Independent host switching events by digenean parasites of cetaceans inferred from ribosomal DNA.

Cetaceans harbour a unique fauna of digeneans whose origin and relationships have sparked considerable debate during recent decades. Disparity in the species reported indicates that they do not share close affinities, but their unusual morphology has made their taxonomic identities and phylogenetic positions uncertain. Here we use sequence data to investigate the phylogenetic relationships of the main species of flukes infecting cetaceans. We sequenced the 18S, 28S and internal transcribed spacer 2 rDNA of digenean species representing all known families reported from cetaceans: Braunina cordiformis (Brauninidae), Ogmogaster antarcticus (Notocotylidae), Pholeter gastrophilus (Heterophyidae), and Campula oblonga, Nasitrema sp. and Oschmarinella rochebruni (Brachycladiidae). The phylogenetic position of the taxa was estimated by Bayesian inference and maximum likelihood incorporating published sequences of 177 species of Digenea. Further Bayesian and maximum likelihood analyses were performed with sequences of 14 Heterophyidae and Opisthorchiidae taxa, incorporating new sequences of P. gastrophilus. Species nominally assigned to the Brachycladiidae formed a clade that was embedded among species of the Acanthocolpidae, thus making the latter family paraphyletic. Braunina cordiformis formed a sister lineage to the Strigeidae and Diplostomidae, whereas O. antarcticus was placed within the Notocotylidae, in agreement with the previous taxonomy of this genus. Similarly, P. gastrophilus was placed within the Heterophyidae as originally described. Our results suggest a paraphyletic relationship between the Heterophyidae and Opisthorchiidae, mirroring the uncertain taxonomic placement of P. gastrophilus, which has been assigned to both families in the past. The digenean families involved are parasites of fish-eating birds and mammals (i.e. Strigeidae, Diplostomidae and Heterophyidae), parasites of marine fish (i.e. Acanthocolpidae) and other herbivorous aquatic birds and mammals (i.e. Notocotylidae). The phylogenetic positions of these taxa indicate that the digenean fauna of cetaceans may have been acquired through independent host-capture events, with two clades showing subsequent diversification exclusively among marine mammals.