Studies on mutagen-sensitive strains of Drosophila melanogaster. II. Detection of qualitative differences between genetic damage induced by X-irradiation of mature spermatozoa in oxygenated and anoxic atmospheres through the use of the repair-deficient mutant mei-9a

Studies on mutagen sensitive strains of Drosophila melanogaster. XI. Survival (dominant lethality) after X-irradiation and relation to recessive lethals and translocations

Muller-5 males of Drosophila melanogaster were irradiated in N2 or O2 and mated to excision repair deficient, post-replication repair deficient (mei-9a, mei-41D5, mus101D1, mus201D1, mus302D1, mus306D1 and mus308D2) or repair proficient females. The surviving fraction (dominant lethality) was estimated in the F1 and used to reassess existing recessive lethal and translocation data. The surviving fraction was found to decrease if repair deficient females were used (maternal effect). The dose-effect curves are often biphasic with a steeper slope at low doses than at high (> or = 5 Gy) doses of X-rays. The high dose part of the curve is sensitive to oxygenation during irradiation and is affected significantly by the mutants with low fertility (mei-9, mus101 and mus302). The low dose component is not sensitive to oxygenation during irradiation and seems influenced by all seven repair deficient mutants. The sensitivity of the high dose part to oxygenation suggests that this part is related mainly to DNA break damage, while in the low dose part base damage seems more important. Existing recessive lethal and translocation data were plotted against the surviving fraction for a reassessment. In excision repair deficient mutants translocation induction is lower compared to repair proficient flies at the same level of survival (i.e., dominant lethality). Likewise in post-replication repair deficient mutants induction of recessive lethals is decreased. However the frequency of respectively induced recessive lethals and translocations obtained at the same level of X-rays was the same in repair deficient and proficient backgrounds. It is concluded that genetic damage recovered in a repair deficient background is likely to be qualitatively different even if the frequency of the damage induced by a given dose is not altered.

Spontaneous second-chromosome recessive lethals in a Drosophila melanogaster repair-deficient (mei-9a) mutant

The frequency of spontaneous second-chromosome recessive lethal mutations was determined in a Drosophila melanogaster strain kept homozygous for the X-linked mei-9 mutant for 50 generations. The data show that the rate of lethals of independent origin is 9 times higher and that of semilethals 4 times higher than the control values. The mei-9 mutant, originally isolated on the ground of its abnormal behaviour during meiosis, was later found to be defective in excision repair in somatic cells and to yield, after exposure to different physical and chemical agents, higher frequencies of genetic damage than mei-9+ flies. Our results are taken as evidence that the normal functioning of the mei-9 locus plays an important role in the repair of spontaneous lesions in the second chromosome and give support to positive findings in the X chromosome. Monitoring the accumulation of mutations in an experimental population could be a useful methodological approach to detect slight, though relevant, genetic damage induced by long-term exposure to environmental mutagens, whose detection is beyond the resolving power of conventional tests in Drosophila.

Studies on mutagen-sensitive strains of Drosophila melanogaster, VIII. Further data on differences between Canton-S and ebony strains with respect to maternal effects for the X-ray induction of autosomal translocations and ring-X chromosome losses in mature spermatozoa

The influence of the maternal genotype (Canton-S, proficient in the repair of X-ray-induced chromosome breaks and ebony, less proficient in this regard) on the recovery of X-ray-induced autosomal (II-III) translocations and ring-X chromosome losses in mature spermatozoa was studied. In the first series of experiments, males carrying appropriate markers on their second and third chromosomes were irradiated and mated to Canton-S or ebony females and the frequencies of II-III translocations were determined. In the second series of experiments, males carrying ring-X chromosomes were irradiated in N2 or in O2, mated to Canton-S or ebony females and the frequencies of XO males were determined; additionally, under similar gas-treatment and radiation conditions, the pattern of egg-mortality was also assessed. The data on translocations show that the yields are higher with ebony than with Canton-S females; these and earlier results on dominant lethals and sex-linked recessive lethals support the interpretation that the maternal repair system in the ebony strain is less proficient and more error-prone than that of the Canton-S strain. Those on the losses of ring-X chromosomes demonstrate that (i) the absolute yields of XO males are lower with ebony than with Canton-S females irrespective of whether the parental males are irradiated in N2 or in O2; (ii) the exposure-frequency relationships are all linear, but the slopes are higher when the males are irradiated in O2 and are consistent with an oxygen-enhancement-ratio of about 1.5 and (iii) the relationships between the logarithm of egg-survival and XO male frequency are also linear, but the slopes for the O2 groups are lower than those for the N2 groups (slope ratios of 0.86-0.87). The finding that at given survival levels, the XO frequencies are lower in the O2 than in the N2 groups of both the Canton-S and ebony series viewed in the context of the mechanisms that have been postulated to explain the loss of ring-X chromosomes in irradiated mature spermatozoa permits the following interpretation for the observed results: (i) a higher proportion of potential XO zygotes is lost through dominant lethality in the O2 groups than in the N2 ones presumably because the chromosome breaks induced in O2 are qualitatively different in the sense that they have a higher probability to undergo reunions relative to restitution, compared with breaks induced under anoxia and (ii) this leads to lower than expected oxygen-enhancement ratios (i.e., expected on the basis of published data on sex-linked recessive lethals, another kind of genetic damage which shows a linear exposure-frequency relationship.

