DEUTEROSTOME PHYLOGENY AND THE SISTER GROUP OF THE CHORDATES - EVIDENCE FROM MOLECULES AND MORPHOLOGY

Complete coding regions of the 18S rRNA gene of an enteropneust hemich ordate and an echinoid and ophiuroid echinoderm were obtained and alig ned with 188 rRNA gene sequences of all major chordate clades and four outgroups. Gene sequences were analyzed to test morphological charact er phylogenies and to assess the strength of the signal. Maximum-parsi mony analysis of the sequences fails to support a monophyletic Chordat a; the urochordates form the sister taxon to the hemichordates, and to gether this clade plus the echinoderms forms the sister taxon to the c ephalochordates plus craniates. Decay, bootstrap, and tree-length dist ribution analyses suggest that the signal for inference of deuterostom e phylogeny is weak in this molecule. Parsimony analysis of morphologi cal plus molecular characters supports both monophyly of echinoderms p lus enteropneust hemichordates and a sister group relationship of this clade to chordates. Evolutionary parsimony does not support chordate monophyly. Neighbor-joining, Fitch-Margoliash, and maximum-likelihood analyses support a chordate lineage that is the sister group to an ech inoderm-plus-hemichordate lineage. The results illustrate both the lim itations of the 188 rRNA molecule alone for high-level phylogeny infer ence and the importance of considering both molecular and morphologica l data in phylogeny reconstruction.