Toward a phylogeny for Dorylaimida (Nematoda): Systematic studies in the subclass Dorylaimia

Free-living soil nematodes, with their ubiquity, small size, and staggering diversity, are attaining prominence as indicators of ecosystem function and health. Their utility as indicators is hampered by lack of systematic knowledge, especially for certain key groups. The order Dorylaimida is the most diverse and numerically important group of terrestrial nematodes; this order represents one of the basal evolutionary lineages within the phylum Nematoda. No coherent phylogenetic hypothesis currently exists for this group. The studies reported here represent the initial steps towards the construction of the evolutionary framework necessary for future studies in ecology and systematics of Dorylaimida. Nucleotide sequences of portions of the 18S small subunit ribosomal DNA (SSU) have been obtained from a large number of nematode taxa representing Dorylaimida and other related groups. These have been employed in phylogenetic studies aimed at defining the study group, assessing its placement relative to other nematode orders, and exploring relationships among its constituent taxa. The systematic position of Campydora has long been uncertain. Morphological studies of C. demonstrans, the only species of Campydora, have placed this taxon either in Dorylaimida or in Enoplida. Phylogenetic analyses, using SSU sequences obtained from representatives of the subclasses Enoplia and Dorylaimia, were conducted to evaluate these competing hypotheses. The results unambiguously indicate that Campydora shares more recent common ancestry with genera currently placed in Enoplida than with any members of Dorylaimida. SSU sequence data also confirm the monophyly of Dorylaimida, comprising the suborders Nygolaimina and Dorylaimina, but reveal a deep division within Nygolaimina, giving rise to the possibility of paraphyly for this suborder. A well-supported clade containing members of the traditional orders Mermithida and Mononchida forms the sister taxon to Dorylaimida, but the topology of this clade indicates the need for a systematic re-evaluation of its component suborders. Vertebrate parasites within Dorylaimia are reconstructed in a sister-taxon relationship to the mononch/dorylaim lineage. The enigmatic order Isolaimida is placed in an ancestral relationship to all other Dorylaimia sampled