Patterns and processes in the evolution of animal parasitic nematodes

The parasitic Nematoda have traditionally been classified distinct from fre e-living species, and animal parasites treated separately from plant parasi tes. In classical concepts of phylogenetic relationships within the phylum, parasitic groups are usually afforded ordinal status and their origins are often obscure. We have been using molecular phylogenetics to examine the i nterrelationships of animal parasites with free-living and plant-parasitic groups, and find that a new view of the origins and radiation of animal par asites is warranted. Using sequence from the nuclear small subunit ribosoma l RNA gene, we have constructed an alignment that allows robust phylogeneti c inference. With this dataset, we place the Strongylida as a monophyletic clade nested within the Rhabditina. The Ascaridida, Oxyurida and Spirurida are closely related, but currently have no clear closest free-living sister taxon. Strongyloides spp. are rooted in a radiation of cephalobid/tylenchi d species. Where available, other sequences in general confirm these relati onships.