Interrelationships and evolution of the tapeworms (Platyhelminthes : Cestoda)

Interrelationships of the tapeworms (Platyhelminthes: Cestoda) were examine d by use of small (SSU) and large (LSU) subunit ribosomal DNA sequences and morphological characters. Fifty new complete SSU sequences were added to 2 1 sequences previously determined, and 71 new LSU (D1-D3) sequences were de termined for the complementary set of taxa representing each of the major l ineages of cestodes as currently understood. New sequences were determined for three amphilinidean taxa, but were removed from both alignments due to their excessively high degree of divergence from other cestode sequences. A morphological character matrix coded for supraspecific taxa was constructe d by the modification of matrices from recently published studies. Maximum- parsimony (MP) analyses were performed on the LSU, SSU, LSU+SSU, and morpho logical data partitions, and minimum-evolution (ME) analyses utilizing a ge neral time reversible model of nucleotide substitution including estimates of among-site rate heterogeneity were performed on the molecular data parti tions. Resulting topologies were rooted at the node separating the Gyrocoty lidea from the Eucestoda. The LSU data were found to be more informative th an the SSU data and were more consistent with inferences from morphology, a lthough nodal support was generally weak for most basal nodes. One class of transitions was found to be saturated for comparisons between the most dis tantly related taxa (gyrocotylideans vs cyclophyllideans and tetrabothriide ans), Differences in the topologies resulting from MP and ME analyses were not statistically significant. Nonstrobilate orders formed the basal lineag es of trees resulting from analysis of LSU data and morphology. Difossate o rders were basal to tetrafossate orders, the latter of which formed a stron gly supported clade. A clade including the orders Cyclophyllidea, Nippotaen iidea, and Tetrabothriidea was supported by all data partitions and methods of analysis. Paraphyly of the orders Pseudophyllidea, Tetraphyllidea, and Trypanorhyncha was consistent among the molecular data partitions. Inferenc es are made regarding a monozoic (nonsegmented) origin of the Eucestoda as represented by the Caryophyllidea and for the evolution of the strobilate a nd acetabulate/tetrafossate conditions having evolved in a stepwise pattern . (C) 2001 Academic Press.