Characterization of the rbcS multigene family in wheat:: subfamily classification, determination of chromosomal location and evolutionary analysis

To elucidate the evolution of a multigene family in plants. nucleotide sequences of members of the rbcS multigene family encoding the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase in wheat were determined. Five genomic clones containing rbcS sequences were isolated. The known members of the rbcS family of common wheat, including the clones reported here, were categorized into two Classes (I and II), which, in turn, were classified into three (A, B and E) and two (C and D) subtypes, respectively. The sequences of the intron and the 3 ' flanking region were divergent between the classes bur not among the types within a class. The differences among the types within a class were caused by large indels (about 100-350 bp) in the intron. The members of Classes I and II were located on homeologous group 2 and 5 chromosomes, respectively. The Southern hybridization data indicated that all types of rhcS were present in the ancestral species of wheat. Thus. it is concluded that the divergence between the classes originated from an interchromosomal duplication and that several intrachromosomal duplications followed: these events occurred before the speciation of diploids. The detection of nonfunctional genes and the elimination of fragments suggests that several mechanisms were involved in reducing the copy number of rbcS during the evolution of wheat.