Slipped-strand mispairing in a plastid gene: rpoC2 in grasses (Poaceae)

An exception to the generally conservative nature of plastid gene evolutionis the gene coding for the beta" subunit of RNA polymerase, rpoC2. Previouswork by others has shown that maize and rice have an insertion in thecoding region of rpoC2, relative to spinach and tobacco. To assess thedistribution of this extra coding sequence, we surveyed a broadphylogenetic sample comprising 55 species from 17 angiosperm families byusing Southern hybridization. The extra coding sequence is restricted tothe grasses (Poaceae). DNA sequence analysis of 11 species from all fivesubfamilies within the grass family demonstrates that the extra sequence inthe coding region of rpoC2 is a repetitive array that exhibits more than atwofold increase in nucleotide substitution, as well as a large number ofinsertion/deletion events, relative to the adjacent flanking sequences. Thestructure of the array suggests that slipped-strand mispairing causes therepeated motifs and adds to the mechanisms through which the codingsequence of plastid genes are known to evolve. Phylogenetic analyses basedon the sequence data from grass species support several relationshipspreviously suggested by morphological work, but they are ambiguous aboutbroad relationships within the family.