Organization of an Arabidopsis thaliana gene cluster on chromosome 4 including the RPS2 gene, in the Brassica nigra genome

 Genetic and physical maps, consisting of a large number of DNA markers for Arabidopsis thaliana chromosomes, represent excellent tools to determine the organization of related genomes such as those of Brassica. In this paper we report the chromosomal localization and physical analysis by pulsed-field gel electrophoresis (PFGE) of a well-defined gene complex of A. thaliana in the Brassica nigra genome (B genome n=8). This complex is approximately 30 kb in length in A. thaliana and contains a cluster of six genes including ABI1 (ABA-responsive), RPS2 (resistance against Pseudomonas syringae, a bacterial disease), CK1 (casein kinase I), NAP (nucleosome-assembly protein), X9 and X14 (both of unknown function). The Arabidopsis chromosomal complex was found to be duplicated and conserved in gene number at different levels in the Brassica genome. Linkage group B1 had the most-conserved arrangement carrying all six genes tightly linked. Group B4 had an almost complete complex except for the absence of RPS2. Other partial complexes of fewer members were found on three other chromosomes. Our studies demonstrate that by this approach it is possible to identify ancestrally related chromosome segments in a complex and duplicated genome, such as the genome of B. nigra, permitting one to draw conclusions as to its origin and evolution. Received: 11 July 1997 / Accepted: 9 October 1997