Evidence for host-specific clades of tetraphyllidean tapeworms (Platyhelminthes : Eucestoda) revealed by analysis of 18S ssrDNA

Sequence data from the V4 and V7-V9 variable regions of the 18S small subun it ribosomal DNA (ssrDNA) gene were used to examine relationships among 26 tetraphyllidean and two lecanicephalidean taxa. Newly collected specimens o f 21 of the tetraphyllidean species were used to generate ssrDNA sequences that were combined with sequences previously available, including those of two diphyllidean taxa used for outgroup rooting. The sequences were aligned by eye according to secondary structural motifs of the conserved core of t he molecule. Of the 1520 sites in the alignment, 874 (58%) were excluded fr om analysis due to alignment gaps and lack of positional homology as inferr ed by manual inspection. Genetic variability of the ssrDNA gene regions com pared was greater than would be expected, based on the present taxonomy of the ingroup species, and the genetic divergences among tetraphyllidean 'fam ilies' and genera were comparable to that among tapeworm orders. Phylogenet ic hypotheses were generated by the methods of maximum parsimony and maximu m likelihood (GTR + I + Gamma nucleotide substitution model). Four most par simonious trees resulted from analysis by maximum parsimony. Strict consens us of the four trees supported the monophyly of the Tetraphyllidea, with th e lecanicephalidean taxa forming a sister lineage. Among the tetraphyllidea n taxa included in the analysis were three major clades: a basal clade incl uding species of the phyllobothriid genera Anthocephalum, Echeneibothrium, Rhinebothrium, Rhodobothrium and Spongiobothrium; a clade uniting the phyll obothriids of the genus Duplicibothrium with the dioecotaeniid genus Dioeco taenia; and a larger sister clade to the Duplicibothrium + Dioecotaenia cla de that included the phyllobothriid genera Caulobothrium, Ceratobothrium, C listobothrium, Paraorygmatobothrium and Plosobothrium, the litobothriid gen us Litobothrium and the onchobothriid genera Acanthobothrium, Calliobothriu m, Phoreiobothrium and Platybothrium. Maximum likelihood analysis resulted in a topology that was congruent where nodes were strongly supported by par simony analysis, but differed in the relative positions of the well-support ed clades. In addition, maximum likelihood analysis grouped the lecanicepha lidean taxa among the tetraphyllidean taxa, indicating paraphyly of the ord er Tetraphyllidea as currently defined. Relationships suggested by both met hods of analysis reflected common host associations of the taxa better than their current classification, suggesting that coevolution has had a signif icant role in the evolution of the group. (C) 1999 Australian Society for P arasitology Inc. Published by Elsevier Science Ltd. All rights reserved.