THE SELF-INCOMPATIBILITY (S) HAPLOTYPES OF BRASSICA CONTAIN HIGHLY DIVERGENT AND REARRANGED SEQUENCES OF ANCIENT ORIGIN

In Brassica, the recognition of self-related pollen by the stigma is c ontrolled by the highly polymorphic S locus that encodes several linke d and coadapted genes and can span several hundred kilobases. We used pulsed-field gel electrophoresis to analyze the structure of different S haplotypes. We show that the S-2 and S-13 haplotypes of Brassica ol eracea contain extensive sequence divergence and rearrangement relativ e to each other. In contrast, haplotypic configuration is more conserv ed between a. oleracea S-13 and a. campestris S-8, two haplotypes that have been proposed to be derived from a common ancestral haplotype ba sed on sequence comparisons. These results support the view that exten sive restructuring of the S locus preceded speciation in Brassica. Thi s structural heteromorphism, together with haplotype-specific sequence s, may suppress recombination within the S locus complex, potentially providing a mechanism for maintaining the linkage of coadapted allelic combinations of genes over time.