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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

[ref. ID; 7080 (M.L. Blaxter et al., 1998)]

A molecular evolutionary framework for the phylum Nematoda

We constructed a database of small subunit (SSU) sequences from 53 taxa. Species were chosen to cover all the major parasitic and free-living taxonomic groups. Sequences were aligned with reference to a secondary-structure model and on the basis of similarity. Model phylogenies were evaluated under the criteria of maximum parsimony (MP), maximum likelihood (ML) and minimum evolution using the neighbour-joining (NJ) method, and were tested for statistical support with careful consideration of unequal rate effects. Our hypothesis is based on a study of a single genetic locus but permits, for the first time, the comparison of all taxa using the same defined measurement.

Nematoda is traditionally divided into two classes, namely, the predominantly terrestrial Secernentea and the predominantly marine Adenophorea. Contrary to this view, our analyses indicate that the Adenophorea may be paraphyletic, as it includes the ancestors of the Secernentea. Phylogenies in which the Adenophorea is monophyletic by outgroup rooting score significantly worse in all methods of analysis. This result is supported by some morphological analyses which indicate the lack of synapomorphies (common characters) for Adenophorea or place Secernentea as a monophyletic clade derived from adenophorean ancestry.

Phylum Nematoda

Class Adenophorea

Class Secernentea