortion, chromosome aberrations and chromosome non-disjunction (Kidwell et al., 1977; Engels and Preston, 1979; Bregliano and Kidwell, 1983). There are some hybrid dysgenesis systems in Drosophila. In the P-M system of D. melanogaster crosses of P strain males with M strain females
Correspondence: Dr. I.S. Gubenko, Institute of Molecular Biology and Genetics, Academy of Sciences of the Ukraine, 150 Zabolotny Str., 252627 Kiev, Ukraine.
promote the mobilization of P elements in the germ lines of their progeny, producing hypermutability and other events reflecting chromosomal damage. The reciprocal cross, M males to P females, was much less effective in promoting instability. The crosses between different M strains lacking full-length P elements are practically stable (Bingham et al., 1982; Rubin et al., 1982; Kidwell, 1983, 1984; Engels, 1983; Engels and Preston, 1984; Engels et al., 1987). A second D. melanogaster hybrid dysgenesis system, the I-R system, gives rise to similar characteristics when an inducer (I) male containing I transposable genetic factors is crossed with a responsive (R) female lacking these factors (Bregliano and Kidwell, 1983). In the progeny of the crosses between defined D. virilis strains Lo-
198
zovskaya and Evgen'ev (1987) have recently found genetic phenomena similar to those displayed in the P-M and I-R syndromes of D. melanogaster but having the involved transposable elements of different origin (Scheinker et al., 1990). Some D. cirilis stocks differing widely in genetic instability were identified by the interstrain crosses. Two of these strains were used in our radiation mutagenesis experiments for cytogenetic analysis of 20CD D. virilis chromosome 2 subdivision activated by heat shock and other stress conditions (Gubenko et al., 1991). In the present paper a correlation between genetic instability and chromosome radiosensitivity of the strains studied is outlined. Materials and methods
Strains of Drosophila virilis A standard sugar-corn meal-yeast-agar medium was employed for maintenance of the stock and all the crosses. Three D. virilis stocks were used in experiments. 9 is a wild-type laboratory strain isolated from a natural Batumi population. 160 is the stock obtained by M.G. Evgen'ev from crosses between some Japanese and American strains and carrying the recessive markers b (2-188.0), gp L 2 (3-118.5), cd (4-32.5), pe (5-203.0), and gl (6-1.0) in all the autosomes. As described
earlier (Lozovskaya and Evgen'ev, 1987), some specific traits of the hybrid dysgenesis syndrome (male and female sterility, gonadal dysgenesis, increased mutation frequency and other abnormalities) were only seen with unidirectional crosses, i.e., when females of wild-type strain 9 were crossed with males of the old laboratory marked strain 160. It was concluded that D. virilis strain 9 is the analogue of D. melanogaster M-type strain; strain 160, on the contrary, can be classified as a strong P-type strain containing specific mobile Ulysses elements characteristic of the D. virilis genome (Scheinker et al., 1990). In the present study we used this classification and detailed characterization of the genetic instability levels of strains 9 and 160 without additional examination. 140 is the strain carrying the two recessive markers eb (2-83.5) and va (2-231.5) on the second linkage group. This stock was used by us as material for the selection of females in radiation experiments. As a rule we also used strain 140 for selection and reproduction of different D. virilis mutant stocks; no features of genetic instability were found in the F l progeny of direct (see Table 1B) and reciprocal (data not shown) crosses of 140 with non-irradiated 9 or 160 flies. Therefore, only the cross of 9 females with 160 males caused hybrid dysgenesis, and all the others produced
TABLE 1 C H A R A C T E R I Z A T I O N O F T H E P R O G E N Y O F X - R A Y I R R A D I A T E D (A) A N D N O N - I R R A D I A T E D (B) M A L E S IN DIFFERENT EXPERIMENTS Experiment
Strain, source of irradiated males
Cross
(A) Crosses with irradiated males 1 9 140 x 9 2 160 140 x 160 3 *a 9 140x 9 b 160 140 x 160 4 *a 9 140x 9 b 160 140 x 160
Number of flies screened
N u m b e r of F 1 offspring carrying Dl eb other phenotypic changes
Total n u m b e r of variations
13,430 9,557 8,950 9,670 26,370 10,960
2 3 1 2 1 2
4 9 7 11 4 6
4 10 18 58 10 20
10 22 26 71 15 28
43,400 30,240
0 0
0 0
1 0
1 0
(B) Crosses with non-irradiated males (control) 140 × 9 140 × 160
* Males from strain 9 or 160 were irradiated simultaneously.
