(A) Top left: Schematic depicting the positions of nuclei just prior to the early-to-late prophase transition (*), which is delayed in mutants deficient in crossing over. Top right: Quantification of RPA-1 and RAD-51 foci in nuclei from the positions indicated in the schematic; foci levels for all mutants are significantly higher than WT (p < 0.0001). Bottom: Representative examples of such nuclei. (B) Early-to-late prophase transition region of WT (top) and cosa-1 mutant (bottom) gonad, stained for MSH-5, HTP-3 and early prophase marker DSB-1. MSH-5 foci are not retained after the transition to late prophase in the cosa-1 mutant (see also Fig S4B). (C) Early prophase (red outline), early-to-late prophase transition (yellow) and late prophase (green) nuclei from a syp-1 mutant gonad. Throughout the prolonged early prophase of syp-1 mutants, MSH-5 colocalizes with BLM foci (left). At the transition to late prophase, MSH-5 foci reduce in number, and BLM is still present (middle). Upon transition to late prophase, BLM is lost from MSH-5-marked sites (right). Scale bar: 5 μm. (D) Left: late prophase nuclei from WT (gfp::cosa-1) and the syp-3 mutant (syp-3; gfp::cosa-1). Right: Quantification of late prophase MSH-5/COSA-1 foci. (E) Representative SIM images of early prophase (red outline), early-to-late-prophase transition (yellow) and late prophase (green) nuclei from syp-1; blm::HA worms (scale bar: 1 μm; with enlarged individually cropped DNA repair sites from the same fields shown below (scale bar: 500 nm). In late prophase nuclei of the syp-1 mutant, MSH-5 doublets are association with and oriented along single unpaired axes without BLM being present, as indicated in the schematic.