Molecular systematics of some North American species of Diplostomum (Digenea) based on rDNA-sequence data and comparisons with European congeners

The systematics of Diplostomum species, common intestinal parasites of piscivorous birds, has long been problematic, owing to phenotypic plasticity and the paucity of morphological features that are often subject to age- and host-induced variation. We sequenced the ITS1–5.8S–ITS2 regions of the rDNA from adult Diplostomum huronense, Diplostomum indistinctum, and Diplostomum baeri obtained from experimentally infected ring-bill gulls (Larus delawarensis) and compared them with partial ITS1 sequences from several species of Diplostomum in GenBank. The three North American species were distinguishable on the basis of ITS sequences. Sequences from D. huronense differed from those of D. indistinctum at 12 sites in ITS1 and 4 sites in ITS2, supporting morphological and morphometric data that indicate the two are distinct species. Sequences of D. huronense and D. indistinctum differed from those of D. baeri at 27 and 24 sites, respectively, in ITS1 and 15 and 12 sites, respectively, in ITS2. Phylogenetic analysis of partial ITS1 sequences revealed that the North American and European species of Diplostomum formed separate groups, with the former being basal to the latter. The results indicated that D. huronense and D. indistinctum from North America are distinct from Diplostomum spathaceum and other similar species from Europe. Furthermore, sequences from specimens identified as D. baeri from North America differed from those of D. baeri from Europe by 3.8% in ITS1 (23 sites). While morphologically similar, the two are not conspecific. Sequences of the North American species have been deposited in GenBank (AY 123042–123044).