Heterozygous Insertions Alter Crossover Distribution but Allow Crossover Interference in Caenorhabditis elegans

Abstract

The normal distribution of crossover events on meiotic bivalents depends on homolog recognition, alignment, and interference. We developed a method for precisely locating all crossovers on Caenorhabditis elegans chromosomes and demonstrated that wild-type animals have essentially complete interference, with each bivalent receiving one and only one crossover. A physical break in one homolog has previously been shown to disrupt interference, suggesting that some aspect of bivalent structure is required for interference. We measured the distribution of crossovers in animals heterozygous for a large insertion to determine whether a break in sequence homology would have the same effect as a physical break. Insertions disrupt crossing over locally. However, every bivalent still experiences essentially one and only one crossover, suggesting that interference can act across a large gap in homology. Although insertions did not affect crossover number, they did have an effect on crossover distribution. Crossing over was consistently higher on the side of the chromosome bearing the homolog recognition region and lower on the other side of the chromosome. We suggest that nonhomologous sequences cause heterosynapsis, which disrupts crossovers along the distal chromosome, even when those regions contain sequences that could otherwise align. However, because crossovers are not completely eliminated distal to insertions, we propose that alignment can be reestablished after a megabase-scale gap in sequence homology.

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

“…Single-nucleotide polymorphism genotyping: Markers used, corresponding primer sequences, and overall SNP genotyping strategy followed the general protocol of Hammarlund et al (2005). Heterozygote F 1 hermaphrodites from the cross AZ212 3 Hawaiian were plated individually at the L4 stage and F 2 worms were picked into PCR tubes containing 10 ml lysis buffer.…”

“…This method reliably detects the majority of DCO products, but as discussed in detail by Hammarlund et al (2005), it underestimates the total number of DCOs because a subset of DCO products cannot be identified unambiguously. For example, the genotypes of some individuals carrying one DCO product and one noncrossover (NCO) product will be indistinguishable from those of some individuals carrying two single-crossover (SCO) products.…”

“…First, essentially all COs in C. elegans depend on conserved meiotic CO-promoting machinery (i.e., Msh4 and Msh5) and on SC central region proteins (SYP-1, -2, -3, and -4), so analysis is generally not complicated by residual COs forming by alternative pathways (Zalevsky et al 1999;Kelly et al 2000;MacQueen et al 2002;Colaiacovo et al 2003;Smolikov et al 2007aSmolikov et al , 2009. Second, C. elegans hermaphrodites exhibit robust CO control, with COs usually being limited to one per homolog pair per meiosis (Hillers and Villeneuve 2003;Nabeshima et al 2004;Hammarlund et al 2005). Consequently, circumstances that give rise to double crossover (DCO) meiotic products can be inferred to represent impairment of mechanisms that normally inhibit CO formation.…”

Crossover Heterogeneity in the Absence of Hotspots inCaenorhabditis elegans

“…With the genome sequence (C. elegans Sequencing Consortium 1998), extensive annotation (Gerstein et al 2010), and characterization of chromatin modification (Liu et al 2011), it has become clear that the low-recombination central regions of C. elegans chromosomes are gene dense and enriched for highly expressed genes and markers of open chromatin, while the high-recombination arms are enriched for repetitive sequences, genes with low expression, and markers of closed chromatin. C. elegans chromosomes exhibit nearly complete crossover interference, with elaborate regulatory mechanisms that ensure that each pair of chromosomes receives exactly one crossover per meiosis (Hillers and Villeneuve 2003;Hammarlund et al 2005). Variation in crossover frequency along the chromosomes therefore reflects the location preferences of the single crossover event.…”

“…elegans chromosomes exhibit nearly complete crossover interference, with elaborate regulatory mechanisms that ensure that each pair of chromosomes receives exactly one crossover per meiosis (Hillers and Villeneuve 2003;Hammarlund et al 2005). Variation in crossover frequency along the chromosomes therefore reflects the location preferences of the single crossover event.…”