Near, T.J.: Phylogeny of the Acanthocephala and insights to the evolution of parasitism

The Acanthocephala are a lineage of obligate helminth parasites of arthropods and vertebrates. Previous phylogenetic investigations utilizing ribosomal and protein coding genes indicate that the Acanthocephala is nested within the Rotifera. This is the only majorparasitic lineage with an indentified free-living sister-taxon. This recovered phylogenetic relationship permits critical examination of several hypotheses concerning the evolution of parasitism in metazoans. The hypothesis that the Acanthocephala are modified rotifers has been questioned. The evidence for this hypothesis is reviewed and both total evidence and consensus approaches are utilized in a phylogenetic reanalysis of the extant morphological and molecular data. Potential confounding variables in the phylogenetic analyses are presented. For example, alignment of the 18S rRNA dataset is problematic. The issue of homology assessment is discussed in relation to previous investigations and recognition of rRNA secondary structure. Revised phylogenetic analyses of relationships within Acanthocephala are used to test the validity of recognized taxonomic groups, character evolution within and among lineages, and diversification of hosts and habitats. The results of phylogenetic reanalysis of acanthocephalan affinities, in relation to the Rotifera, are utilized to examine the evolution of parasitism in the Acanthocephala. The results from this examination provide a critical test to several long-standing hypotheses concerning the evolution of parasitism in metazoans.