Characterization and Sequence Variation in the rDNA Region of Six Nematode Species of the Genus Longidorus (Nematoda).

Total DNA was isolated from individual nematodes of the species Longidorus helveticus, L. macrosoma, L. arthensis, L. profundorum, L. elongatus, and L. raskii collected in Switzerland. The ITS region and D1-D2 expansion segments of the 26S rDNA were amplified and cloned. The sequences obtained were aligned in order to investigate sequence diversity and to infer the phylogenetic relationships among the six Longidorus species. D1-D2 sequences were more conserved than the ITS sequences that varied widely in primary structure and length, and no consensus was observed. Phylogenetic analyses using the neighbor-joining, maximum parsimony and maximum likelihood methods were performed with three different sequence data sets: ITS1-ITS2, 5.8S-D1-D2, and combining ITS1-ITS2+5.8S-D1-D2 sequences. All multiple alignments yielded similar basic trees supporting the existence of the six species established using morphological characters. These sequence data also provided evidence that the different regions of the rDNA are characterized by different evolution rates and by different factors associated with the generation of extreme size variation.