C. elegans NBS-1 is required for meiotic DSBR but dispensable for DSB formation. (A) DAPI-stained diakinesis oocytes from nbs-1 and nbs-1; spo-11 mutant worms unirradiated or exposed to 5,000-rad γ-irradiation. In contrast to the abnormal chromosome aggregates present in the nbs-1 oocytes (with or without irradiation), 12 intact chromosomes (univalents) are observed in the unirradiated nbs-1; spo-11 oocyte. Aggregation of chromosomes is, however, observed in the nbs-1; spo-11 oocyte upon introduction of exogenous DSBs by irradiation. (B) Quantification of the number of DAPI bodies in diakinesis nuclei. Fewer than 5 countable DAPI bodies reflect aggregation of chromosomes, whereas 12 DAPI bodies typically reflect intact univalents. Numbers of nuclei counted: WT, n = 92; spo-11, n = 37; nbs-1(me102), n = 42; nbs-1; spo11, n = 39; mre-11, n = 66; and nbs-1; mre-11, n = 59. (C) Quantification of DAPI bodies as in B after exposure of worms to 5,000-rad γ-irradiation, showing that irradiation-induced breaks rescue chiasma formation in the spo-11 mutant but induce chromosome aggregation in nbs-1; spo-11 mutant oocytes. Numbers of oocytes counted: WT, n = 105; spo-11, n = 57; nbs-1(me102), n = 144; and nbs-1; spo-11, n = 45. (D) Graph showing indistinguishable profiles of diakinesis DAPI body counts in nbs-1(me102) mutant worms derived from heterozygous nbs-1/+ mothers (m+z−) and nbs-1 m−z− mutant worms, which were derived from a cross using homozygous nbs-1; cku-80 double-mutant mothers (m−z−). See Fig. S3 for more details.