Nematodes

Figure 1. Our current understanding of the phylogenetic relationships of nematodes. The main panel shows the phylogeny of the major nematode groups, information about ecological range and presence of parasites (black bars indicate definitive hosts, grey bars intermediate hosts), and some representative genera. Phylogenetic relationships within Nematoda currently rely mostly on small-subunit rRNA sequences, but this gene provides surprisingly good resolution even when hundreds of species are included. There are three large monophyletic groups: the mostly marine Enoplia, Dorylaimia that include the parasitic trichinellids and mermithids, and Chromadoria with nematodes of all habitats and life styles. Most animal parasites belong to Spirurina, but hookworms (Strongylida) are found in Rhabditina along with Caenorhabditis elegans. Recent analyses with whole genomes for a smaller number of species as well as features of early embryonic development support Enoplia as the first branch of the nematode tree. (Adapted with permission from De Ley, P.A. (2006).) (Inset) Nematodes in the tree of life. All recent phylogenetic studies, using sequence data from a variety of genes and species, place Nematoda together with its sistergroup Nematomorpha (Gordian worms) as the closest relatives of Panarthropoda (arthropods, onychophorans and tardigrades) in a clade that is sometimes called Ecdysozoa. Nematodes