A comparative analysis of the Lactuca and Helianthus (Asteraceae) plastid genomes: identification of divergent regions and categorization of shared repeats.

We have sequenced two complete chloroplast genomes in the Asteraceae, Helianthus annuus (sunflower), and Lactuca sativa (lettuce), which belong to the distantly related subfamilies, Asteroideae and Cichorioideae, respectively. The Helianthus chloroplast genome is 151 104 bp and the Lactuca genome is 152 772 bp long, which is within the usual size range for chloroplast genomes in flowering plants. When compared to tobacco, both genomes have two inversions: a large 22.8-kb inversion and a smaller 3.3-kb inversion nested within it. Pairwise sequence divergence across all genes, introns, and spacers in Helianthus and Lactuca has resulted in the discovery of new, fast-evolving DNA sequences for use in species-level phylogenetics, such as the trnY-rpoB, trnL-rpl32, and ndhC-trnV spacers. Analysis and categorization of shared repeats resulted in seven classes useful for future repeat studies: double tandem repeats, three or more tandem repeats, direct repeats dispersed in the genome, repeats found in reverse complement orientation, hairpin loops, runs of A's or T's in excess of 12 bp, and gene or tRNA similarity. Results from BLAST searches of our genomic sequence against expressed sequence tag (EST) databases for both genomes produced eight likely RNA edited sites (C → U changes). These detailed analyses in Asteraceae contribute to a broader understanding of plastid evolution across flowering plants.