Mechanistic and methodological aspects of chemically-induced somatic mutation and recombination in Drosophila melanogaster

Abstract

This study presents the analysis of chemically-induced somatic mutations and chromosomal damage in the eye imaginal discs of Drosophila larvae, assayed later as twin (TS) and single light (LS) mosaic spots in the adult eyes. Regarding the question as to what kind of DNA alterations contribute to somatic cell mutagenicity, the approach followed here has been to investigate the possible differences in response between male (hemizgous for an X) and female (homozygous) larvae, rod-X / rod-X versus ring-X / rod-X genotypes and inversion-heterozygotes versus genotypes not carrying an inversion. The systems chosen for analysis were the white-coral/white (w co / w) and the white (w + / w) eye mosaic system. The principle findings with 12 mutagens of different modes of action are as follows: 1. (1) At least 98% of all TS and LS induced by cisplatin (DDP) in w co / w female larvae and about 95% of those by formaldehyde (FA) appear as the result of recombinogenic activity between the two homologous X-chromosomes. The corresponding estimates for MMS, EMS and ENU are 81%, 73% and 61%, respectively. 2. (2) The long sc S1L sc 8R inversion, which also contains In(1)dl-49, suppresses induction of TS to 83–93%. There was also a sharp decline in the frequency of LS in inversion heterozygotes for DDP (91%), FA (86%), MMS (52%) and EMS (47%). 3. (3) Ethylnitrosourea (ENU) was the mutagen for which introduction of the inverted chromosome reduced only slightly (23%) the frequency of LS, indicating that the majority of them were somatic mutations (and deletions) at the white locus. 4. (4) In w/RX females heterozygous for a ring-X chromosome, the frequency of LS was only approximately one tenth of that of the control ( w + / w) group, after exposure to MMS or DDP. The explanation is that exchange processes involving the ring frequently lead to genetic imbalance with subsequent cell killing. 5. (5) Low frequencies of TS in w co males (one fifth to one tenth of the rates in females) after exposure to MMS, EMS, DEN, ENNG, ENU and DDP are indicative of unequal sister-strand recombination in the X-chromosome of males. Unequal mitotic recombination had been predicted by Auerbach (1945) to occur in males, and this event was later demonstrated after exposure to X-rays and triethylenemelamine (Becker, 1975). 6. (6) Frequencies of LS were clearly higher in w co / w females than in w co males for MMS (2 : 1) and DDP (7 : 1) but not for EMS and ENU. After correcting for that portion of LS due to recombinogenic activity, there was no significant departure from a 1 : 1 ratio for MMS and ENU. EMS seems to represent a mutagen which is more genotoxic in males than in females, both in germinal and somatic tissue. Part of this study also dealt with methodological aspects: 1. (7) Four different routes of application, i.e., chronic exposure, acute treatment, surface treatment and inhalation, were examined in order to devise suitable protocols for chemicals with different properties. Exposure doses as low as 1.4 ppm 1,2-dibromoethane were still mutagenic in inhalation experiments with w co / w and w + / w female larvae. 2. (8) The white coral / white bioassay gave problems in the sense that small mosaic spots, i.e., size classes 1, 2 and 3–4, but also 5–8 (ommatidia), mostly escaped detection. This drawback could be overcome by replacing the w co marker by the wild-type allele w +. Thus, we recommend for mutagenicity testing the use of w + / w crosses because this test enables the quantitative scoring of both small and large mosaic sectors (LS). 3. (9) Comparative experiments with MMS and ENU revealed that eye mosaics can be induced in first, second and third larval instar. However, the high number of small spots produced when treating third-instar larvae could only be detected in wild-type ( w + / w) phenotypes.