Phylogenetic relationships among species of Contracaecum Railliet & Henry,1912 and Phocascaris Host, 1932 (Nematoda : Ascaridoidea) based on nuclearrDNA sequence data

Nuclear-encoded large-subunit ribosomal DNA sequences were used to infer a phylogenetic hypothesis for 17 taxa (16 nominal species) of the genera Cont racaecum and Phocascaris. Phylogenetic trees based on these data have been used to assess the validity of the taxonomic distinction between these gene ra, which was based on the presence or absence of certain structural featur es, rather than on explicit hypotheses of evolutionary history. Phylogeneti c hypotheses based on parsimony, likelihood, and neighbor-joining analyses of these sequence data strongly support the hypothesis that species of Phoc ascaris are nested within the clade of Contracaecum species hosted by phoci d seals, and are more closely related to species of the Contracaecum oscula tum complex than to other Contracaecum species. Alternative tree topologies representing Phocascaris as not nested within the C. osculatum complex wer e significantly worse interpretations of these sequence data. Phylogenetic analysis also provides strong support for the monophyly of all taxa (Contra caecum and Phocascaris) from phocid seals, which is consistent with Berland 's (1964) proposal that such species form a natural group; however, his pro posal to recognize all species in phocid seals as Phocascaris, with all spe cies from birds as Contracaecum would result in a paraphyletic Contracaecum , according to the molecular phylogenetic hypothesis.