Partial small subunit (18S) rRNA gene sequences from fish parasites of the families Lepocreadiidae and Fellodistomidae (Digenea) and their use in phylogenetic analyses

Abstract

Digeneans of the families Lepocreadiidae and Fellodistomidae are common parasites of shallow and deep-water marine fishes. Worms recovered from these contrasting environments may be indistinguishable or very similar morphologically and little is known concerning their inter-relationships. To investigate the systematic status of these forms, partial sequences from a small subunit 18S rRNA gene were generated from four species of lepocreadiids (Lepidapedon elongatum, L. gaevskayae, L. rachion and Opechona bacillaris) and two fellodistomids (Fellodistomum fellis and Steringophorus agnotus). Approximately 1,637 bases were sequenced for L. elongatum, L. rachion and F. fellis. A comparison of these sequences identified regions which varied between the two families. As a result, two regions were identified as containing potential phylogenetically informative sites and sequenced from the remaining species. Analyses of these regions by both maximum parsimony and distance methods showed that nucleic acid sequence data can be used to resolve the two families. The results indicate that 18S rRNA sequences are likely to be more satisfactory at resolving higher levels of phylogeny, i.e. at the family level, than at the generic level due to the similarity shown between the sequences of congeners.