PHYSICAL MAPPING OF MEIOTIC CROSSOVER EVENTS IN A 200-KB REGION OF NEUROSPORA-CRASSA LINKAGE GROUP-I

We propose a general restriction fragment length polymorphism-based st rategy to analyze the distribution of meiotic crossover events through out specific genetic intervals. We have isolated 64 recombinant chromo somes carrying independent meiotic crossover events in the genetic int erval eth-1-un-2 on linkage group I of Neurospora crassa. Thirty-eight crossover events were physically mapped with reference to a 200-kb re gion cloned by chromosome walking, using N. crassa lambda and cosmid l ibraries. Crossovers were homogeneously distributed at intervals of 5. 0 +/- 2.3 kb along the entire cloned interval. The ratio of physical t o genetic distance appears to be higher in the region than in the over all N. crassa genome, suggesting that recombinational activity is less in large chromosomes than in small ones. The present work provides a method for defining the centromeric-telomeric orientation of single cl oned DNA fragments. Their physical distance can also be estimated with respect to linked loci, provided that crossover events are distribute d homogeneously in the interval. This strategy overcomes typical diffi culties in defining the position and direction of chromosome walking s teps on conventional linkage maps.