A molecular phylogeny of the Eucestoda.

Evolutionary relationships among the orders of the Eucestoda were examined based on data derived from partial sequences of genes encoding 18S rRNA. Considered in this study were 47 species-level taxa, including 1 from Monogenea, 1 from Amphilinidea, both of which were used as outgroups, and 45 from 10 orders of Eucestoda. Parsimony analysis of 1.1 kbp of aligned fragments yielded 480 shortest trees (length 704; consistency index 0.41) the strict consensus of which shows the following relationships: (Monogenea, (Amphilinidea, (Caryophyllidea, (Spathebothriidea, (Trypanorhyncha, (Pseudophyllidea, (Tetraphyllidea, ((Diphyllidea, Proteocephalidea), ((Cyclophyllidea, Tetrabothriidea), Nippotaeniidea))))))))). The Tetraphyllidea, Pseudophyllidea (because of the Diphyllobothriidae), and Cyclophyllidea (because of the Mesocestoididae) were found to be paraphyletic. This tree is almost completely congruent with the most recent hypotheses based on characters derived from comparative morphology, ontogeny, and ultrastructural studies. The only discrepancies between these separate analyses were in the position of the Trypanorhyncha and Diphyllidea. Among the main conclusions corroborated here are the following: (1) the basal position of the Caryophyllidea (and Spathebothriidea), and hence the plesiomorphic condition of monozoism; (2) the monophyly of the "higher tetrafossates" provided the nippotaeniids and tetrabothriids are included; (3) the close relationship between the Tetrabothriidea and Cyclophyllidea; and (4) the paraphyly of the Tetraphyllidea. At lower taxonomic levels, structure could not be defined within Proteocephalidea; within the Cyclophyllidea, all families represented by several taxa (Hymenolepididae, Davaineidae, and Dilepididae) were found to be monophyletic, and Taeniidae may be the most basal family. Furthermore, the validity of the Gryporhynchidae is confirmed. and it is suggested that the Mesocestoididae should be excluded from the Cyclophyllidea.