Here we probe the relationships between assembly of the synaptonemal complex (SC) and progression of recombination between homologous chromosomes during Caenorhabditis elegans meiosis. We identify SYP-2 as a structural component of the SC central region and show that central region assembly depends on proper morphogenesis of chromosome axes. We find that the SC central region is dispensable for initiation of recombination and for loading of DNA strand-exchange protein RAD-51, despite the fact that extensive RAD-51 loading normally occurs in the context of assembled SC. Further, persistence of RAD-51 foci and absence of crossover products in meiotic mutants suggests that SC central region components and recombination proteins MSH-4 and MSH-5 are required to promote conversion of resected double-strand breaks into stable post-strand exchange intermediates. Our data also suggest that early prophase barriers to utilization of sister chromatids as repair templates do not depend on central region assembly.

Fanconi anemia (FA) is a complex cancer susceptibility disorder associated with DNA repair defects and infertility, yet the precise function of the FA proteins in genome maintenance remains unclear. Here we report that C. elegans FANCD2 (fcd-2) is dispensable for normal meiotic recombination but is required in crossover defective mutants to prevent illegitimate repair of meiotic breaks by nonhomologous end joining (NHEJ). In mitotic cells, we show that DNA repair defects of C. elegans fcd-2 mutants and FA-deficient human cells are significantly suppressed by eliminating NHEJ. Moreover, NHEJ factors are inappropriately recruited to sites of replication stress in the absence of FANCD2. Our findings are consistent with the interpretation that FA results from the promiscuous action of NHEJ during DNA repair. We propose that a critical function of the FA pathway is to channel lesions into accurate, as opposed to error-prone, repair pathways.

The breast and ovarian cancer susceptibility protein BRCA1 is evolutionarily conserved and functions in DNA double-strand break (DSB) repair through homologous recombination, but its role in meiosis is poorly understood. By using genetic analysis, we investigated the role of the Caenorhabditis elegans BRCA1 orthologue (brc-1) during meiotic prophase. The null mutant in the brc-1 gene is viable, fertile and shows the wild-type complement of six bivalents in most diakinetic nuclei, which is indicative of successful crossover recombination. However, brc-1 mutants show an abnormal increase in apoptosis and RAD-51 foci at pachytene that are abolished by loss of spo-11 function, suggesting a defect in meiosis rather than during premeiotic DNA replication. In genetic backgrounds in which chiasma formation is abrogated, such as him-14/MSH4 and syp-2, loss of brc-1 leads to chromosome fragmentation suggesting that brc-1 is dispensable for crossing over but essential for DSB repair through inter-sister recombination.

Faithful chromosome segregation during meiosis I depends on the establishment of a crossover between homologous chromosomes. This requires induction of DNA double-strand breaks (DSBs), alignment of homologs, homolog association by synapsis, and repair of DSBs via homologous recombination. The success of these events requires coordination between chromosomal events and meiotic progression. The conserved SUN/KASH nuclear envelope bridge establishes transient linkages between chromosome ends and cytoskeletal forces during meiosis. In Caenorhabditis elegans, this bridge is essential for bringing homologs together and preventing nonhomologous synapsis. Chromosome movement takes place during synapsis and recombination. Concomitant with the onset of chromosome movement, SUN-1 clusters at chromosome ends associated with the nuclear envelope, and it is phosphorylated in a chk-2- and plk-2-dependent manner. Identification of all SUN-1 phosphomodifications at its nuclear N terminus allowed us to address their role in prophase I. Failures in recombination and synapsis led to persistent phosphorylations, which are required to elicit a delay in progression. Unfinished meiotic tasks elicited sustained recruitment of PLK-2 to chromosome ends in a SUN-1 phosphorylation–dependent manner that is required for continued chromosome movement and characteristic of a zygotene arrest. Furthermore, SUN-1 phosphorylation supported efficient synapsis. We propose that signals emanating from a failure to successfully finish meiotic tasks are integrated at the nuclear periphery to regulate chromosome end–led movement and meiotic progression. The single unsynapsed X chromosome in male meiosis is precluded from inducing a progression delay, and we found it was devoid of a population of phosphorylated SUN-1. This suggests that SUN-1 phosphorylation is critical to delaying meiosis in response to perturbed synapsis. SUN-1 may be an integral part of a checkpoint system to monitor establishment of the obligate crossover, inducible only in leptotene/zygotene. Unrepaired DSBs and unsynapsed chromosomes maintain this checkpoint, but a crossover intermediate is necessary to shut it down.

In most cells, the DNA damage checkpoint delays cell division when replication is stalled by DNA damage. In early Caenorhabditis elegans embryos, however, the checkpoint responds to developmental signals that control the timing of cell division, and checkpoint activation by nondevelopmental inputs disrupts cell cycle timing and causes embryonic lethality. Given this sensitivity to inappropriate checkpoint activation, we were interested in how embryos respond to DNA damage. We demonstrate that the checkpoint response to DNA damage is actively silenced in embryos but not in the germ line. Silencing requires rad-2, gei-17, and the polh-1 translesion DNA polymerase, which suppress replication fork stalling and thereby eliminate the checkpoint-activating signal. These results explain how checkpoint activation is restricted to developmental signals during embryogenesis and insulated from DNA damage. They also show that checkpoint activation is not an obligatory response to DNA damage and that pathways exist to bypass the checkpoint when survival depends on uninterrupted progression through the cell cycle.