Heterozygous insertions alter crossover distribution but allow crossover interference in Caenorhabditis elegans.

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.