The effect of themus309mutation, defective in DNA double-strand break repair, on crossing over inDrosophila melanogastersuggests a mechanism for interference

Abstract

The mus309 gene in Drosophila melanogaster encodes a recQ helicase which is involved in DnA double-strand break (DSB) repair. In a brood pattern analysis, it was observed that in mus309 mutant females, the frequency of single crossovers in the central cv -v interval of the X chromosome was decreased in young females but returned to the level of the wild type control as the females aged. In the proximal v -f interval, the frequency of single crossovers was increased during the whole experimental period. In particular, it was observed that the frequency of double crossovers, as well as the coefficient of coincidence first increased but then gradually decreased, finally reaching the level of the control flies, as the females aged. Map distances increased due to the mus309 mutation in both gene interval studies, but they did not change as the females aged, a result suggesting that the mus309 gene controls the distribution of DSBs to be repaired as crossovers instead of non-crossovers. The results are consistent with the hypothesis that in general the DSBs are initially independently distributed on the chromosome but non-randomly repaired so that the distribution of crossovers in the mutant flies becomes uniform, but uneven in wild-type flies. The results are consistent with the counting number model of crossover interference, based on genetic distance. On the other hand, the data are not consistent with the reaction-diffusion model based on physical distance. Consequently, the view that crossover interference in Drosophila is tightly tied to genetic distance is supported. The occurrence of double-strand DnA breaks is a necessary condition for crossing over in a variety of organisms (reviewed by Boyd et al. 1987). Present molecular models of meiotic crossing over and gene conversion suggest that crossing over is initiated by the formation double-strand breaks (DSBs) of DnA, catalyzed most likely in all eukaryotes by the topoisomerase-like Spo11 protein in cooperation with other enzymes (Keeney et al. 1997 Based on the most recent studies in Saccharomyces cerevisae, it appears most likely that in all eukaryotes the formation of single-strand DnA (ssDnA) tails at a double-strand break is a key step in homologous recombination. The resection of DSBs is in all probability a two-step process. First, after the formation of a DSB, the ends are trimmed by an endonuclease to an intermediate form.This intermediate form is subsequently processed alternatively by either the 5′ → 3′ exonuclease activity of exo1, or by the Sgs1 helicase, the yeast counterpart of the MUS309 protein in Drosophila melanogaster, to generate extensive tracts of ssDnA to initiate homologous recombination (gravel et al. 2008; mimitou and symington 2008; raynard et al. 2008; zhu et al. 2008).Until the last year, only two alternative pathways resolving the dHJs were known: the synthesis-dependent strand annealing (SDSA) pathway and the double-strandbreak repair (DSBr) pathway. The former pathway leads exclusively to non-crossover products and the latter t...