A quick tour of nematode diversity and the backbone of nematode phylogeny

Abstract

Contrary to textbook dogma, nematodes are not only highly diverse, but often also complex and biologically specialized metazoans. Just a few of the many fascinating adaptations are reviewed in this chapter, as a prelude to a quick tour through phylogenetic relationships within the phylum. Small Subunit rDNA sequences have confirmed several controversial prior hypotheses, as well as revealing some unexpected relationships, resulting in a recent proposal for revised classification. Three major lineages exist within the phylum: Chromadoria, Enoplia and Dorylaimia. The exact order of appearance of these lineages is not yet resolved, which also leaves room for uncertainty about the biology and morphology of the exclusive common ancestor of nematodes. Enoplia and Dorylaimia differ considerably in many respects from C. elegans, which is a member of Chromadoria. The latter group is extremely diverse in its own right, for example in ecological range, in properties of the cuticle and in structure of the pharynx. The formerly relatively widely accepted class Secernentea is deeply nested within Chromadoria, and has therefore recently been relegated to the rank and name of order Rhabditida. Within this order, closer relatives of C. elegans include strongylids, diplogasterids and bunonematids. Tylenchs, cephalobs and panagrolaimids are also members of Rhabditida, albeit probably more distantly related to C. elegans.

It takes nerves to fight infections: insights on neuro-immune interactions fromC. elegans

“…Alternatively, they might represent regulatory pathways that are unique to C. elegans owing to its evolutionary history. Regarding the latter point, although this information might not be directly relevant to human health, it could be useful for the control of parasitic nematodes, which share a high degree of similarity with C. elegans (De Ley, 2006). Finally, the neuroendocrine system also affects the function of B and T cells of the adaptive immune system, and molecular insights into the antigen-specific immune responses in which these cell types are involved can only be gained from studies of higher vertebrates.…”

Technological advancements and their importance for nematode identification

“…According to Coomans (2002), morphology can still provide useful diagnostic characters, especially if we are able to overcome the limited resolution light microscopy provides. And despite all its limitations, morphology-based study when carried out diligently can be as good as any biochemical or molecular method used in identifying taxa (Mayr and Ashlock, 1991;De Ley, 2006;Agatha and Strüder-Kypke, 2007). What is lacking, however, is the technical and taxonomic expertise required to correctly utilise phenotypic characters to effectively make a decision about the identity of an organism .…”

The embryonic cell lineage of the nematode Halicephalobus gingivalis (Nematoda: Cephalobina: Panagrolaimoidea)

“…nanus-like' configuration, in order to avoid compensatory migrations and to speed up the cell cycle periods and thereby developmental rate. However, recently Lahl et al (2003) observed a reversal of polarity in P 2 and P 3 in several species of the Plectidae, an outgroup of the Rhabditida (Blaxter et al, 1998;De Ley & Blaxter, 2002;Meldal et al, 2007). Furthermore, descriptions of the early development of members of other families from clades I, II and III suggest the presence of small primordial germ precursor cells at the ventral side that contact the intestinal precursor (Malakhov, 1994).…”

“…Throughout this order, the adult body plan has undergone a wide range of evolutionary modifications that allows them to flourish in terrestrial habitats, e.g., those subject to frequent episodes of rapid de-and rehydration, such as mosses and lichens or extremely xeric and/or cryogenic soils (De Ley, 2006). For example, adaptations in buccal and pharynx morphologies and different food strategies for different adult species must occur late in development after all cells are formed in the first part of embryonic development.…”

“…This diversity allows them to thrive in a wide range of ecological habitats, ranging from deep-sea sediments to arid deserts (De Ley, 2006). However, it is still unclear how the morphological diversity of nematodes relates to their developmental diversity.…”