Loss of ATM activates the mei-41–dependent meiotic DSB repair checkpoint. Drosophila ovaries of the indicated genotypes are shown. Each oocyte develops within a 16-cell cyst (Walker and Hawley, 2000; Page and Hawley, 2001). (A) In controls, GRK protein localizes in a ring around the oocyte nucleus (arrows) in stage 5 and 6 egg chambers. In similarly staged tefu⁸ mutant egg chambers, GRK staining is much weaker or absent altogether. A, anterior; P, posterior; WT, wild type. (B, top) In control oocytes, chromatin becomes condensed during stage 4 of oogenesis into a spherical structure called the karyosome (arrow). Antibodies to ORB, a protein that localizes to the oocyte cytoplasm, identified the oocyte nucleus. (bottom) Similarly aged ORB-labeled tefu⁸ mutant egg chambers contain misshapen or fragmented karyosomes (arrow). (C) The GRK localization and karyosome morphology defects are suppressed in mei-41^D3;tefu⁸ mutants. All cysts are oriented with the anterior end to the left and posterior to the right. Arrows denote the oocytes. Bars, 10 µm.