Abstract
Homologous recombination is essential for crossover (CO) formation and accurate chromosome segregation during meiosis. It is of considerable importance to work out how recombination intermediates are processed, leading to CO and non-crossover (NCO) outcome. Genetic analysis in budding yeast and Caenorhabditis elegans indicates that the processing of meiotic recombination intermediates involves a combination of nucleases and DNA repair enzymes. We previously reported that in C. elegans meiotic joint molecule resolution is mediated by two redundant pathways, conferred by the SLX-1 and MUS-81 nucleases, and by the HIM-6 Bloom helicase in conjunction with the XPF-1 endonuclease, respectively. Both pathways require the scaffold protein SLX-4. However, in the absence of all these enzymes, residual processing of meiotic recombination intermediates still occurs and CO formation is reduced but not abolished. Here we show that the LEM-3 nuclease, mutation of which by itself does not have an overt meiotic phenotype, genetically interacts with slx-1 and mus-81 mutants, the respective double mutants displaying 100% embryonic lethality. The combined loss of LEM-3 and MUS-81 leads to altered processing of recombination intermediates, a delayed disassembly of foci associated with CO designated sites, and the formation of univalents linked by SPO-11 dependent chromatin bridges (dissociated bivalents). However, LEM-3 foci do not colocalize with ZHP-3, a marker that congresses into CO designated sites. In addition, neither CO frequency nor distribution is altered in lem-3 single mutants or in combination with mus-81 or slx-4 mutations. Finally, we found persistent chromatin bridges during meiotic divisions in lem-3; slx-4 double mutants. Supported by the localization of LEM-3 between dividing meiotic nuclei, this data suggest that LEM-3 is able to process erroneous recombination intermediates that persist into the second meiotic division.
Highlights
Meiosis is comprised of two specialized cell divisions that elicit the reduction of the diploid genome to haploid gametes
While an excess of meiotic double-strand breaks is generated, most breaks are repaired without leading to a CO outcome and usually only one break for each chromosome pair matures into a CO-designated site in Caenorhabditis elegans
We found that a conserved nuclease LEM-3/Ankle1 acts in parallel to the SLX-1/MUS-81 pathway to process meiotic recombination intermediates
Summary
Meiosis is comprised of two specialized cell divisions that elicit the reduction of the diploid genome to haploid gametes. It is thought that the excessive number of DSBs is required to ensure that at least one CO occurs on each homologue, a notion supported by checkpoint mechanisms that delay meiotic prophase progression when the number of DSBs is reduced [9,10,11]. It is unclear how the obligate CO is selected from the pool of DSBs. The CO selection (or designation) correlates with the congression of several pro-CO factors into six distinct foci, one on each paired chromosome starting from the mid-pachytene stage. These include the cyclin-related protein COSA1/CNTD1, MSH-4/MSH-5 components of the MutSγ complex, the predicted ubiquitin ligases ZHP-3/RNF212 and HEI10, the Bloom (BLM) helicase HIM-6 as well as its regulatory subunit RMH-1 [12,13,14,15,16,17]
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