CHARACTERIZATION OF A MEIOTIC CROSSOVER IN MAIZE IDENTIFIED BY A RESTRICTION FRAGMENT LENGTH POLYMORPHISM-BASED METHOD

Genetic map lengths do not correlate directly with genome size, sugges ting that meiotic recombination is not uniform throughout the genome. Further, the abundance of repeated sequences in plant genomes requires that crossing over is restricted to particular genomic regions. We us ed a physical mapping approach to identify these regions without the b ias introduced by phenotypic selection. This approach is based on the detection of nonparental polymorphisms formed by recombination between polymorphic alleles. In an F-2 population of 48 maize planes, we iden tified a crossover at two of the seven restriction fragment length pol ymorphism loci tested. Characterization of one recombination event rev ealed that the crossover mapped within a 534-bp region of perfect homo logy between the parental alleles embedded in a 2773-bp unique sequenc e. No transcripts from this region could be detected. Sequences immedi ately surrounding the crossover site were not detectably methylated, e xcept for an SstI site probably methylated via non-CpG or CpXpG cytosi ne methylation. Parental methylation patterns at this SstI site and at the flanking repetitive sequences were faithfully inherited by the re combinant allele. Our observations suggest that meiotic recombination in maize occurs between perfectly homologous sequences, within unmethy lated, nonrepetitive regions of the genome.