MEIOTIC RECOMBINATION, NONCODING DNA AND GENOMIC ORGANIZATION IN CAENORHABDITIS-ELEGANS

The genetic map of each Caenorhabditis elegans chromosome has a centra l gene cluster (less pronounced on the X chromosome) that contains mos t of the mutationally defined genes. Many linkage group termini also h ave clusters, though involving fewer loci. We examine the factors shap ing the genetic map by analyzing the rate of recombination and gene de nsity across the genome using the positions of cloned genes and random cDNA clones from the physical map. Each chromosome has a central gene -dense region (more diffuse on the X) with discrete boundaries, flanke d by gene-poor regions. Only autosomes have reduced rates of recombina tion in these gene-dense regions. Cluster boundaries appear discrete a lso by recombination rate, and the boundaries defined by recombination rate and gene density mostly, but not always, coincide. Terminal clus ters have greater gene densities than the adjoining arm but similar re combination rates. Thus, unlike in other species, most exchange in C. elegans occurs in gene-poor regions. The recombination rate across eac h cluster is constant and similar; and cluster size and gene number pe r chromosome are independent of the physical size of chromosomes. We p ropose a model of how this genome organization arose.