Studies on mutagen-sensitive strains of Drosophila melanogaster. IV. Modification of genetic damage induced by X-irradiation of spermatozoa and spermatids in N2 or O2 by mei-9a, mei-41D5 and mus(1)101D1

The influence of defects in DNA repair processes on X-ray-induced genetic damage in post-meiotic male germ cell stages of Drosophila melanogaster was studied using the 'maternal effects approach'. Basc males were irradiated in N2, air or O2 either as 48-h-old pupae (to sample spermatids) or as 3-4-day-old adults (to sample mature spermatozoa) and mated to females of 3 repair-deficient strains (mei-9a: excision-repair-deficient; mei-41D5: post-replication-repair-deficient; mus(1)101D1: post-replication-repair-deficient and impaired in DNA synthesis). Simultaneous controls involving mating of males to repair-proficient females (mei+) were run. The frequencies of sex-linked recessive lethals and of autosomal translocations were determined following standard genetic procedures. The responses elicited in the different crosses with repair-deficient females were compared with those in mei+ crosses. The main findings are the following: with mei-9 females, the frequencies of recessive lethals are higher after irradiation of spermatids in N2, but not after irradiation in air of O2 (relative to those in the mei+ crosses); this result is different from that obtained in earlier work with spermatozoa, in which cell stage, higher yields of recessive lethals were obtained after irradiation of males in either N2 or air; in the mei-9 crosses, there are no significant differences in response (relative to mei+) after irradiation of either spermatozoa or spermatids in O2; the translocation frequencies in the mei-9 crosses are similar to those in the mei+ crosses, irrespective of the treated germ cell stage or the irradiation atmosphere; irradiation of either spermatozoa or spermatids in N2, air or O2 does not result in any differential recovery of recessive lethals in the mei-41 relative to mei+ crosses; irradiation of spermatids in N2 and of spermatozoa in air leads to a higher recovery of translocations in the mei-41 crosses; and after irradiation of spermatids or spermatozoa in any of the gaseous atmospheres, the frequencies of recessive lethals and of translocations are lower in the mus-101 crosses. The differences in responses (between cell stages, in different gaseous atmospheres and with different repair-deficient females) are explained on the basis of both qualitative and quantitative differences in the composition of the initial lesions and the extent to which their repair may be affected by the defects present in the different repair-deficient females. Several discrepancies between expectations based on biochemical results and the genetic results are pointed out.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies on mutagen-sensitive strains of Drosophila melanogaster. VII. Effects of repair deficiency in males on X-ray-induced sex-linked recessive lethals in spermatozoa

The response of mature spermatozoa to the X-ray induction (500 R and 3000 R) of sex-linked recessive lethals was studied in Drosophila melanogaster males known to be deficient in excision- or post-replication repair of UV damage in somatic cells. The results show that the induced frequencies of recessive lethals in the excision-repair-deficient males (mei-9a and mei-9L1) are similar to those in the appropriate repair-proficient males (mei+ and Berlin-K). However, in the post-replication-repair-deficient males (w mus(1)101D1), these frequencies are significantly lower than in the comparable repair-proficient males (w) after 500 R, but not after 3000 R.

Studies on mutagen-sensitive strains of Drosophila melanogaster. VI. The effect of DNA-repair deficiencies in spermatids, spermatocytes and spermatogonia irradiated in N2 or O2

This study was aimed at ascertaining the extent to which paternal repair processes possibly deficient in mei-9a, mei-41D5 and mus-101D1 genotypes would affect the recovery of radiation-induced recessive lethals in early spermatids, spermatocytes and spermatogonia. These germ cell stages were sampled in two 2-day broods from freshly hatched males, that were irradiated as 24-h old pupae in O2, or N2 followed by N2 or O2 post-treatment. Spontaneous mutation frequencies were higher in mei-9 and mei-41 males, and thus appropriate corrections were applied to the radiation data. Only with mei-9 males a clear and consistent increase of the radiation-induced mutation frequency was observed. The effect is somewhat more pronounced in brood B, presumably representing spermatogonia, than in brood A and is observed after radiation in either O2 or N2. The paternal repair process thus differs from the maternal one in that it also responds to radiation damage induced in O2. The finding that, following irradiation under anoxia, post-treatment with O2 (versus that with N2), also lowers the mutation frequency in mei-9 males, indicates that the repair defect in mei-9 does not interfere with oxygen-dependent post-radiation repair. Thus there are two different paternal repair processes in these early stages of spermatogenesis: that is, one controlled by mei-9 and one depending on oxygen. Mei-41 and mus-101 do not appear to interfere with the paternal repair process. The frequency of translocations recovered from these stages was likewise not affected by mus-101.