COMPARATIVE MAPPING BETWEEN ARABIDOPSIS-THALIANA AND BRASSICA-NIGRA INDICATES THAT BRASSICA GENOMES HAVE EVOLVED THROUGH EXTENSIVE GENOME REPLICATION ACCOMPANIED BY CHROMOSOME FUSIONS AND FREQUENT REARRANGEMENTS

Chromosome organization and evolution in the Brassicaceae family was s tudied using comparative linkage mapping. A total of 160 mapped Arabid opsis thaliana DNA fragments identified 284 homologous loci covering 7 51 cM in Brassica nigra. The data support that modern diploid Brassica species are descended from a hexaploid ancestor, and that the A. thal iana genome is similar in structure and complexity to those of each of the hypothetical diploid progenitors of the proposed hexaploid. Thus, the Brassica lineage probably went through a triplication after the d ivergence of die lineages leading to A. thaliana and B. nigra. These d uplications were also accompanied by an exceptionally high rate of chr omosomal rearrangements. The average length of conserved segments betw een A. thaliana and B. nigra was estimated at 8 cM. This estimate corr esponds to similar to 90 rearrangements since the divergence of the tw o species. The estimated rate of chromosomal rearrangements is higher than any previously reported data based on comparative mapping. Despit e the large number of rearrangements, fine-scale comparative mapping b etween model plant A. thaliana and Brassica crops is likely to result in the identification of a large number of genes that affect important traits in Brassica crops.