199
non-dysgenic progeny. All the strains listed above were obtained from the Institute of Development Biology of the Academy of Sciences of Russia (Moscow). For other information on genetic markers see Gubenko and Evgen'ev (1984).
X-irradiation Irradiation of the males from stock 9 or 160 was carried out with a RUM-17 X-ray source. The dose used for all the experiments reported here was 6000 R. Crosses In all the experiments virgin strain 140 females were crossed to X-ray irradiated males from strain 9 or 160. Mass mating was carried out in large vials with 10-15 irradiated males and 20-30 females per vial. In F 1 progeny, eb and DI mutants were monitored by the appearance of the recessive ebony and the dominant Delta phenotype respectively. These mutants were collected for further cytological analysis. We also counted all the visible alterations or modifications of phenotypes in the progeny. Cytology Salivary glands of third-instar larvae were isolated in Hanks' medium, stained with acetoorcein, and squash preparations were made. For cytological identification of chromosome regions we used photographic maps of D. virilis salivary gland chromosomes (Gubenko and Evgen'ev, 1984).
Results and discussion
The effect of X-rays on the variability of the visible phenotypic characters of F1 (140 x 9 or 140 x 160) progeny is presented in Tables 1 and 2. Because of the use of two markers (see Materials and methods) to distinguish chromosome rearrangements at the distal end of chromosome 2, flies carrying the mutant phenotype Delta (L veins fused at the wing margin, lethal homozygous) or ebony (very dark body color) were isolated from the F 1 progeny, and carefully genetically and cytologically analyzed. Other changes in the structure of eyes, wings, thorax, abdomen, and legs were also found. Very likely, some of them are of mutational origin, but these characters were not analyzed in detail, and therefore they can only be classified as other phenotypic variations (see Table 1). The results of the crosses with X-ray treated males are shown separately for each experiment in Table 1. During the first two experiments (1 and 2 in Table 1) we already isolated many more flies having D1 and eb mutations and other phenotypic damages in the (140 × 160) F 1 than in the (140 x 9) F 1 progeny. In order to remove differences in experimental conditions, throughout the two subsequent experiments (3 and 4 in Table 1) we tried to standardize them as much as possible: the X-ray treatment of the males from strain 9 or 160, the selection of virgin females, the interstrain crosses, and all the other procedures were carried out simultaneously in the same conditions. Table 2 summarizes the results of these two experiments.
TABLE 2 C O M P A R I S O N O F V A R I A T I O N F R E Q U E N C Y IN T H E P R O G E N Y O F X - R A Y T R E A T E D M A L E S F R O M S T R A I N 9 O R 160 Experiment
Cross with irradiated males
N u m b e r of flies screened
3a+4a 3b+4b
140× 9 140× 160
35,320 20,630
N u m b e r of phenotypically variant flies total
per 1000 of F 1 offspring
carrying more than two variations simultaneously
41 99
1.16 4.8
17 (17.1%)
200
Here too we observed a great deal of phenotypic variability among the offspring from the crosses of 140 females with irradiated 160 males. The total number of flies carrying the D1 or eb mutation and other phenotypic changes in (140 × 160) F~ was about 4 times as high as in the (140 x 9) F~ progeny per 1000 offspring. Furthermore, about 17% of the flies of (140 x 160) F l had more than two different phenotypic anomalies simultaneously (Table 2). In salivary gland nuclei of all the newly isolated D1 and eb mutants cytologically visible chromosome rearrangements were routinely observed.
