[
Zh Obshch Biol,
2004]
The early embryonic development of Nematoda proceeds by three ways, which strictly correspond to three phylogenetic lineages. Under the first way the endodermal precursor is localized in the posterior blastomere at the two-cells stage (such a determination is the peculiarity of all the Chromadoria, including Secernentea and Caenorhabditis elegans). Under the second way the endodermal precursor is localized in the anterior blastomere of the egg. This feature is very unusual for Metazoa, but it is the only way of entoderm determination in all the Dorylaimia orders (Mononchida, Mermithida, Trichinellida, Dioctophymida, Dorylaimida). The third way described for the sea Enoplida is characterized with variable location of blastomers and changeable localization of endodermal precursor before eight-cells stage. It is still unknown of these three variants was typical the most recent common ancestor of present Nematoda. D.A. Voronov (2001) produced argument in favour of variable cleavage as primitive one for Nematoda. This opinion is rejected because of the similarity in development between sea Enoplida and C. elegans. Both of them share such features as low-cell gastrula and neurula, identical phylotypic lima bean stage of embryogenesis, identity of some geometrical figures 4 or 8 blastomers, isolating of the endodermal precursor at the eight-cells stage, the lack in development of any plesiomorphous features, which are widely distributed outside Nematoda (under the variable cleavage of Enoplida there are no such locations of blastomers, which are typical for spiral or radial cleavage, there are no embryonic leaves as well). One can see the homology of separate cells at adult Enoplida and Rhabditia. Cell lineage of Triplonchida as far as it is described at Tobrilus gracilis doesn't exclude the hypothesis on their origin from the cleavage similar to one of present Dorylaimia with localization of the endodermal precursor in the anterior blastomere. In view of all the considerations mentioned above one should interpret variable cleavage of Enoplida as derivation from invariant cleavage
[
FEBS Lett,
1998]
Analysis of the secondary structure of 18S rRNA molecules in nematodes revealed some new traits in the secondary structure peculiar to their hairpin 17. Some of them are characteristic of all the nematodes, whereas others are characteristic exclusively of the order Rhabditida. The loss of a nucleotide pair in the highly conservative region of hairpin 17 distinguishes 18S rRNA of the Strongylida and some species of the Rhabditida from other nematodes and, moreover, from all other organisms. Hence, it is possible to regard the Strongylida and a part of the Rhabditida including Caenorhabditis elegans as a new monophyletic taxon.
[
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.