The C. elegans DSB-2 protein reveals a regulatory network that controls competence for meiotic DSB formation and promotes crossover assurance

Evidence that masking of synapsis imperfections counterbalances quality control to promote efficient meiosis

Reduction in ploidy to generate haploid gametes during sexual reproduction is accomplished by the specialized cell division program of meiosis. Pairing between homologous chromosomes and assembly of the synaptonemal complex at their interface (synapsis) represent intermediate steps in the meiotic program that are essential to form crossover recombination-based linkages between homologs, which in turn enable segregation of the homologs to opposite poles at the meiosis I division. Here, we challenge the mechanisms of pairing and synapsis during C. elegans meiosis by disrupting the normal 1∶1 correspondence between homologs through karyotype manipulation. Using a combination of cytological tools, including S-phase labeling to specifically identify X chromosome territories in highly synchronous cohorts of nuclei and 3D rendering to visualize meiotic chromosome structures and organization, our analysis of trisomic (triplo-X) and polyploid meiosis provides insight into the principles governing pairing and synapsis and how the meiotic program is “wired” to maximize successful sexual reproduction. We show that chromosomes sort into homologous groups regardless of chromosome number, then preferentially achieve pairwise synapsis during a period of active chromosome mobilization. Further, comparisons of synapsis configurations in triplo-X germ cells that are proficient or defective for initiating recombination suggest a role for recombination in restricting chromosomal interactions to a pairwise state. Increased numbers of homologs prolong markers of the chromosome mobilization phase and/or boost germline apoptosis, consistent with triggering quality control mechanisms that promote resolution of synapsis problems and/or cull meiocytes containing synapsis defects. However, we also uncover evidence for the existence of mechanisms that “mask” defects, thus allowing resumption of prophase progression and survival of germ cells despite some asynapsis. We propose that coupling of saturable masking mechanisms with stringent quality controls maximizes meiotic success by making progression and survival dependent on achieving a level of synapsis sufficient for crossover formation without requiring perfect synapsis.

Diploid organisms must produce haploid gametes prior to sexual reproduction in order to maintain a constant number of chromosomes from one generation to the next. Ploidy reduction is accomplished during meiosis and requires crossover recombination-based linkages between homologous chromosomes. Here, we manipulate karyotype in C. elegans to probe the mechanisms that govern stable, pairwise, homologous associations essential for crossover formation. We find that chromosomes sort into homolog groups regardless of number prior to stabilizing interactions (“synapsing”) in a preferentially pairwise manner. Increased numbers of homologs delay meiotic progression and/or boost cell death, reflecting operation of quality control mechanisms that either buy time to correct synapsis problems or eliminate defective cells. Moreover, we found evidence for mechanisms that can “mask” synapsis imperfections, thus allowing resumption of meiotic progression and survival of germ cells when synapsis is “good enough”, albeit imperfect. This strategy would maximize meiotic success by making progression and survival contingent on achieving a level of synapsis sufficient for crossover formation without imposing an onerous and unnecessary requirement for perfect synapsis. We suggest that the regulatory logic of coupling saturable masking mechanisms with stringent quality controls may be employed widely to maximize efficiency of biological circuits.

Identification of DSB-1, a protein required for initiation of meiotic recombination in Caenorhabditis elegans, illuminates a crossover assurance checkpoint

Meiotic recombination, an essential aspect of sexual reproduction, is initiated by programmed DNA double-strand breaks (DSBs). DSBs are catalyzed by the widely-conserved Spo11 enzyme; however, the activity of Spo11 is regulated by additional factors that are poorly conserved through evolution. To expand our understanding of meiotic regulation, we have characterized a novel gene, dsb-1, that is specifically required for meiotic DSB formation in the nematode Caenorhabditis elegans. DSB-1 localizes to chromosomes during early meiotic prophase, coincident with the timing of DSB formation. DSB-1 also promotes normal protein levels and chromosome localization of DSB-2, a paralogous protein that plays a related role in initiating recombination. Mutations that disrupt crossover formation result in prolonged DSB-1 association with chromosomes, suggesting that nuclei may remain in a DSB-permissive state. Extended DSB-1 localization is seen even in mutants with defects in early recombination steps, including spo-11, suggesting that the absence of crossover precursors triggers the extension. Strikingly, failure to form a crossover precursor on a single chromosome pair is sufficient to extend the localization of DSB-1 on all chromosomes in the same nucleus. Based on these observations we propose a model for crossover assurance that acts through DSB-1 to maintain a DSB-permissive state until all chromosome pairs acquire crossover precursors. This work identifies a novel component of the DSB machinery in C. elegans, and sheds light on an important pathway that regulates DSB formation for crossover assurance.

For most eukaryotes, recombination between homologous chromosomes during meiosis is an essential aspect of sexual reproduction. Meiotic recombination is initiated by programmed double-strand breaks in DNA, which have the potential to induce mutations if not efficiently repaired. To better understand the mechanisms that govern the initiation of recombination and regulate the formation of double-strand breaks, we use the nematode Caenorhabditis elegans as a model system. Here we describe a new gene, dsb-1, that is required for double-strand break formation in C. elegans. Through analysis of the encoded DSB-1 protein we illuminate an important regulatory pathway that promotes crossover recombination events on all chromosome pairs to ensure successful meiosis.