MUTANT REC-1 ELIMINATES THE MEIOTIC PATTERN OF CROSSING-OVER IN CAENORHABDITIS-ELEGANS

Meiotic crossovers are not randomly distributed along the chromosome. In Caenorhabditis elegans the central portions of the autosomes have r elatively few crossovers compared to the flanking regions. We have mea sured the frequency of crossing over for several intervals across chro mosome lin strains mutant for rec-1. The chromosome is similar to 50 m ap units in both wild-type and rec-1 homozygotes, however, the distrib ution of exchanges is very different in rec-1. Map distances expand ac ross the gene cluster and contract near the right end of the chromosom e, resulting in a genetic map more consistent with the physical map. M utations in two other genes, him-6 and him-14, also disrupted the dist ribution of exchanges. Unlike rec-1, individuals homozygous for him-G and him-14 had an overall reduction in the amount of crossing over acc ompanied by a high frequency of nondisjunction and reduced egg hatchin g. In rec-1; him-6 and rec-1; him-14 homozygotes the frequency of cros sing over was characteristic of the Him mutant phenotype, whereas the distribution of the reduced number of exchanges was characteristic of the Rec-1 pattern. It appears that these gene products play a role in establishing the meiotic pattern of exchange events.