Whereas there were single chromosome aberrations in the salivary gland nuclei of D1 or eb mutants from the (140 x 9) F~ progeny, multiple chromosome damage was found in the polytene nuclei of the (140 x 160) F 1 offspring. For example, in the salivary gland chromosomes of the eb TG-52 mutant isolated from the (140 x 160) F1 progeny, besides the Dr(2) eb Tc52 20C;20D deficiency causing the ebony phenotype, three additional translocations T (2;5) 26B; 55A; T (2;6?) 22D; chromocenter and T(3;5) 32F; 58F were present (Fig. 1). During the course of the polytene chromosome analysis in the (140 x 160) Ft
Fig. 1. Salivary gland chromosomes of D. virilis eb TG52 mutant from (140x 160) F I progeny. I(X), 2, 3, 4, 5, 6 are the numbers of the chromosomes. The break sites of chromosome rearrangement Dr(2) 20B;20D, T(2;5) 26B; 55A, T(2;6?) 22D; chromocenter T(3;5) 32F;58F are indicated by arrows.
201
progeny several mutants carrying three (one offspring) or two (three offspring) chromosome rearrangements were also detected. The problems of the mechanism and the role of genetic factors increasing chromosome radiosensitivity remain unsolved. A potential genome instability appears to be one of such factors in D. virilis strain 160. The chromosomes of strain 160 flies carry specific Ulysses elements, the long terminal repeatcontaining retrotransposons structurally similar to the proviral form of vertebrate retroviruses and mobilized during hybrid dysgenesis. Ulysses elements were isolated, cloned, sequenced and localized in Drosophila virilis salivary gland chromosomes (Lozovskaya and Evgen'ev, 1987; Scheinker et al., 1990). Using in situ hybridization of these cloned DNA sequences to polytene chromosomes will make it possible to check the correlation between their chromosome localization and the disposition in the mutant genome of the rearrangement breakpoints, and also to clarify whether the new chromosome Ulysses sites appeared because of a possible activation of Ulysses elements during irradiation. The most important investigation will be to test a synergistic effect of irradiation and transposon mobility, i.e., the possible interaction between X-ray and Ulysses induced damage. Probably, all these variabilities are due to a common mechanism (Engels and Preston, 1984), namely the induction of gene and chromosome mutations by chromosomal breakage events. But, considering all the data, it seems that the level of mutability, phenotypic variation and chromosome aberration in the progeny of X-ray irradiated males from the genetically unstable strain 160, which induces the hybrid dysgenesis syndrome, is much higher than in the progeny of the wild-type strain 9 males. Additional analysis will be necessary to determine the genetic, cellular and environmental factors modulating the chromosome radiosensitivity associated with transposon mobility in the Drosophila genome. Two conclusions may be drawn from the experimental data reported in this paper.
First, our results indicate that Drosophila stock carrying genome instability traits is more irradiation sensitive than the stable wild-type strain. This system can be fruitfully used in radiation mutagenesis experiments to effectively induce mutations and rearrangements in definite chromosome subdivisions. Second, taking into account that genetic instability depends on the mobility of specific transposable elements, the danger is to be stressed of some manipulations concerning genome changing and insertion of exogenous genetic material into Eucaryota including the human genome, especially in the region of higher irradiation levels. References Bingham, P.M., M.G. Kidwell and G.M. Rubin (1982) The molecular basis of P-M hybrid dysgenesis: the role of the P elements, a P strain specific transposon family, Cell, 29, 995 - 1004. Bregliano, J.C., and M.G. Kidwell (1983) Hybrid dysgenesis determinants, in: J.A. Shapiro (Ed.), Mobile Genetic Elements, Academic Press, New York, pp. 363-410. Engels, W.R. (1983) The P family of transposable elements in Drosophila, Annu. Rev. Genet., 17, 315-344. Engels, W.R., and C.R. Preston (1979) Hybrid dysgenesis in Drosophila melanogaster: the biology of female and male sterility, Genetics, 92, 161-174. Engels, W.R., and C.R. Preston (1984) Formation of chromosome rearrangements by P factor in Drosophila, Genetics, 107, 657-678. Engels, W.R., W.K. Benz, C.R. Preston, P.L. Graham, R.W. Phillis and H.M. Robertson (1987) Somatic effect of P element activity in Drosophila rnelanogaster, Genetics, 117, 745-757, Gubenko, I.S., and M.B. Evgen'ev (1984) Cytological and linkage maps of Drosophila virilis chromosomes, Genetica, 65, 127-139. Gubenko, I.S., R.P. Subbota and V.F. Semeshin (1991) Unusual 20CD Drosophila virilis stress puff: cytological localization of heat sensitive locus and some peculiarities of heat shock response, Genetika, 27, 61-70. Kidwell, M.G. (1983) Hybrid dysgenesis in Drosophila melanogaster: factors affecting chromosomal contamination in the P-M system, Genetics, 104, 317-341. Kidwell, M.G. (1984) Hybrid dysgenesis in Drosophila melanogaster: partial sterility associated with embryo lethality in the P-M system, Genet. Res., 44, 11-28. Kidwell, M.G., F. Kidwell and J.A. Sved (1977) Hybrid dysgenesis in Drosophila melanogaster: a syndrome of aberrant traits including mutations, sterility and male recombination, Genetics, 86, 813-833.
