Evidence based on studies of the mus309 mutant, deficient in DNA double-strand break repair, that meiotic crossing over in Drosophila melanogaster is a two-phase process.

The mus309 gene in Drosophila melanogaster encodes a RecQ helicase which is involved in DNA double-strand break (DSB) repair and specifically in the choice between the different pathways of the repair. In a brood pattern analysis of mus309 and wild type females which either had or had not experienced a temperature shock, different parameters of meiotic crossing over including map distances and crossover interference in the X chromosome were measured. The results suggest that, like in other eukaryotes studied, the control of meiotic crossover formation also in D. melanogaster is a two-phase process. The first phase seems to be temperature shock sensitive, independent of the mus309 gene and coincidental with the premeiotic DNA synthesis, thus most likely representing the formation of DSBs. The second phase seems to be temperature shock tolerant, dependent on the mus309 gene, occurring during the meiotic prophase and most likely representing the choice made by the oocyte between the different pathways of the DSB repair. A hypothesis of the localization of chiasmata is also presented, combining the mechanisms of interference and the so-called centromere effect, and based on the balance between the SDSA and DSBR pathways of DSB repair.