An improved molecular phylogeny of the Nematoda with special emphasis on marine taxa

Phylogenetic reconstructions of relations within the phylum Nematoda are inherently difficult but have been advanced with the introduction of large-scale molecular-based techniques. However, the most recent revisions were heavily biased towards terrestrial and parasitic species and greater representation of clades containing marine species (e.g. Araeolaimida, Chromadorida, Desmodorida, Desmoscolecida, Enoplida, and Monhysterida) is needed for accurate coverage of known taxonomic diversity. We now add small subunit ribosomal DNA (SSU rDNA) sequences for 100 previously un-sequenced species of nematodes, including 46 marine taxa. SSU rDNA sequences for >200 taxa have been analysed based on Bayesian inference and LogDet-transformed distances. The resulting phylogenies provide support for (i) the re-classification of the Secernentea as the order Rhabditida that derived from a common ancestor of chromadorean orders Araeolaimida, Chromadorida, Desmodorida, Desmoscolecida, and Monhysterida and (ii) the position of Bunonema close to the Diplogasteroidea in the Rhabditina. Other, previously controversial relationships can now be resolved more clearly: (a) Alaimus, Campydora, and Trischistoma belong in the Enoplida, (b) Isolaimium is placed basally to a big clade containing the Axonolaimidae, Plectidae, and Rhabditida, (c) Xyzzors belongs in the Desmodoridae, (d) Comesomatidae and Cyartonema belongs in the Monhysterida, (e) Globodera belongs in the Hoplolaimidae and (f) Paratylenchus dianeae belongs in the Criconematoidea. However, the SSU gene did not provide significant support for the class Chromadoria or clear evidence for the relationship between the three classes, Enoplia, Dorylaimia, and Chromadoria. Furthermore, across the whole phylum, the phylogenetically informative characters of the SSU gene are not informative in a parsimony analysis, highlighting the short-comings of the parsimony method for large-scale phylogenetic modelling.     

The meiofauna community of two small German streams as indicator of pollution

In order to validate the results of biomarkerstudies as tools for the assessment of smallstreams, the meiofauna community and inparticular the nematodes of two small Germanstreams (Baden-Württemberg) were analysedduring 1998. One site of Krähenbach (AB)and three sites of Körsch (KA, KD, KE) weresampled in March, May, July and November. Thelimnological analysis indicates that two sitesof Körsch (KD, KA: polluted sites) weredistinctly enriched by inorganic and organicsubstances compared with KE and AB (unpollutedsites). In the Krähenbach, nematodes,chironomids, oligochaetes and tardigradesdominated the meiofauna community (92%),whereas in the Körsch nematodes andoligochaetes were dominant (93%). The totalmeiofauna density showed seasonal patterns inboth streams. Nematodes were the most abundantgroup at the four investigated locations. Themean abundance of nematodes ranged from 28individuals per 10 cm² to 1205 individualsper 10 cm² and at least 103 species wereidentified, belonging to 25 families. Nosubstantial difference occurred in thedistribution of nematode feeding-types at thepolluted and unpolluted sites. All communitieswere clearly dominated by bacteria-feedingnematodes with percentages ranging from 70% to75%. In order to assess the sediment quality,the nematode community of each sampling sitewas interpreted using the ratio ofSecernentea/Adenophorea as well as the MaturityIndex (MI). Both the percentage of Adenophoreaand the MI decreased towards the pollutedsites, detecting a more disturbed nematodecommunity at the polluted than unpollutedsites. However, there are nematode species(e.g. Tobrilus diversipapillatus) whichdo not fit in this assessment concept. In thisstudy the disturbance of the nematode communitywas related to eutrophication, rather than tothe effect of contaminants. In summary, thisinvestigation confirmed that meiofaunaorganisms and in particular nematodes aresuitable for the assessment of the sedimentquality of running waters.     

Trefusiidae Are a Subtaxon of Marine Enoplida (Nematoda): Evidence from Primary Structure of Hairpin 35 and 48 Loops of SSU rRNA Gene

A rare nucleotide substitution was found in the evolutionarily conserved loop of hairpin 35 of the 18S rRNA gene of marine free-living nematode, Trefusia zostericola(Nematoda: Enoplida). The same substitution was found in all the marine Enoplida studied but not in other nematodes. Such a molecular synapomorphy indicates that marine enoplids are more closely related to T. zostericolathan to freshwater Triplonchida. Maximum parsimony, neighbor-joining, and maximum likelihood analyses of complete nucleotide sequences of the gene, with the heterogeneity of nucleotide sites in evolution rates taken into account, support this conclusion. Hence, the hypothesis of particular primitiveness of Trefusiidae among nematodes should be rejected. Phylogenies based on molecular data support the morphological reduction of metanemes in Trefusiidae. Alongside with the unique change in hairpin 35 loop among marine Enoplida (including T. zostericola), hairpin 48 is also modified by a rare transversion which could be found among Mesorhabditoidea nematodes, in related genera Pelodera, Mesorhabditis, Teratorhabditis, Parasitorhabditis, Crustorhabditis, and Distolabrellus, and in 11 orders of Rhodophyta. Rare mutations in hairpins 35 and 48 tend to be fixed correlatively in evolution and could be found in all the Acanthocephala species. X-Ray data show that these regions (H31 and H43, in alternative nomenclature) are spatially brought together in native ribosomes. The nature and distribution of molecular autoapomorphies in phylogenetic trees of high-rank taxa are discussed.