202 Lozovskaya, E.R., and M.B. Evgen'ev (1987) Hybrid dysgenesis in Drosophila virilis, Dokl. Acad. Sci. USSR, 296, 727-731. Rubin, G.M., M.G. Kidwell and P.M. Bingham (1982) The molecular basis of P-M hybrid dysgenesis: the nature of induced mutation, Cell, 29, 987-994. Scheinker, V.Sh., E.R. Lozovskaya, J.G. Bishop, V.G. Corces
and M.B. Evgen'ev (1990) A long terminal repeat-containing retrotransposon is mobilized during hybrid dysgenesis in Drosophila virilis, Proc. Natl. Acad. Sci. (U.S.A.), 87, 9615-9619.
Communicated by K. Sankaranarayanan
197
© 1992 Elsevier Science Publishers B.V. All rights reserved 0165-7992/92/$05.00
MUTLET 0679
Characterization of the progeny of X-ray irradiated males from two Drosophila virilis strains differing in genetic instability I.S. Gubenko and R.P. Subbota Department of Molecular Genetics, Institute of Molecular Biology and Genetics, Academy of Sciences of the Ukraine, 252627 Kiev, Ukraine (Received 19 August 1991) (Revision received 17 March 1992) (Accepted 27 March 1992)
Keywords: Genome instability; X-ray mutagenesis; Chromosome radiosensitivity
Summary Significant effects of X-ray treatment on the increase in the number of phenotypic variations, two visible mutations, and chromosome aberrations were found in the progeny of irradiated males from the D. virilis laboratory stock that is capable of hybrid dysgenesis syndrome induction. This effect is much more pronounced than in the progeny of irradiated males from strong wild-type strains studied. A correlation between genetic instability and chromosome radiosensitivity was outlined. The mechanism of this phenomenon and the possibilities of using the property of genome instability for the productive induction of gene and chromosome damage in radiation mutagenesis experiments are discussed.
The noticeable phenotypical variation depending on the hypermutability and instability of the mutants is considered to be one of several characteristic hybrid dysgenesis syndrome traits such as male recombination, sterility, gonadal dysgenesis, transmission ration distortion, chromosome aberrations and chromosome non-disjunction (Kidwell et al., 1977; Engels and Preston, 1979; Bregliano and Kidwell, 1983). There are some hybrid dysgenesis systems in Drosophila. In the P-M system of D. melanogaster crosses of P strain males with M strain females
Correspondence: Dr. I.S. Gubenko, Institute of Molecular Biology and Genetics, Academy of Sciences of the Ukraine, 150 Zabolotny Str., 252627 Kiev, Ukraine.
