Nematode Feeding Habits

Nematodes feed on a wide range of foods. A general trophic grouping is: bacterial feeders, fungal feeders, plant feeders, and predators and omnivores. For the purposes of our overview, one can use anterior (stomal or mouth) structures to differentiate feeding, or trophic, groups (Fig. 7.5) (Yeates and Coleman, 1982; Yeates et al., 1993). Plant-feeding nematodes have a hollow stylet that pierces cell walls of higher plants. Some species are facultative, feeding occasionally on plant roots or root hairs. Others, more recognized for their damage to agricultural crops and forest plantations, are obligate parasites of plants and feed internally or externally on plant roots. The effect nematodes have on plants is generally species-specific and can include alterations in root architecture, water transport, and plant metabolism, or all of these. Recently, the sedentary obligate parasites were found to

FIGURE 7.4 Structures of a Rhabditis sp., a secernentean microbotrophic nematode of the order Rhabditida. (Left) Female. (Right) Male. St, stoma; C, corpus area of the pharynx; N, nerve ring; E.p, excretory pore; B.b, basal bulb of the pharynx; I, intestine; T, testis; E, eggs; V, vulva; Va, vagina; U, uterus; O, ovary; Sp, sperm; V.d, vas deferens; R.g, rectal glands; R, rectum; A, anus; S, spicules; G, gubernaculum; B, bursa; P, phasmids; G.P., genital papillae; Cl, cloaca (courtesy of Proceedings of the Helminthological Society of Washington) (from Poinar, 1983).

FIGURE 7.4 Structures of a Rhabditis sp., a secernentean microbotrophic nematode of the order Rhabditida. (Left) Female. (Right) Male. St, stoma; C, corpus area of the pharynx; N, nerve ring; E.p, excretory pore; B.b, basal bulb of the pharynx; I, intestine; T, testis; E, eggs; V, vulva; Va, vagina; U, uterus; O, ovary; Sp, sperm; V.d, vas deferens; R.g, rectal glands; R, rectum; A, anus; S, spicules; G, gubernaculum; B, bursa; P, phasmids; G.P., genital papillae; Cl, cloaca (courtesy of Proceedings of the Helminthological Society of Washington) (from Poinar, 1983).

have multiple parasitic genes. Some of the genes for secretion of endogluconases (cellulases) appear to play direct roles in the nematode parasitic process. Their enzyme products modify plant cell walls and cell metabolism (Davis et al., 2000, 2004). These genes have greatest similarity to microbial genes for cellulases, but

FIGURE 7.5 Head structures of a range of soil nematodes. (a) Rhabditis (bacterial feeding); (b) Acrobeles (bacterial feeding); (c) Diplogaster (bacterial feeding, predator); (d) tylenchid (plant feeding, fungal feeding, predator); (e) Dorylaimus (feeding poorly known, omnivore); (f) Xiphinema (plant feeding); (g) Trichodorus (plant feeding); (h) Mononchus (predator) (from Yeates and Coleman, 1982).

coevolution of plant and parasite seems more likely than horizontal gene transfer from microbes to parasite.

Some of the stylet-bearing nematodes (e.g., the Family Neotylenchidae) may feed on roots, root hairs, and fungal hyphae (Yeates and Coleman, 1982). Some bacterial feeders (e.g., Alaimus) may ingest 10-^m-wide cyanobacterial cells (Oscillatoria) despite the mouth of the nematode being only 1 ^m wide. This indicates that the cyanobacterial cells can be compressed markedly by the nematode (Yeates, 1998). The population growth of bacterial-feeding nematodes is strongly dependent on the species of bacteria ingested (Venette and Ferris, 1998). Immature forms of certain nematodes may be bacterial feeders and then become predators or parasites on other fauna once they have matured.

The feeding habits and impacts of entomopathogenic nematodes, nematodes carrying symbiotic bacteria that are lethal to their insect host, are distributed worldwide. They have been cultured and sold commercially to control garden pests and mosquitoes (Gaugler, 2002; Hominick, 2002). The nonfeeding, infective juveniles, or third instar (dauer) larvae, of nematodes in the Family Heterorhabditidae and Steinernematidae, live in the soil and search for insect hosts (Gaugler, 2002). The infective juvenile enters the insect host (which it senses along a CO2 gradient

(Strong et al., 1996)) through a body opening, punctures a membrane, and releases its symbiotic bacteria, which kill the host within 24-48 h. A rapidly growing bacterial population then digests the insect cadaver and provides food for the exponentially growing adult nematode population. The symbiotic bacteria produce antibiotics and other antimicrobial substances that protect the host cadaver and adult nematodes inside from invasion by alien bacteria and fungi from the soil (Strong et al., 1999). When the cadaver is exhausted of resources, reproduction shunts to infective juveniles, which break through the host integument and disperse into the soil. As many as 410,000 Heterorhabditis hepialus infective juveniles are produced in a large ghost moth caterpillar.

Ruess et al. (2002) have traced the fatty acids specific to fungi to the body tissues of fungal-feeding nematodes. This technique shows considerable promise for more detailed biochemical delineation of food sources of specific feeding groups of nematodes. Fungal-feeding nematodes are known to feed preferentially on different fungal species (Mankau and Mankau, 1963), including mycorrhizas and yeasts. Because of the wide range of feeding types and the fact that they seem to reflect ages of the systems in which they occur, i.e., annual vs perennial crops, old fields and pastures, and more mature forests, nematodes have been used as indicators of overall ecological condition (Bongers, 1990; Freckman and Ettema, 1993; Ferris et al., 2001). This is a growing area of research in soil ecology, one in which the intersection between community analysis and ecosystem function could prove to be quite fruitful.

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