The differential roles of rad9 alternatively spliced forms in double- strand DNA break repair during Drosophila meiosis.

The 9-1-1 complex, comprising the Rad9, Hus1 and Rad1 proteins, is believed to operate as a component of a DNA damage checkpoint pathway. Our initial analysis of the Drosophila hus1 gene showed that Hus1 plays a dual role in meiosis, regulating both meiotic DNA damage checkpoint and homologous recombination repair. In this study, we further analyzed the meiotic roles of another protein in the complex, Rad9, which has two alternatively spliced forms, Rad9A and Rad9B. Using CRISPR/Cas9, we generated flies mutant for both rad9 isoforms. We found that, similarly to hus1, mutations in rad9 lead to female sterility. Also, double-strand DNA breaks (DSBs) that form during meiosis are not processed efficiently, and the DNA within the oocyte nucleus fails to form its characteristic shape in rad9 mutants. On the other hand, the hus1 mutation completely disrupts checkpoint activation in DSB repair enzyme mutants, whereas the rad9 mutation only partially impairs checkpoint activation in this context. Moreover, spatial rescue experiments revealed that Rad9B is efficient in repairing meiotic DSBs, while Rad9A is not. Furthermore, we found that female fertility in rad9 mutants depends on early efficient meiotic DSB repair but not on karyosome formation. In summary, our results demonstrate a differential role of Rad9 alternatively spliced forms during Drosophila meiosis in oogenesis, and while former studies showed that Hus1 is sufficient for the effective activation of the meiotic recombination checkpoint, our results revealed that this is not true for Rad9.