[
European Worm Meeting,
1998]
We are continuing in our quest to define the evolutionary history of the phylum Nematoda using molecular phylogenetic methods. The published analysis (Blaxter et al, Nature 392:71-75 (1998)) has now been complemented with over 100 additional sequenced taxa, arising from our work and that of others (Kampfer et al, Invert. Biol. 117:29-36 (1998) and Aleshin et al, Russ. J. Nematol. in press (1998)). Our analysis continues to support the division of the Nematoda into five major clades, and does not support the division of the phylum into two classes (Adenophorea and Secernentea). The origin of the Secernentea has been more closely defined as residing within the Chromadorida, although a separate chromadorid radiation is now evident. Caenorhabditis remains a close relative of the parasitic strongyles. We are extending the dataset using sequence data derived from fixed museum specimens. Formalin fixation preserves nematode structures very well, but is not good for DNA preservation. We have been able to extract PCR-amplifiable DNA from single 12-year fixed specimens of Strongyloides species using an amino acid titration method. Fragments up to 600 bp are recoverable and sequencable. For specimens preserved in alcohol, even at low temperatures, it has been much more difficult to obtain amplifiable DNAs, but we now have a rehydration-extraction method which works on larger samples. Current projects are focussing on the phylogenetics of the genus Strongyloides, parasites of vertebrate guts which have a facultative free-living generation. This group has turned out to be entertainingly complex, with the species divided into two distinct clades separated by freeliving and parasitic taxa from other genera. We find no correlation between host and parasite phylogenies, suggesting that horizontal transfer between hosts has been common in these parasites. We are examining in detail the coevolution of filarial nematodes and an endosymbiotic Wolbachia-like bacterium. In this case, we have strong evidence for vertical transmission of the endosymbiont within the filarial lineage. We are also providing a phylogenetic framework for comparative EST-based genome projects on additional filarial and other nematode species.