MU2 and Double Strand Break Repair Figure 4. Gamma H2Av foci in S2 cells are depleted by mu2 dsRNA. RNAi was performed in S2 cells targeting two different regions of MU2. After four days of continuous treatment cells were irradiated with 25 Gy at 5 Gy/min, fixed immediately, and stained with rabbit anti-cH2Av (red) antibodies and DAPI (blue). dsRNA treatment with the FHA or the BTCT domain of MU2 are represented as FHA RNAi and BRCT RNAi, respectively, along with a control unrelated RNAi as described earlier. The lower set of panels show cH2Av foci after a chase of 15 min. doi:10.1371/journal.pgen.1000473.g004 ylated on Ser 10 (PH3). As the eye-antennal imaginal discs in mu2a larvae were larger than those in Oregon R, an effort was made to score discs of equal size. The HU treated control discs did not show a discernible mitotic wave (Figure 8A) and had very few PH3 spots. However, there was a 10 fold increase in the number of cells undergoing mitosis in the mu2a discs after HU treatment compared with Oregon R (Figure 8B). As a positive control, we tested mei41D3 mutant discs for progression to mitosis after treatment with HU. These discs exhibited a 3 fold increase in mitotic cells over mu2a and a 30 fold increase over Oregon R (Figure 8B). Thus, our studies of the intra-S checkpoint using eye-antennal imaginal discs indicate that MU2 plays a role in the control of this process. Discussion Although rare, terminal deficiency chromosomes that have lost a native telomere and gained a neotelomere have been demonstrated in humans [30], plants [31], ciliates [14], yeast [32], and plasmodium [33]. One classic example is the Drosophila mu2a mutation that enhances the recovery of terminal deficiencies from irradiated oocytes [12]. Genetic studies suggested that mu2a mutants reduce the rate of DNA repair during oogenesis, that the unrepaired broken chromosomes bypass cell cycle control, and that the broken ends are healed by the addition of new telomeres after fertilization [17]. Differences in the repair capacity of lesions induced in sperm PLoS Genetics | www.plosgenetics.org 7 May 2009 | Volume 5 | Issue 5 | e1000473