promote the mobilization of P elements in the germ lines of their progeny, producing hypermutability and other events reflecting chromosomal damage. The reciprocal cross, M males to P females, was much less effective in promoting instability. The crosses between different M strains lacking full-length P elements are practically stable (Bingham et al., 1982; Rubin et al., 1982; Kidwell, 1983, 1984; Engels, 1983; Engels and Preston, 1984; Engels et al., 1987). A second D. melanogaster hybrid dysgenesis system, the I-R system, gives rise to similar characteristics when an inducer (I) male containing I transposable genetic factors is crossed with a responsive (R) female lacking these factors (Bregliano and Kidwell, 1983). In the progeny of the crosses between defined D. virilis strains Lo-
198
zovskaya and Evgen'ev (1987) have recently found genetic phenomena similar to those displayed in the P-M and I-R syndromes of D. melanogaster but having the involved transposable elements of different origin (Scheinker et al., 1990). Some D. cirilis stocks differing widely in genetic instability were identified by the interstrain crosses. Two of these strains were used in our radiation mutagenesis experiments for cytogenetic analysis of 20CD D. virilis chromosome 2 subdivision activated by heat shock and other stress conditions (Gubenko et al., 1991). In the present paper a correlation between genetic instability and chromosome radiosensitivity of the strains studied is outlined. Materials and methods
Strains of Drosophila virilis A standard sugar-corn meal-yeast-agar medium was employed for maintenance of the stock and all the crosses. Three D. virilis stocks were used in experiments. 9 is a wild-type laboratory strain isolated from a natural Batumi population. 160 is the stock obtained by M.G. Evgen'ev from crosses between some Japanese and American strains and carrying the recessive markers b (2-188.0), gp L 2 (3-118.5), cd (4-32.5), pe (5-203.0), and gl (6-1.0) in all the autosomes. As described
earlier (Lozovskaya and Evgen'ev, 1987), some specific traits of the hybrid dysgenesis syndrome (male and female sterility, gonadal dysgenesis, increased mutation frequency and other abnormalities) were only seen with unidirectional crosses, i.e., when females of wild-type strain 9 were crossed with males of the old laboratory marked strain 160. It was concluded that D. virilis strain 9 is the analogue of D. melanogaster M-type strain; strain 160, on the contrary, can be classified as a strong P-type strain containing specific mobile Ulysses elements characteristic of the D. virilis genome (Scheinker et al., 1990). In the present study we used this classification and detailed characterization of the genetic instability levels of strains 9 and 160 without additional examination. 140 is the strain carrying the two recessive markers eb (2-83.5) and va (2-231.5) on the second linkage group. This stock was used by us as material for the selection of females in radiation experiments. As a rule we also used strain 140 for selection and reproduction of different D. virilis mutant stocks; no features of genetic instability were found in the F l progeny of direct (see Table 1B) and reciprocal (data not shown) crosses of 140 with non-irradiated 9 or 160 flies. Therefore, only the cross of 9 females with 160 males caused hybrid dysgenesis, and all the others produced
TABLE 1 C H A R A C T E R I Z A T I O N O F T H E P R O G E N Y O F X - R A Y I R R A D I A T E D (A) A N D N O N - I R R A D I A T E D (B) M A L E S IN DIFFERENT EXPERIMENTS Experiment
Strain, source of irradiated males
Cross
(A) Crosses with irradiated males 1 9 140 x 9 2 160 140 x 160 3 *a 9 140x 9 b 160 140 x 160 4 *a 9 140x 9 b 160 140 x 160
Number of flies screened
N u m b e r of F 1 offspring carrying Dl eb other phenotypic changes
Total n u m b e r of variations
13,430 9,557 8,950 9,670 26,370 10,960
2 3 1 2 1 2
4 9 7 11 4 6
4 10 18 58 10 20
10 22 26 71 15 28
43,400 30,240
0 0
0 0
1 0
1 0
(B) Crosses with non-irradiated males (control) 140 × 9 140 × 160
* Males from strain 9 or 160 were irradiated simultaneously.
199
non-dysgenic progeny. All the strains listed above were obtained from the Institute of Development Biology of the Academy of Sciences of Russia (Moscow). For other information on genetic markers see Gubenko and Evgen'ev (1984).
X-irradiation Irradiation of the males from stock 9 or 160 was carried out with a RUM-17 X-ray source. The dose used for all the experiments reported here was 6000 R. Crosses In all the experiments virgin strain 140 females were crossed to X-ray irradiated males from strain 9 or 160. Mass mating was carried out in large vials with 10-15 irradiated males and 20-30 females per vial. In F 1 progeny, eb and DI mutants were monitored by the appearance of the recessive ebony and the dominant Delta phenotype respectively. These mutants were collected for further cytological analysis. We also counted all the visible alterations or modifications of phenotypes in the progeny. Cytology Salivary glands of third-instar larvae were isolated in Hanks' medium, stained with acetoorcein, and squash preparations were made. For cytological identification of chromosome regions we used photographic maps of D. virilis salivary gland chromosomes (Gubenko and Evgen'ev, 1984).
