THE EFFECTIVENESS OF MITOCHONDRIAL RIBOSOMAL-RNA GENE-SEQUENCES FOR THE RECONSTRUCTION OF THE PHYLOGENY OF AN INSECT ORDER (ORTHOPTERA)

We investigated the value of mitochondrial rRNA sequences for analyzin g pre-Cainozoic divergence events in insects. Using small subunit and large subunit rRNA sequences from 38 orthopteroid species, we examined several aspects of sequence evolution including secondary structure, substitution rate, and base composition. Substitution matrices calcula ted from the two genes were very similar, though differences were dete cted in rates of C-T transitions between paired and unpaired positions in secondary structures. By contrast, extreme disparities between sub stitution frequencies at different phylogenetic levels make character- transition weighting essential in parsimony reconstruction. The analys is of base composition indicated that branch attraction of at least tw o important lineages was due to shared base composition biases and to long branch attraction, The importance of taxonomic sampling and seque nce length for the effectiveness of phylogenetic recovery using the rR NA fragments was also assessed. Significantly, combining the two seque nces seemed both justifiable and necessary for this taxonomic sample. However, for reconstructing deep branches of phylogeny, it seems that increasing either or both the number of taxa or nucleotide positions w ill not necessarily solve all problems. Instead, the resolution of anc ient branching events using mtDNA sequences probably depends upon the development and application of better specified reconstruction methods . (C) 1997 Academic Press.