Phylogeny of the Trichostrongylina (Nematoda) inferred from 28S rDNA sequences

We produced a molecular phylogeny of species within the order Strongylida ( bursate nematodes) using the D1 and D2 domains of 28S rDNA, with 23 new seq uences for each domain. A first analysis using Caenorhabditis elegans as an outgroup produced a tree with low resolution in which three taxa (Dictyoca ulus filaria, Dictyocaulus noerneri, and Metastrongylus pudendotectus) show ed highly divergent sequences. In a second analysis, these three species an d C. elegans were removed and an Ancylostomatina, Bunostomum trigonocephalu m, was chosen (on the basis of previous morphological analyses) as the outg roup for an analysis of the phylogenetic relationships between and within t he Strongylina (strongyles) and Trichostrongylina (trichostrongyles). A ver y robust tree was obtained. The Trichostrongylina were monophyletic, but th e Strongylina were paraphyletic, though this requires confirmation. Within the Trichostrongylina, the three superfamilies defined from morphological c haracters are confirmed, with the Trichostrongyloidea sister group to a cla de including the Molineoidea and Heligmosomoidea. Within the Trichostrongyl oidea, the Cooperiidae, Trichostrongylidae, and Haemonchidae were polytomou s, the Haemonchinae were monophyletic, but the Ostertagiinae were paraphyle tic. The sister-group relationships between Molineoidea and Heligmosomoidea were unsuspected from previous morphological analysis. No unequivocal morp hological synapomorphy could be found for the grouping Molineoidea + Heligm osomoidea, but none was found which contradicted it. (C) 2001 Academic Pres s.