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Riddle DL, Blumenthal T, Meyer BJ, et al., editors. C. elegans II. 2nd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997.

Cover of C. elegans II

C. elegans II. 2nd edition.

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Section IThe Nematode World

Of every five animals on the planet, four are nematodes (Platt 1994). The vast majority are free-living microbivores, but many species have adopted a parasitic lifestyle. Most plants and animals have at least one parasitic nematode species uniquely adapted to exploit the concentration of food and resources that the host species represents. The reasons for the success of the Nematoda as parasites probably include the presence of an environmentally protective cuticle, facultative diapause (like the dauer stage of Caenorhabditis elegans), biochemical adaptations to existence in extreme conditions, and the use of a variety of reproductive strategies. Some parasitic nematodes are the subject of extensive research efforts, often aimed at understanding their lifestyle and ecology so as to control or manage them better.

The choice of the free-living rhabditid C. elegans as a model for metazoan development has been fortunate for the study of other nematodes, particularly parasites. Although only distantly related to most parasitic species, C. elegans displays the morphological conservatism of the phylum and shares structures found to be important to parasitic species. It also has a characteristic nematode biochemistry and has proved to be sensitive to all the major nematicidal drugs. In this chapter, we first provide an overview of the diversity and importance of parasitic nematodes and subsequently describe three systems where research on C. elegans is having a significant impact on parasitology.

Historically, the study of nematodes has fragmented into three major branches (plant nematology, animal parasitology, and free-living nematology) that have had little impact on one another. Plant nematology emphasizes the study of plant pathology and crop protection. Veterinary and human nematode parasitology have been closely linked through the use of model parasite-host combinations in the elucidation of pathogenesis and immunity. Research on free-living nematodes has emphasized taxonomic, ecological, and nutritional studies. Because research on C. elegans impinges on all these areas, the three fields are growing closer together while the horizons of traditional nematology are expanding.

Nematodes infect plants and invertebrate and vertebrate animals. The relationships between nematodes and their hosts vary from necromenic association (where the nematode uses the host both to transport it to new food sources and as a source of food upon its death) to complex life cycles involving multiple intermediate hosts. In this spectrum, it is often difficult to distinguish a “true” parasitic lifestyle from that of a predator which invades its food source and devours it from the inside out. Xiphinema index, a dorylaimid plant “parasite” that grazes upon plant roots by piercing them with a pharyngeal stylet, remains outside its hosts and moves from root to root. Other plant-eating species, such as Criconemella xenoplax, are sedentary ectoparasites. True parasites are mainly internal, in the gut (e.g., Haemonchus contortus), in roots (e.g., Meloidogyne incognita), or within tissues (e.g., Brugia malayi), or even intracellular (e.g., the human-infective muscle-cell parasite Trichinella spiralis). Host ranges vary widely. Some animal parasites such as the rhabditid Heterorhabditis bacteriophora (which invades insect larvae) are promiscuous and will enter and reproduce in several species, whereas others are exquisitely specific to a single host or host-vector combination. Some nematodes will enter and developmentally arrest in many species (known as paratenic hosts) but will only complete their reproductive cycle in a limited range of hosts (e.g., the ascarid Toxocara canis reproduces only in dogs and cats but can infect and cause pathology in many vertebrates including humans).

The phylogenetic relationship of parasitic nematode groups to free-living species such as C. elegans remains problematic. Although it is clear that there are relatively easily defined groups of parasites and nonparasites (Anderson 1992), the morphological conservation within the phylum coupled with remarkable instances of convergence makes deep-level phylogenetics difficult (Andrássy 1976; Lorenzen 1994; Malakhov 1994). Some species such as Steinernema (insect hosts) and Strongyloides (mammalian hosts) are clearly closely related to C. elegans, but the relationships of others such as the ascarids and filarids are obscure (Fig. 1) (see Fitch and Thomas, this volume). Molecular phylogenies have been constructed for local groups, including the rhabditids (Fitch et al. 1995), ascarids (Nadler 1992), and filarids (Xie et al. 1994), but a robust phylum-wide analysis is yet to be performed.

Figure 1. Relationships of the nematodes mentioned in this chapter.

Figure 1

Relationships of the nematodes mentioned in this chapter. Taxonomy taken from Anderson (1992) and Malakhov (1994).

Copyright © 1997, Cold Spring Harbor Laboratory Press.
Bookshelf ID: NBK20038

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