A review of arthropod phylogeny: New data based on ribosomal DNA sequencesand direct character optimization

Ribosomal gene sequence data are used to explore phylogenetic relationships among higher arthropod groups. Sequences of 139 taxa (23 outgroup and 116 ingroup taxa) representing all extant arthropod "classes" except Remipedia and Cephalocarida are analyzed using direct character optimization explorin g six parameter sets. Parameter choice appears to be crucial to phylogeneti c inference. The high level of sequence heterogeneity in the 18S rRNA gene (sequence length from 1350 to 2700 bp) makes placement of certain taxa with "unusual" sequences difficult and underscores the necessity of combining r ibosomal gene data with other sources of information. Monophyly of Pycnogon ida, Chelicerata, Chilopoda, Chilognatha, Malacostraca, Branchiopoda (exclu ding Daphnia), and Ectognatha are among the higher groups that are supporte d in most of the analyses. The positions of the Pauropoda, Symphyla, Protur a, Collembola, Diplura, Onychophora, Tardigrada, and Daphnia are unstable t hroughout the parameter space examined. (C) 2000The Wllli Hennig Society.