Abstract

Evolutionary relationships within the Strigeidae and Diplostomidae (Digenea: Diplostomoidea), which are cosmopolitan parasites of vertebrates, are poorly understood. In this study, the phylogenetic relationships of genera within these groups were studied using full small (SSU), partial large (LSU), and full internal transcribed spacer regions 1 and 2 (ITS) sequences of ribosomal DNA and partial sequences of cytochrome oxidase I (COI) from mitochondrial DNA. Sequences from nine diplostomid genera (18 species) and five strigeid genera (8 species) were analyzed using maximum parsimony and maximum likelihood methods. Markers were analyzed independently and in total evidence combinations and all molecular topologies indicated paraphyletic relationships. A maximum likelihood analysis of concatenated sequences of SSU, LSU, and COI produced a tree concordant with the fewest evolutionary changes based on a matrix of 32 morphological and life-history characters. The Strigeidae and Diplostomidae form two clades. A group comprising the diplostomids Diplostomum, Tylodelphys, Alaria, Fibricola and Hysteromorpha was basal to a paraphyletic clade in which the strigeids Apharyngostrigea, Apatemon and Cotylurus, were separated from other strigeids, Ichthyocotylurus and Cardiocephaloides by the diplostomids Ornithodiplostomum, Posthodiplostomum, Uvulifer and Bolbophorus. Metacercariae of the two clades differ in type, encystment and limebody enclosure; however no other characters differed in the two strigeid groups. These results provide further evidence that the classification of these groups needs to be reassessed.