The phylogenetic position of Udonella (Platyhelminthes).

Phylogenetic analysis of molecular data from complete 18S rRNA and partial 28S rRNA genes, of a variety of platyhelminths, places the enigmatic Udonella caligorum firmly as a monopisthocotylean monogenean. Both maximum parsimony and a modified distance measure, operating under a maximum likelihood model, gave identical solutions for each data set. These data further support morphological evidence from ultrastructural studies indicating the neodermatan affinities of Udonella, namely shared features in sensory receptors, surface tegument, sperm structure and spermiogenesis. The molecular data reject the class Udonellidea and the placement of udonellids as sister-group to the Neodermata. As shown previously with molecular data, the monogeneans appear as a paraphyletic assemblage comprising strongly monophyletic Monopisthocotylea and Polyopisthocotylea. Their relationships with the trematodes and cestodes are not resolved with 28S rDNA or 18S rDNA alone.