Introduction to Nematodes

Nematodes are astonishing organisms. Despite their deceptively simple morphology and the fact that they are essentially aquatic, requiring at least a film of liquid for active life, they have been successful in colonising an enormous range of environments. Irrespective of their habitat, nematodes have a similar external morphology, with a worm shaped, bilaterally symmetrical, unsegmented body. The phylum Nematoda comprises >25,000 described species and the importance of nematodes should never be underestimated. Species parasitic on plants and animals have a massive deleterious social and economic impact on man. As will be discussed below, one major attribute that contributes to the undoubted success of nematodes is the amazing ability of some of the life cycle stages to survive adverse environmental conditions. Free-living nematodes of the species Caenorhabditis elegans, carried as part of the experimental payload on the Columbia spacecraft, even survived when the spacecraft broke up on re-entry in 2003 (Szewczyk and Lamb 2005).

Free-living species, which make up the bulk of the phylum Nematoda, feed primarily on bacteria and fungi, and are found in soil, marine and freshwater habitats; species have been thawed out of Antarctic ice (Cobb 1914) and others have been found in hot water springs in New Zealand (Rahm 1937). The free-living forms in the soil are beneficial as they are involved in nutrient turnover; in addition, they may be of use as indicator species for pollution monitoring (Wilson and Khakouli-Duarte 2009). The parasitic forms have devastating effects on man, his crops and his livestock as well as infecting wild plants and animals. Nematodes infecting plants are also known as 'eelworms' and some species infecting animals are colloquially called 'roundworms'.

Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK e-mail: [email protected]

Biology Department, Ghent University, K.L. Ledeganckstraat 35, 9000 Ghent, Belgium

J. Jones et al. (eds.), Genomics and Molecular Genetics of Plant-Nematode Interactions, 3

DOI 10.1007/978-94-007-0434-3_1, © Springer Science+Business Media B.V. 2011

Entomopathogenic nematodes of the genera Steinernema and Heterorhabditis have been commercialised as environmentally acceptable control agents for several insect pests (Ehlers 2001; Gaugler and Han 2002). Like many free-living nematodes, individuals of these genera feed on bacteria but they have become specialised in using insect larval stages as a basis for culturing their food supply. The infective juvenile nematodes invade the target insects and release symbiotic bacteria in the haemocoel of the host. The bacteria multiply and kill the insect by septicaemia, usually within 48h, and the nematodes feed on the proliferating bacteria, develop and reproduce. The research information on these nematodes is extensive (Gaugler 2002).

Although the majority of nematodes are microscopic in size (less than 1 mm in length and between 15 and 20 ^m in diameter; Fig. 1.1), the animal-parasitic species are often considerably larger. The largest nematode is Placentonema gigantisma, discovered in the placenta of a sperm whale; the adult nematode can grow to 8 m in length. In humans, nematodes are of great medical importance and it has been estimated that a quarter of the world's population suffer from a nematode infection of some sort. One of the most familiar diseases caused by nematodes is elephantiasis, caused by Wuchereria bancrofti. Ascaris lumbricoides is a major parasite of the intestine as is Enterobius vermicularis, which is probably familiar to many mothers as the 'pin worm' parasite of children. Dogs and cats are infected by several nematodes, among which are the microscopic Toxocara cati (in cats) and T. canis (in dogs). If the animals are not wormed the eggs voided in the faeces in parks and play areas, for example, can attach to fingers and, if ingested by humans, the juvenile nematode will hatch. The juvenile does not develop further, but not being in the proper host will wander around the body causing serious damage to organs. If the nematodes enter the cerebrospinal fluid and migrate to the brain, the victim can suffer brain damage and blindness. Deaths, usually of small children, have been reported. These are only a few examples of animal-parasitic nematodes; for further reading illustrating these pests and the horrific diseases they cause see Matthews (1998) and Lee (2001).

The subjects of this book, plant-parasitic nematodes, do not have such obvious and unpleasant effects. However, their economic and social impacts are no less severe,

Fig. 1.1 Free-living nematodes and free-living stages of plant-parasitic nematodes obtained from a field soil extraction. (Courtesy Wim ML Wesemael, Institute for Agricultural and Fisheries Research, Belgium)

Fig. 1.1 Free-living nematodes and free-living stages of plant-parasitic nematodes obtained from a field soil extraction. (Courtesy Wim ML Wesemael, Institute for Agricultural and Fisheries Research, Belgium)

especially in developing countries where crop loss due to nematodes may be disastrous. All crop plants have one or more species of nematodes that feed on the roots as ectoparasites or invade the roots and feed internally as endoparasites. Some species are migratory endoparasitic, moving in and out of the plants and there are also species that feed on the aerial parts of plants (stems, leaves, buds and seeds). As well as the detrimental effects on the growth of the plants, causing stunting, early senescence and in severe cases total crop loss, the damage caused, especially to root crops such as carrots, can render the produce unmarketable and eliminate income. A major difficulty in controlling the plant-parasitic species is convincing farmers, growers and advisors that the crop problems are actually caused by these microscopic pests. With good reason, plant-parasitic nematodes have been called 'The Invisible Enemy'.

Among the most economically important nematodes are those endoparasitic species that form complex feeding structures in the roots of their host plants. The most damaging are the root-knot (Meloidogyne spp.) and cyst (Heterodera and Glo-bodera spp.) nematodes. The root-knot nematodes cause most damage worldwide (Moens et al. 2009). In general, species of Meloidogyne have broad host ranges,

Fig. 1.2 Galls of the false root-knot nematode, Nacob-bus bolivianus, on potato roots. (Courtesy Rosa H Manzanilla-Lopez, Rotham-sted Research, UK; from Manzanilla-Lopez 2010)

Fig. 1.2 Galls of the false root-knot nematode, Nacob-bus bolivianus, on potato roots. (Courtesy Rosa H Manzanilla-Lopez, Rotham-sted Research, UK; from Manzanilla-Lopez 2010)

and are able to infect almost all species of flowering plants. The world-wide spread of root-knot and cyst nematodes and their enormous economic impact has formed the justification for much research; information on these two groups is extensive and this bias is, of necessity, reflected in the following sections. However, there are other groups that have a major agricultural impact, especially species of the genus Nacobbus (Manzanilla-López et al. 2002; Manzanilla-López 2010; Fig. 1.2) and the migratory endoparasitic root lesion nematodes of the genus Pratylenchus (Castillo and Vovlas 2007).

With the decline in use or banning of many chemicals because of adverse environmental impacts, it is imperative that new strategies for nematode control and management are developed and implemented. In this context, understanding plant-nematode interactions will be vital, and the ability to exploit genomics will not only indicate novel control targets, but also justify re-examination of some older suggestions for control based on interrupting certain phases of the life cycle.

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