Phylogenetic relationships within the phylum Nematoda as revealed by ribosomal DNA, and their biological implications

Nematodes – “eel worms”; members of the phylum Nematoda – can be considered as a success story within the Metazoa (multicellular, heterotrophic eukaryotes in which cells lack cell walls): they are speciose and – probably - the most numerous group of multicellular animals on our planet. Nematodes are present in virtually all terrestrial, freshwater and marine habitats. Nematodes are trophically diverse; they may feed on bacteria, fungi/oomycetes, algae and protozoa, other nematodes or on a combination of these (omnivores), or live as facultative or obligatory parasites of plants or animals. As they are abundant, ubiquitous and occupy several trophic levels, they play an important role in the soil food web. Nematode parasites of animals affect billions of humans and livestock, while plant parasites such as cyst, root knot and lesion nematodes cause large agricultural losses worldwide. Despite their undisputed ecological and economical relevance, the systematics of the phylum Nematoda is far from established. One of the aims of this research was to further elucidate nematode phylogeny using molecular data. First a phylogenetic tree was constructed of 349 taxa, spanning the entire phylum Nematoda, on the basis of full length small subunit ribosomal DNA (SSU rDNA) sequences. A series of mostly well-supported bifurcations defined twelve major clades, whereas the most basal clade was defined by representatives of the Enoplida and Triplonchida. Our analysis confirmed the paraphyly of the Adenophorea. Furthermore it was found that the SSU rDNA from representatives of the distal clades evolved at a higher rate than the SSU rDNA from the basal clades. In the meantime, a substantial number of sequences was added to our overall SSU rDNA nematode alignment - both public data (GenBank) and data generated by ourselves (≈ 1,500 sequences in total; February 2008). It is noted that the clade division as proposed in 2006 on the basis of “only” 349 taxa still seems to be valid. Subseque