INFERENCES ABOUT ORTHOPTEROID PHYLOGENY AND MOLECULAR EVOLUTION FROM SMALL-SUBUNIT NUCLEAR RIBOSOMAL DNA-SEQUENCES

We determined DNA sequences of SSU rRNA genes in twenty-nine polyneopt eran insect species and aligned these with homologues from eight other insects. In a phylogenetic analysis we recovered the classic division s of Palaeoptera and Neoptera, with the latter divided into monophylet ic Paraneoptera and Polyneoptera. The polyneopterans divided into thre e lineages: one includes the Grylloblattodea, Dermaptera and Plecopter a, the second contains the Blattodea, and the third (Orthopteroidea se nsu Hennig) contains the Embiidina, Phasmida, and Orthoptera, in that order. The monophyly of the Orthoptera is supported by the analyses, a s is the separation between taxa from its suborders Caelifera and Ensi fera. The Caelifera are not always supported as a monophyletic group; the basal Tridactyloidea are separated from the rest of the Caelifera in some analyses. Inside of Tridactyloidea, the Acridoidea, Pamphagoid ea, Pneumoroidea and Trigonopterygoidea are always recovered as a mono phyletic group. We also examined the basal orthopteran relationships, with the specific aim of assessing the antiquity of the Ensifera, Char acter state reconstructions indicated that the ancestral ensiferan seq uence is very similar to the ancestral orthopteran sequence. However, likelihood ratio tests rejected the null hypothesis of a molecular clo ck and we conclude that a change in substitution rate has occurred wit hin the Orthoptera and several of the other polyneopteran orders. Simi lar observations have been made in holometabolous insects, suggesting that variation in substitution rate is a general feature of insect nuc lear rRNA evolution.