Finally, we used our kinetic information to derive the total number of DSBs. We found an average of 40 DSBs per nucleus in wild-type meiosis suggesting a ratio of 7:1 of DSBs to crossovers, and more DSBs in mutant scenarios.

Second, strand invasion kinetics were similar for repair events templated by the homolog versus the sister. This was surprising, since a leading hypothesis is that DSBs that don't find the homologs stall and are only repaired at the end of meiosis using the sister chromatid.

We made 3 crucial findings. First, we found that most repair events finish the strand-invasion step in 1-2 hours. This is true for both endogenous (SPO-11-induced) and irradiation-induced DSBs.

New preprint from the lab! We defined the kinetics of strand invasion during meiosis in C. elegans. Great work from Antonia, Henry and Divya.
www.biorxiv.org/content/10.1...

In both nematode, Skp1 is not only necessary for assembly of the SC onto chromosomes - without dimerization-competent Skp1, SC proteins are absent.

Lisa turned to the distantly related nematode P. pacificus, and found that the answer is a resounding ‘yes’. Ppa-SKR-1 localize to the middle of the SC, and a conserved dimerization interface in Skp1 is specifically required for SC assembly in Pristi, as it is in elegans.

That has crucial implications: ZHP-3 can sample the entire 6um chromosome in tens of minutes, whereas SYP-3 cannot. By extension, ZHP-3 is capable of efficiently transducing a crossover signal, whereas SYP-3 would be unlikely to.

The second important finding came from comparing the diffusion of an SC component (SYP-3) vs a regulator of crossovers (ZHP-3). ZHP-3 diffuses 4-9 times faster than SYP-3 (depending on meiotic stage).

However, a crucial piece of this model has not been tested: do molecules diffuse within the SC? Lexy directly tested that. By sparsely labeling SC components and crossover regulators, she was able to observe single molecules in live gonads.

Our data revealed sliding of cohesin on DNA (presumably loop extrusion); positioning of nucleosomes; and, AFAIK, the first binding patterns of cohesins in the rDNA locus. Much more data inside.

We also performed a CheC-PLS on isolated nuclei, using Rec8-GFP and nanobodies targeted to GFP and tethered to a methyltransferase. (This variation is related to recently published techniques like DiMeLo-seq and SAMOSA). cc @astraight.bsky.social

#1: CheC-PLS (pronounced ‘check, please’) is a new genomic technique. It relates to ChIP, but adds 4 crucial features: 1. contiguity (how distant events are regulated) 2. history (where a protein has been) 3. heterogeneity (single cell data) 4. context (mapping repeats).

The story starts with a saturated suppressor screen for a temperature-sensitive mutation that destabilizes the SC. Lisa identified dozens of suppressors, that all mapped to three short stretches in SC proteins.