The Synaptonemal Complex Shapes the Crossover Landscape Through Cooperative Assembly, Crossover Promotion and Crossover Inhibition During Caenorhabditis elegans Meiosis

Abstract

The synaptonemal complex (SC) is a highly ordered proteinaceous structure that assembles at the interface between aligned homologous chromosomes during meiotic prophase. The SC has been demonstrated to function both in stabilization of homolog pairing and in promoting the formation of interhomolog crossovers (COs). How the SC provides these functions and whether it also plays a role in inhibiting CO formation has been a matter of debate. Here we provide new insight into assembly and function of the SC by investigating the consequences of reducing (but not eliminating) SYP-1, a major structural component of the SC central region, during meiosis in Caenorhabditis elegans. First, we find an increased incidence of double CO (DCO) meiotic products following partial depletion of SYP-1 by RNAi, indicating a role for SYP-1 in mechanisms that normally limit crossovers to one per homolog pair per meiosis. Second, syp-1 RNAi worms exhibit both a strong preference for COs to occur on the left half of the X chromosome and a significant bias for SYP-1 protein to be associated with the left half of the chromosome, implying that the SC functions locally in promoting COs. Distribution of SYP-1 on chromosomes in syp-1 RNAi germ cells provides strong corroboration for cooperative assembly of the SC central region and indicates that SYP-1 preferentially associates with X chromosomes when it is present in limiting quantities. Further, the observed biases in the distribution of both COs and SYP-1 protein support models in which synapsis initiates predominantly in the vicinity of pairing centers (PCs). However, discontinuities in SC structure and clear gaps between localized foci of PC-binding protein HIM-8 and X chromosome-associated SYP-1 stretches allow refinement of models for the role of PCs in promoting synapsis. Our data suggest that the CO landscape is shaped by a combination of three attributes of the SC central region: a CO-promoting activity that functions locally at CO sites, a cooperative assembly process that enables CO formation in regions distant from prominent sites of synapsis initiation, and CO-inhibitory role(s) that limit CO number.

Numerical and spatial patterning of yeast meiotic DNA breaks by Tel1

“…This and findings from other organisms (Wojtasz et al 2009;Hayashi et al 2010;Kauppi et al 2013) have led to the hypothesis that homologous chromosomes that successfully engage one another largely stop making DSBs (Kauppi et al 2013;Keeney et al 2014;Thacker et al 2014). This negative feedback regulation, which (Thacker et al 2014), is proposed to be distinct from Tel1-mediated DSB control .…”

Manipulation of Karyotype inCaenorhabditis elegansReveals Multiple Inputs Driving Pairwise Chromosome Synapsis During Meiosis

“…Recent work has demonstrated that the C. elegans SC central region proteins are important not only for promoting the formation of meiotic COs, but also for limiting their numbers by contributing to CO interference (Hayashi et al 2010;Libuda et al 2013). Inherent in the concept of CO interference is the ability of a nascent CO to change the environment in which it occurs in a manner that inhibits other recombination events in its vicinity from maturing into COs.…”

Proteomic identification of germline proteins inCaenorhabditis elegans

“…It is possible that this increased efficiency reflects our use of the C. elegans strain AZ212 as host for feeding RNAi, as this strain has been demonstrated to be more sensitive to RNAi. 28 However, it also likely reflects the direct advantage of targeted screening (the genes assayed were preselected for likely involvement in fertility); further, smaller-scale screens such as this also allow increased sensitivity of assays-by screening only a limited number of candidates, we can carry out assays allowing detection of more modest reductions in fertility than could be assayed in genomewide screening.…”

“…Feeding RNAi was done in strain AZ212 (reported to have higher sensitivity to RNAi). 28 Standard strain propagation for proteomics took place at 16 C on nematode growth medium (NGM) plates seeded with E. coli strain OP50; liquid culture was done in S medium supplemented with OP50 at 25 C. 30 Feeding RNAi was carried out at 20 C.…”