The mid-pachytene transition between early and late double-strand breaks in C. elegans meiosis is conserved throughout early adulthood and coincides with a peak in recombination intermediates

Abstract In meiosis, SPO-11-induced DNA double-strand breaks (DSBs) are repaired via two main homologous recombination pathways: the double Holliday Junction pathway, which yields crossovers (COs), and the synthesis-dependent strand annealing pathway, which generates non-crossover (NCOs). The standard model assumes that the choice between these two outcomes occurs simultaneously and late in the pathway, following strand invasion. We previously demonstrated that in Caenorhabditis elegans this decision is made early: DSBs formed early in meiotic prophase are destined for NCOs, while late DSBs are committed to COs. In this study we aimed to identify if and how the time window for early-to-late DSB transition is altered when the germline grows. It is established that the germline grows as a worm develops through early adulthood. We find that the expansion of the pachytene stage of meiotic prophase I is the main driver for germline growth. We further characterize the movement of meiotic nuclei and identify that while the average movement rate is consistent with previous studies, surprisingly, nuclear movement gradually slows during meiotic prophase I. Furthermore, the late DSB (CO-fated) region expands proportionally with the growing pachytene, consistently corresponding to the proximal half of the pachytene region. These findings indicate that the early-to-late DSB transition coincides with the peak of RAD-51 foci numbers, which remains stably positioned at the middle of pachytene during germline growth. Thus, our data not only provide a precise measurement of early adult germline development but also suggest that the time window for early-to-late DSB transition coincides with mid-pachytene signaling.