A MOLECULAR EVOLUTIONARY FRAMEWORK FOR THE PHYLUM NEMATODA

Nematodes are important: parasitic nematodes threaten the health of pl ants, animals and humans on a global scale(1,2); interstitial nematode s pervade sediment and soil ecosystems in overwhelming numbers(3); and Caenorhabditis elegans is a favourite experimental model system(4). A lack of dearly homologous characters and the absence of an informativ e fossil record have prevented us from deriving a consistent evolution ary framework for the phylum. Here we present a phylogenetic analysis, using 53 small subunit ribosomal DNA sequences from a wide range of n ematodes. With this analysis, we can compare animal-parasitic, plant-p arasitic and free-living taxa using a common measurement. Our results indicate that convergent morphological evolution may be extensive and that present higher-level classification of the Nematoda will need rev ision. We identify five major clades within the phylum, all of which i nclude parasitic species. We suggest that animal parasitism arose inde pendently at least four times, and plant parasitism three times. We cl arify the relationship of C. elegans to major parasitic groups; this w ill allow more effective exploitation of our genetic and biological kn owledge of this model species.