What Is A Nematophagous Fungus

Nematodes constitute a food source for the nematophagous (nematode-destroying) fungi. Living stages of nematodes (eggs, juveniles, vermiform adults, and feeding sedentary females) can be attacked, penetrated, and digested by several types of nematophagous fungi (Jansson and Lopez-Llorca 2001). Dead nematodes (juveniles, vermiform adults, cysts, and root-knot nematode mature females) may also be invaded by nematophagous as well as other (saprotrophic) fungi, but the latter may not be regarded as proper nematode-destroying fungi. For instance, dead vermiform nematodes may be invaded by nematode-trapping fungi, but then the fungi enter the natural openings (mouth, anus, etc.) of the nematodes and never penetrate their cuticles (NordbringHertz and Stalhammar-Carlemalm 1978), and thus represent the saprophytic growth stage of these fungi. Most nematophagous fungi are facultative parasites and exist in both saprophytic and parasitic (with infection structures, proper lytic enzyme systems, etc.) stages induced by external and/or internal signals.

The nematophagous fungi are divided into groups depending on their mode of infecting nematodes: nematode-trapping (Figure 2A), endoparasitic (Figure 2B), egg- and female-parasitic (Figure 2C), and toxin-producing fungi (Jansson et al. 1997). Irrespective of the mode of infection, the final result is always the same—a complete digestion of the host. Nematode-trapping fungi capture vermiform nematodes in special trapping organs formed on the hyphae (Barron 1977). The traps can have either an adhesive function where the nematodes stick to the trap, or a mechanical function. These fungi, e.g., Arthrobotrys oligospora, are characterized by low host specificity and lower parasitic ability. The endoparasites, e.g., Drechmeria coniospora, use their spores to infect the nematode hosts. The spores of these fungi can either be motile, e.g., zoospores, or nonmotile, e.g., conidia of various fungi. The endoparasites have high host specificity and are mostly obligate parasites. The toxin-producing fungi, e.g., Pleurotus ostreatus, immobilize their victims using toxins prior to penetration. These three groups of fungi all attack vermiform nematode stages. Females of

Figure 2 Scanning electron micrographs of nematophagous fungi. (A) Adhesive network trap of Arthrobotrys oligospora. Bar = 20 mm. (Courtesy of Birgit Nordbring-Hertz). (B) Adhesive conidia of the endoparasitic fungus Drechmeria coniospora adhering to the nematode head. Bar = 5 mm. (From Jansson and Nordbring-Hertz 1983, courtesy of Society of General Microbiology). (C) Protoappressorium formed at the end of a germ-tube of Pochonia rubescens on the surface of an egg of Heterodera schachtii. Bar = 4 mm. (From Lopez-Llorca and Claugher 1990, courtesy of Pergamon Press).

Figure 2 Scanning electron micrographs of nematophagous fungi. (A) Adhesive network trap of Arthrobotrys oligospora. Bar = 20 mm. (Courtesy of Birgit Nordbring-Hertz). (B) Adhesive conidia of the endoparasitic fungus Drechmeria coniospora adhering to the nematode head. Bar = 5 mm. (From Jansson and Nordbring-Hertz 1983, courtesy of Society of General Microbiology). (C) Protoappressorium formed at the end of a germ-tube of Pochonia rubescens on the surface of an egg of Heterodera schachtii. Bar = 4 mm. (From Lopez-Llorca and Claugher 1990, courtesy of Pergamon Press).

sedentary endoparasitic nematodes, as well as their eggs, can be infected by facultative egg-parasites, e.g., Pochonia chlamydosporia (syn. Verticillium chlamydosporium).

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