Assessing the usefulness of histone H3, U2 snRNA and 28S rDNA in analyses of polychaete relationships

DNA sequence data from for histone H3 (34 species), U2 snRNA (34 species) a nd two segments (D1 and D9-10 expansion regions) of 28S rDNA (28 and 26 spe cies, respectively) have been collected to investigate the relationships of polychaetes. Representatives of all of the major morphologically identifie d clades were used, as well as members of the Sipuncula, Echiura, Turbellar ia, Clitellata and Siboglinidae (formerly the phyla Pogonophora and Vestime ntifera). Maximum parsimony analyses of the separate data sets gave conflicting resul ts and none conformed closely to previous results based on morphology. Inst ead each data set provided corroboration of a few of the morphological grou pings, usually pairing, though inconsistently, members of the same family. Higher groupings proposed on morphological grounds were rarely recovered. Maximum parsimony analysis of the combined data, excluding areas of uncerta in alignment, recovered some morphological groupings such as Cirratulidae, Terebellidae, scale worms and eunicimorphs, and did not significantly contr adict others. However, some expected groupings were not recovered. Surprisi ngly, the fanworms (Sabellidae and Serpulidae) were not shown as sister tax a, and monophyly of Phyllodocida, a morphologically well corroborated clade , required four more steps than most parsimonious trees. Aciculata was not seen in our analyses, although it was the most strongly supported large cla de in Rouse and Fauchald (1997, Cladistics and polychaetes. Zoologica Scrip ta 26, 138-204). Trees constrained to show Aciculata as monophyletic were 1 8 steps longer than the most parsimonious trees. If trees are rooted on sip unculans rather than the nematode, Aciculata is nearly recovered, being ren dered paraphyletic by the inclusion of the sister-pair of Oweniidae and Cha etopteridae. As suggested by some recent morphological and molecular analyses, Siboglini dae and Clitellata may well have sister groups among polychaetes. The morph ologically aberrant Sternaspidae are closest to members of Terebellida in t he present analyses, supporting the placement of Rouse and Fauchald. Intere sting results deserving further assessment concern the placement of Chaetop teridae, Oweniidae and Sipuncula.