COLLINEARITY BETWEEN A 30-CENTIMORGAN SEGMENT OF ARABIDOPSIS-THALIANACHROMOSOME-4 AND DUPLICATED REGIONS WITHIN THE BRASSICA-NAPUS GENOME

Arabidopsis thaliana (the model dicotyledonous plant) is closely relat ed to Brassica crop species. Genome collinearity, or conservation of m arker order, between Brassica napus (oilseed rape) and A. thaliana was assessed over a 7.5-Mbp region of the long arm of A. thaliana chromos ome 4, equivalent to 30 cM. Estimates of copy number indicated that se quences present in a single copy in the haploid genome of A. thaliana (n = 5) were present in 2-8 copies in the haploid genome of B. napus ( n = 19), while sequences present in multiple copies in A. thaliana wer e present in over 10 copies in B. napus. Genetic mapping in B. napus o f DNA markers derived from a segment of A. thaliana chromosome 4 revea led duplicated homologous segments in the B. napus genome. Physical ma pping in A. thaliana of homologues of Brassica clones derived from the se regions confirmed the identity of sic duplicated segments with subs tantial homology to the 7.5-Mbp region of chromosome 4 in A. thaliana. These six duplicated Brassica regions (on average 22 cM in length) ar e collinear, except that two of the six copies contain the same large internal inversion. These results have encouraging implications for th e feasibility of shuttling between the physical map of A. thaliana and genetic maps of Brassica species, for identifying candidate genes and for map based gene cloning in Brassica crops.