Evolutionary relationships of trichostrongyloid nematodes (strongylida) inferred from ribosomal DNA sequence data

The evolutionary relationships of 21 species of trichostrongyloid nematodes were determined by use of sequence data of the second internal transcribed spacer of the ribosomal DNA aligned according to secondary structure infor mation. Irrespective of the method of analysis used, the topologies of the phylogenetic trees derived from the molecular data differed with respect to all four hypotheses proposed previously for the evolutionary relationships of the different subfamilies within the Trichostrongylidae based on morpho logical data. Thus, the molecular data set did not resolve the conflict bet ween the four previous proposals for the subfamilial relationships. Nonethe less, all trees derived from the molecular data showed strong support for t he exclusion of the genera Filarinema and Amidostomum from the clade contai ning the species within the family Trichostrongylidae. This represents a ma jor difference from the most;recent proposal of the systematics of the Tric hostrongyloidea in which these two genera were included within the Trichost rongylidae. Therefore, the molecular data support an earlier systematic fra mework in which Filarinema and Amidostomum were considered to be sister gro ups of the Tricho strongyloidea. (C) 2001 Academic Press.