THE USE OF CHLOROPLAST DNA TO RESOLVE PLANT PHYLOGENIES - NONCODING VERSUS RBCL SEQUENCES

Direct sequencing of polymerase chain reaction products is now an expa nding area of plant systematics and evolution. Within angiosperms the rbcL gene has been widely sequenced and used for inferring plant phylo genies at higher taxonomic levels. Unfortunately rbcL does not usually contain enough information to resolve relationships between closely r elated genera, such as Hordeum, Triticum, and Aegilops. One solution t o this problem could be to analyze noncoding regions of chloroplast DN A, which are supposed to evolve more rapidly than coding regions. Here we present pairwise comparisons among dicots and monocots for rbcL an d two noncoding sequences of cpDNA (the trnL (UAA) intron and the inte rgenic spacer between the trnL (UAA) 3' exon and the trnF (GAA) gene). It appears that these regions evolve faster (more than three times fa ster, on average) than rbcL, as previously reported, and that the trnL intron evolves at a rate that is the same as that of the intergenic s pacer. By the analysis of these regions, the genera Hordeum, Triticum, and Aegilops clearly could be distinguished. A phylogeny using trnL ( UAA) intron sequences is also inferred far some species of the genus G entiana L., clearly illustrating the phylogenetic utility of these zon es at the generic level. The advantages and the disadvantages of the u se of these regions to resolve plant phylogenies are discussed, as wel l as the desirability of a preliminary study before every large-scale analysis.