One crossover is enough

When it comes to crossover recombination during meiosis, once is usually enough. Most worm chromosomes form one and only one crossover between homologues during meiosis I, and other eukaryotes spread out their crossovers so that only a few form. Now, Kenneth Hillers and Anne Villeneuve (Stanford University, Stanford, CA) find that the worm's once-only rule applies even to fusion chromosomes that contain nearly half of the genome.

The fusions—between either two or three worm chromosomes—resulted in a shrinkage of the genetic map, so that the giant chromosomes showed an average of one crossover each. This suggests that crossover formation is not sequence specific, and that the mechanism that inhibits formation of more than one crossover can spread over entire enlarged chromosomes.

The spreading capability is tempered in heterozygotes. In these cases, with one giant chromosome lined up...