M. Garcia-varela et al., Phylogenetic relationships of Acanthocephala based on analysis of 18S ribosomal RNA gene sequences, J MOL EVOL, 50(6), 2000, pp. 532-540
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.