Analysis of meiosis in Pristionchus pacificus reveals plasticity in homolog pairing and synapsis within the nematode lineage

The goal of meiosis is to produce haploid gametes from diploid progenitor cells. While meiosis was likely present in the last eukaryotic common ancestor (LECA), diversity in meiotic mechanisms has long been observed among sexually reproducing eukaryotes. Here we describe a new, comparative model system for molecular analysis of meiosis, the nematode Pristionchus pacificus , a distant relative of the widely studied model organism Caenorhabditis elegans . Despite superficial similarities in germline organization and meiotic progression between P. pacificus and C. elegans , we identify fundamental differences in the molecular mechanisms underlying homolog pairing, synapsis, and crossover regulation. Whereas C. elegans has lost the meiosis-specific recombinase Dmc1, P. pacificus expresses both DMC-1 and RAD-51, which localize sequentially to meiotic chromosomes during prophase. We find that Ppa-spo-11 and Ppa-dmc-1 are required for stable homolog pairing, synapsis, and crossover formation, while Ppa-rad-51 is dispensable for these key processes during early prophase and plays a supporting role in meiotic double-strand break repair. Additionally, we show that elevated crossover recombination in P. pacificus likely arises through a Class II pathway normally inactive in C. elegans , shedding light on crossover control and the evolution of recombination rates.