Phylogenetic relationships of Acanthocephala based on analysis of 18S ribosomal RNA gene sequences

Acanthocephala (thorny-headed worms) is a phylum of endoparasites of verteb rates and arthropods, included among the most phylogenetically basal tripob lastic pseudocoelomates. The phylum is divided into three classes: Archiaca nthocephala, Palaeacanthocephala, and Eoacanthocephala. These classes are d istinguished by morphological characters such as location of lacunar canals , persistence of ligament sacs in females, number and type of cement glands in males, number and size of proboscis hooks, host taxonomy, and ecology. To understand better the phylogenetic relationships within Acanthocephala, and between Acanthocephala and Rotifera, we sequenced the nearly complete 1 8S rRNA genes of nine species from the three classes of Acanthocephala and four species of Rotifera from the classes Bdelloidea and Monogononta, Phylo genetic relationships were inferred by maximum-likelihood analyses of these new sequences and others previously determined. The analyses showed that A canthocephala is the sister group to a clade including Eoacanthocephala and Palaeacanthocephala. Archiacanthocephala exhibited a slower rate of evolut ion at the nucleotide level, as evidenced by shorter branch lengths for the group. We found statistically: significant support for the monophyly of Ro tifera, represented in our analysis by species from the clade Eurotatoria, which includes the classes Bdelloidea and Monogononta. Eurotatoria also app ears as the sister group to Acanthocephala.