Inference of evolutionary relationships between nematodes is severely hampered by their conserved morphology, the high frequency of homoplasy, and the scarcity of phylum-wide molecular data. To study the origin of nematode radiation and to unravel the phylogenetic relationships between distantly related species, 339 nearly full-length small-subunit rDNA sequences were analyzed from a diverse range of nematodes. Bayesian inference revealed a backbone comprising 12 consecutive dichotomies that subdivided the phylum Nematoda into 12 clades. The most basal clade is dominated by the subclass Enoplia, and members of the order Triplonchida occupy positions most close to the common ancestor of the nematodes. Crown Clades 8-12, a group formerly indicated as "Secernentea" that includes Caenorhabditis elegans and virtually all major plant and animal parasites, show significantly higher nucleotide substitution rates than the more basal Clades 1-7. Accelerated substitution rates are associated with parasitic lifestyles (Clades 8 and 12) or short generation times (Clades 9-11). The relatively high substitution rates in the distal clades resulted in numerous autapomorphies that allow in most cases DNA barcode-based species identification. Teratocephalus, a genus comprising terrestrial bacterivores, was shown to be most close to the starting point of Secernentean radiation. Notably, fungal feeding nematodes were exclusively found basal to or as sister taxon next to the 3 groups of plant parasitic nematodes, namely, Trichodoridae, Longidoridae, and Tylenchomorpha. The exclusive common presence of fungivorous and plant parasitic nematodes supports a long-standing hypothesis that states that plant parasitic nematodes arose from fungivorous ancestors.

As a result of the scarcity of informative morphological and anatomical characters, nematode systematics have always been volatile. Differences in the appreciation of these characters have resulted in numerous classifications and this greatly confuses scientific communication. An advantage of the use of molecular data is that it allows for an enormous expansion of the number of characters. Here we present a phylogenetic tree based on 1215 small subunit ribosomal DNA sequences (ca 1700 bp each) covering a wide range of nematode taxa. Of the 19 nematode orders mentioned by De Ley et al. (2006) 15 are represented here. Compared with Holterman et al. (2006) the number of taxa analysed has been tripled. This did not result in major changes in the clade subdivision of the phylum, although a decrease in the number of well supported nodes was observed. Especially at the family level and below we observed a considerable congruence between morphology and ribosomal DNA-based nematode systematics and, in case of discrepancies, morphological or anatomical support could be found for the alternative grouping in most instances. The extensiveness of convergent evolution is one of the most striking phenomena observed in the phylogenetic tree presented here – it is hard to find a morphological, ecological or biological characteristic that has not arisen at least twice during nematode evolution. Convergent evolution appears to be an important additional explanation for the seemingly persistent volatility of nematode systematics.

Cyst (Heteroderidae), root knot (Meloidogyne spp.), and lesion (Pratylenchus spp.) nematodes all belong to a single nematode order, Tylenchida. However, the relationships between and within these economically highly relevant groups, and their relatedness to other parasitic Tylenchida is unclear. We constructed a phylogeny of 116 Tylenchida taxa based on full length small subunit ribosomal DNA (small subunit [SSU] rDNA) sequences. Ancestral state reconstruction points at a gradual development of simple to more complex forms of plant parasitism. Good resolution was observed in distal clades that include cyst, root knot, and lesion nematodes, and monophyly of most families was confirmed. Our data suggest that root knot nematodes have evolved from an ancestral member of the genus Pratylenchus, but it remains unclear which species is closest to this branching point. Contrary to the notoriously polyphagous distal representatives, basal members of the genus Meloidogyne (and probably, their common ancestor) have narrow host ranges. Our analysis also shows that mitotic parthenogeny has arisen at least two times independently among root knot nematodes. In many cases resolution till species was observed, suggesting that SSU rDNA sequences have a potential for DNA barcode-based species identification with, due to the overall conserved nature of this gene, limited intra-species variation.

Indigenous communities of soil-resident nematodes have a high potential for soil health assessment as nematodes are diverse, abundant, trophically heterogeneous and easily extractable from soil. The conserved morphology of nematodes is the main operational reason for their under-exploitation as soil health indicators, and a user-friendly biosensor system should preferably be based on nonmorphological traits. More than 80% of the most environmental stress-sensitive nematode families belong to the orders Mononchida and Dorylaimida. The phylogenetic resolution offered by full-length small subunit ribosomal DNA (SSU rDNA) sequences within these two orders is highly different. Notwithstanding several discrepancies between morphology and SSU rDNA-based systematics, Mononchida families (indicated here as M1-M5) are relatively well-supported and, consequently, family-specific DNA sequences signatures could be defined. Apart from Nygolaimidae and Longidoridae, the resolution among Dorylaimida families was poor. Therefore, a part of the more variable large subunit rDNA (≈ 1000 bp from the 5'-end) was sequenced for 72 Dorylaimida species. Sequence analysis revealed a subclade division among Dorylaimida (here defined as D1-D9, PP1-PP3) that shows only distant similarity with 'classical' Dorylaimid systematics. Most subclades were trophically homogeneous, and - in most cases - specific morphological characteristics could be pinpointed that support the proposed division. To illustrate the practicability of the proposed molecular framework, we designed primers for the detection of individual subclades within the order Mononchida in a complex DNA background (viz. in terrestrial or freshwater nematode communities) and tested them in quantitative assays (real-time polymerase chain reaction). Our results constitute proof-of-principle for the concept of DNA sequence signatures-based monitoring of stress sensitive nematode families in environmental samples.

Nematodes are the only major metazoan group which is persistently abundant and diverse across marine, freshwater and terrestrial ecosystems. This could be the result of a few major habitat transitions followed by extensive diversification, or numerous habitat transitions followed by moderate diversification. To pinpoint habitat transitions, we superposed nematode habitat associations on an available phylum-wide phylogenetic tree based on small subunit ribosomal DNA sequences (≈2730 SSU rDNA sequences covering ≈1750 nematode taxa). Our analysis revealed at least 30 major habitat transitions within the phylum Nematoda. These transitions as well as their directionality were unevenly spread over the 12 major clades. Most transitions reside in Clades 1–6, and these transitions are bidirectional. Members of Clades 8–12 showed five full transitions, and these took place exclusively from terrestrial to marine systems. We relate our results to the distinct secretory–excretory systems in Clades 1–6 and Clades 8–12, as well as to differences in water permeability of the nematode cuticle. Hence, the phylum Nematoda is characterized by a relatively large number of habitat transitions followed by moderate diversification. The identification of multiple habitat transitions at a low taxonomic level will facilitate future investigations into the mechanisms underlying this unusual ecological flexibility.