Results and discussion
The effect of X-rays on the variability of the visible phenotypic characters of F1 (140 x 9 or 140 x 160) progeny is presented in Tables 1 and 2. Because of the use of two markers (see Materials and methods) to distinguish chromosome rearrangements at the distal end of chromosome 2, flies carrying the mutant phenotype Delta (L veins fused at the wing margin, lethal homozygous) or ebony (very dark body color) were isolated from the F 1 progeny, and carefully genetically and cytologically analyzed. Other changes in the structure of eyes, wings, thorax, abdomen, and legs were also found. Very likely, some of them are of mutational origin, but these characters were not analyzed in detail, and therefore they can only be classified as other phenotypic variations (see Table 1). The results of the crosses with X-ray treated males are shown separately for each experiment in Table 1. During the first two experiments (1 and 2 in Table 1) we already isolated many more flies having D1 and eb mutations and other phenotypic damages in the (140 × 160) F 1 than in the (140 x 9) F 1 progeny. In order to remove differences in experimental conditions, throughout the two subsequent experiments (3 and 4 in Table 1) we tried to standardize them as much as possible: the X-ray treatment of the males from strain 9 or 160, the selection of virgin females, the interstrain crosses, and all the other procedures were carried out simultaneously in the same conditions. Table 2 summarizes the results of these two experiments.
TABLE 2 C O M P A R I S O N O F V A R I A T I O N F R E Q U E N C Y IN T H E P R O G E N Y O F X - R A Y T R E A T E D M A L E S F R O M S T R A I N 9 O R 160 Experiment
Cross with irradiated males
N u m b e r of flies screened
3a+4a 3b+4b
140× 9 140× 160
35,320 20,630
N u m b e r of phenotypically variant flies total
per 1000 of F 1 offspring
carrying more than two variations simultaneously
41 99
1.16 4.8
17 (17.1%)
200
Here too we observed a great deal of phenotypic variability among the offspring from the crosses of 140 females with irradiated 160 males. The total number of flies carrying the D1 or eb mutation and other phenotypic changes in (140 × 160) F~ was about 4 times as high as in the (140 x 9) F~ progeny per 1000 offspring. Furthermore, about 17% of the flies of (140 x 160) F l had more than two different phenotypic anomalies simultaneously (Table 2). In salivary gland nuclei of all the newly isolated D1 and eb mutants cytologically visible chromosome rearrangements were routinely observed.
Whereas there were single chromosome aberrations in the salivary gland nuclei of D1 or eb mutants from the (140 x 9) F~ progeny, multiple chromosome damage was found in the polytene nuclei of the (140 x 160) F 1 offspring. For example, in the salivary gland chromosomes of the eb TG-52 mutant isolated from the (140 x 160) F1 progeny, besides the Dr(2) eb Tc52 20C;20D deficiency causing the ebony phenotype, three additional translocations T (2;5) 26B; 55A; T (2;6?) 22D; chromocenter and T(3;5) 32F; 58F were present (Fig. 1). During the course of the polytene chromosome analysis in the (140 x 160) Ft
Fig. 1. Salivary gland chromosomes of D. virilis eb TG52 mutant from (140x 160) F I progeny. I(X), 2, 3, 4, 5, 6 are the numbers of the chromosomes. The break sites of chromosome rearrangement Dr(2) 20B;20D, T(2;5) 26B; 55A, T(2;6?) 22D; chromocenter T(3;5) 32F;58F are indicated by arrows.
