Meiotic sister chromatid exchanges are rare inC. elegans

Abstract

Sexual reproduction shuffles the parental genomes to generate new genetic combinations. To achieve that, the genome is subjected to numerous double-strand breaks, the repair of which involves two crucial decisions: repair pathway and repair template. Use of crossover pathways with the homologous chromosome as template exchanges genetic information and directs chromosome segregation. Crossover repair, however, can compromise the integrity of the repair template and is therefore tightly regulated. The extent to which crossover pathways are used during sister-directed repair is unclear, because the identical sister chromatids are difficult to distinguish. Nonetheless, indirect assays have led to the suggestion that inter-sister crossovers, or sister chromatid exchanges (SCEs), are quite common. Here we devised a technique to directly score physiological SCEs in the C. elegans germline using selective sister chromatid labeling with the thymidine analog 5-ethynyl-2’-deoxyuridine (EdU). Surprisingly, we find SCEs to be rare in meiosis, accounting for <2% of repair events. SCEs remain rare even when the homologous chromosome is unavailable, indicating that almost all sister-directed repair is channeled into noncrossover pathways. We identify two mechanisms that limit SCEs. First, sister-directed repair intermediates are efficiently inhibited by the RecQ helicase BLMHIM-6. Second, the Synaptonemal Complex–a conserved interface that promotes crossover repair– localizes between the homologous chromosomes and not the sister chromatids. Our data suggest that in C. elegans crossover pathways are only used to generate the single necessary link between the homologous chromosomes. Almost all other breaks, regardless of which repair template they use, are repaired by noncrossover pathways.

Synaptonemal Complex dimerization regulates chromosome alignment and crossover patterning in meiosis

“…In addition to bringing homologs together, pro-crossover factors are recruited to the SC, and in that way the SC limits crossovers to synapsed chromosomal regions. This idea receives support in this work, where we show that the SC that assembles on unpaired axes in syp-1 K42E (25°C) animals can recruit GFP-COSA-1 (see also [ 6 , 41 ]). Our work also strengthens the idea that, at least in C .…”

“…This was particularly striking in the case of syp-1 K42E (25°C) animals, where despite lack of homolog zipping, and hence of actual crossovers, we observed an average of 16.4 GFP-COSA-1 foci per nucleus ( Fig 5A–5C ). Based on observations of differentially labeled sister-chromatids, at least some of these foci likely mark inter-sister exchanges [ 41 ]. Tracing of GFP-COSA-1 foci in syp-1 Δ8 animals indicated that the increased overall foci number is not due to association of multiple foci per chromosome–like in wild-type animals, every chromosome harbors only a single focus–but likely due to association of foci with the SC on both zipped and unzipped chromosomes ( Fig 5A–5C ).…”

Conditional immobilization for live imaging C. elegans using auxin-dependent protein depletion

“…2011; ROSU et al . 2011; S eidel and K imble 2015; A lmanzar et al . 2021).…”

“…EdU staining was performed as described in (A lmanzar et al . 2021) with minor changes.…”