Nematodes are important: parasitic nematodes threaten the health of plants, animals and humans on a global scale; interstitial nematodes pervade sediment and soil ecosystems in overwhelming numbers; and Caenorhabditis elegans is a favourite experimental model system. A lack of clearly homologous characters and the absence of an informative fossil record have prevented us from deriving a consistent evolutionary framework for the phylum. Here we present a phylogenetic analysis, using 53 small subunit ribosomal DNA sequences from a wide range of nematodes. With this analysis, we can compare animal-parasitic, plant-parasitic 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 revision. We identify five major clades within the phylum, all of which include parasitic species. We suggest that animal parasitism arose independently at least four times, and plant parasitism three times. We clarify the relationship of C. elegans to major parasitic groups; this will allow more effective exploitation of our genetic and biological knowledge of this model species.

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A solution containing dimethyl sulphoxide, disodium EDTA, and saturated NaCl (abbreviated here as DESS) was tested for various applications in the preservation of nematodes for combined morphological and molecular analyses. The solution can be used to preserve individual nematodes, nematode extracts, or entire soil/sediment samples. Preserved material can be easily stored for months at room temperature, shipped by mail, or carried in luggage. Morphological features are usually well preserved; specimen quality being comparable to formalin-based fixatives and much better than ethanol fixation. Specimens can be transferred to glycerin with little or no modification of traditional protocols. Unlike formalin-preserved material, routine PCR can be performed on individual specimens after any of these procedures with success rates and amplification sizes comparable to PCR of fresh specimens. At this point we have no data on long-term preservation quality. Nevertheless, DESS solution clearly enhances and simplifies a wide range of nematological studies due to its combined suitability for morphological and molecular analyses, as well as its less hazardous chemical properties.

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