201
progeny several mutants carrying three (one offspring) or two (three offspring) chromosome rearrangements were also detected. The problems of the mechanism and the role of genetic factors increasing chromosome radiosensitivity remain unsolved. A potential genome instability appears to be one of such factors in D. virilis strain 160. The chromosomes of strain 160 flies carry specific Ulysses elements, the long terminal repeatcontaining retrotransposons structurally similar to the proviral form of vertebrate retroviruses and mobilized during hybrid dysgenesis. Ulysses elements were isolated, cloned, sequenced and localized in Drosophila virilis salivary gland chromosomes (Lozovskaya and Evgen'ev, 1987; Scheinker et al., 1990). Using in situ hybridization of these cloned DNA sequences to polytene chromosomes will make it possible to check the correlation between their chromosome localization and the disposition in the mutant genome of the rearrangement breakpoints, and also to clarify whether the new chromosome Ulysses sites appeared because of a possible activation of Ulysses elements during irradiation. The most important investigation will be to test a synergistic effect of irradiation and transposon mobility, i.e., the possible interaction between X-ray and Ulysses induced damage. Probably, all these variabilities are due to a common mechanism (Engels and Preston, 1984), namely the induction of gene and chromosome mutations by chromosomal breakage events. But, considering all the data, it seems that the level of mutability, phenotypic variation and chromosome aberration in the progeny of X-ray irradiated males from the genetically unstable strain 160, which induces the hybrid dysgenesis syndrome, is much higher than in the progeny of the wild-type strain 9 males. Additional analysis will be necessary to determine the genetic, cellular and environmental factors modulating the chromosome radiosensitivity associated with transposon mobility in the Drosophila genome. Two conclusions may be drawn from the experimental data reported in this paper.
First, our results indicate that Drosophila stock carrying genome instability traits is more irradiation sensitive than the stable wild-type strain. This system can be fruitfully used in radiation mutagenesis experiments to effectively induce mutations and rearrangements in definite chromosome subdivisions. Second, taking into account that genetic instability depends on the mobility of specific transposable elements, the danger is to be stressed of some manipulations concerning genome changing and insertion of exogenous genetic material into Eucaryota including the human genome, especially in the region of higher irradiation levels. References Bingham, P.M., M.G. Kidwell and G.M. Rubin (1982) The molecular basis of P-M hybrid dysgenesis: the role of the P elements, a P strain specific transposon family, Cell, 29, 995 - 1004. Bregliano, J.C., and M.G. Kidwell (1983) Hybrid dysgenesis determinants, in: J.A. Shapiro (Ed.), Mobile Genetic Elements, Academic Press, New York, pp. 363-410. Engels, W.R. (1983) The P family of transposable elements in Drosophila, Annu. Rev. Genet., 17, 315-344. Engels, W.R., and C.R. Preston (1979) Hybrid dysgenesis in Drosophila melanogaster: the biology of female and male sterility, Genetics, 92, 161-174. Engels, W.R., and C.R. Preston (1984) Formation of chromosome rearrangements by P factor in Drosophila, Genetics, 107, 657-678. Engels, W.R., W.K. Benz, C.R. Preston, P.L. Graham, R.W. Phillis and H.M. Robertson (1987) Somatic effect of P element activity in Drosophila rnelanogaster, Genetics, 117, 745-757, Gubenko, I.S., and M.B. Evgen'ev (1984) Cytological and linkage maps of Drosophila virilis chromosomes, Genetica, 65, 127-139. Gubenko, I.S., R.P. Subbota and V.F. Semeshin (1991) Unusual 20CD Drosophila virilis stress puff: cytological localization of heat sensitive locus and some peculiarities of heat shock response, Genetika, 27, 61-70. Kidwell, M.G. (1983) Hybrid dysgenesis in Drosophila melanogaster: factors affecting chromosomal contamination in the P-M system, Genetics, 104, 317-341. Kidwell, M.G. (1984) Hybrid dysgenesis in Drosophila melanogaster: partial sterility associated with embryo lethality in the P-M system, Genet. Res., 44, 11-28. Kidwell, M.G., F. Kidwell and J.A. Sved (1977) Hybrid dysgenesis in Drosophila melanogaster: a syndrome of aberrant traits including mutations, sterility and male recombination, Genetics, 86, 813-833.
202 Lozovskaya, E.R., and M.B. Evgen'ev (1987) Hybrid dysgenesis in Drosophila virilis, Dokl. Acad. Sci. USSR, 296, 727-731. Rubin, G.M., M.G. Kidwell and P.M. Bingham (1982) The molecular basis of P-M hybrid dysgenesis: the nature of induced mutation, Cell, 29, 987-994. Scheinker, V.Sh., E.R. Lozovskaya, J.G. Bishop, V.G. Corces
and M.B. Evgen'ev (1990) A long terminal repeat-containing retrotransposon is mobilized during hybrid dysgenesis in Drosophila virilis, Proc. Natl. Acad. Sci. (U.S.A.), 87, 9615-9619.
Communicated by K. Sankaranarayanan