Comparative fluorescence in situ hybridization mapping of a 431-kb Arabidopsis thaliana bacterial artificial chromosome contig reveals the role of chromosomal duplications in the expansion of the Brassica rapa genome

Comparative genome studies are important contributors to our understanding of genome evolution. Most comparative genome studies in plants have been ba sed on genetic mapping of homologous DNA loci in different genomes. Large-s cale comparative physical mapping has been hindered by the lack of efficien t and affordable techniques. We report here the adaptation of fluorescence in situ hybridization (FISH) techniques for comparative physical mapping be tween Arabidopsis thaliana and Brassica rapa. A set of six bacterial artifi cial chromosomes (BACs) representing a 431-kb contiguous region of chromoso me 2 of A. thaliana was mapped on both chromosomes and DNA fibers of B. rap a. This DNA fragment has a single location in the A. thaliana genome, but h ybridized to four to six B. rapa chromosomes, indicating multiple duplicati ons in the B. ra;ba genome. The sizes of the fiber-FISH signals from the sa me BACs were not longer in B. rapa than those in A. thaliana, suggesting th at this genomic region is duplicated but not expanded in the B. mpa genome. The comparative fiber-FISH mapping results support that chromosomal duplic ations, rather than regional expansion due to accumulation of repetitive se quences in the intergenic regions, played the major role in the evolution o f the B. rapa genome.