Biology of Nematode Trapping Fungi

Nematode-trapping fungi are found in all regions of the world, from the tropics to Antarctica. They are commonly found in soils and decaying leaf litter, decaying wood, dung, compost and mosses. When grown in soils, nematode-trapping fungi can grow as saprophytes as vegetative hyphae (mycelium). Traps are initiated either spontaneously or in response to signals from the environment, including peptides and other compounds secreted by the host nematode (Dijksterhuis et al. 1994). There is a large variation in the morphology of trapping structures, even between closely related species (Barron 1977) (Fig. 6.1). In some species, the trap consists of an erect branch that is covered with an adhesive material. In other species, such as the well-studied Arthrobotrys oligospora, the trap is a complex three-dimensional net. A third type of trap is the adhesive knob. The knob is a morphologically distinct cell, often produced on the apex of a slender hyphal stalk. A layer of adhesive polymers covers the knob, which is not present on the support stalks. Finally, there are some species of nematode-trapping fungi that capture nematodes in mechanical traps called "constricting rings". When a nematode enters this type of trap, the three ring cells are triggered to swell rapidly and close around the nematode (Barron 1977).

Despite the large morphological variation in trapping structures, phylogenies inferred from molecular data have shown that a majority of nematode-trapping fungi belong to a monophyletic group placed in the family of Orbiliales, Ascomycota. These studies have shown that the trapping mechanisms have evolved along two major lineages, one leading to the constricting rings, and the other into adhesive traps. Among species with adhesive traps, those with adhesive networks separated early from the species with adhesive knobs and branches (Liou and Tzean 1997; Ahren et al. 1998; Hagedorn and Scholler 1999; Li et al. 2005; Yang et al. 2007b) (Fig. 6.1).

The phylogenetic relationships of the various types of nematode-trapping fungi are supported by ultra structural studies. For example, one feature, which is common to the traps of all adhesive species, is the presence of numerous cytosolic organelles, the so-called "dense bodies". Although the function of these organelles is not yet clear, the fact that they exhibit catalase and D-amino acid oxidase activity indicates that the dense bodies are peroxisomal in nature. Furthermore, adhesive trap cells have extensive layers of extracellular polymers, which are thought to be important for attachment of the traps to the surface of the nematode. The trap cells of the inflating constricting rings have a unique, highly ordered structure that are not observed in cells of adhesive traps (Dijksterhuis et al. 1994).

Detailed microscopic studies of net-forming species, in particular A. oligospora, by Birgit Nordbring-Hertz, Martin Veenhuis and colleagues, have revealed that the infection of nematodes occurs by a sequence of events (Dijksterhuis et al. 1994). Following a physical contact between the trap cells and the nematode, the nema-todes become attached to the trap surface. In the electron microscope, the trap cells are surrounded by a layer of extracellular fibrils. After contact, these fibrils become directed perpendicularly to the nematode surface (i.e. cuticle). Subsequently, the fungus pierces the cuticle by forming a penetration tube. This step probably

Orbilia auricolor - Arthrobotrys oligospora - Arthrobotrys conoides Duddingtonia flagrans Arthrobotrys musiformis

Arthrobotrys oligospora (hyphae) Arthrobotrys pyriformis

Arthrobotrys superba Monacrosporium psychrophilum Monacrosporium gephyropagum Monacrosporium ellipsosporum Monacrosporium haptotylum

¬° - Dactylella oxyspora c

Dactylella rhopalota Arthrobotrys dactyloides Monacrosporium doedycoides Peziza badia Cazia flexiascus Peziza quelepidotia

Discoitis venosa Discina macrospora Spathularia flavida Cudonia confusa Neurospora crassa Inermisia aggregate Sphaerophorus globosus Ascobolus lineolatus Leotialubrica Choiromyces venosus Rhizinia undulata Aspergillus niger Saccharomyces cerevisiae Neolecta vitellina

Orbilia auricolor - Arthrobotrys oligospora - Arthrobotrys conoides Duddingtonia flagrans Arthrobotrys musiformis

Fig. 6.1 A cladistic tree showing the relationship between various nematode-trapping fungi and other ascomycetes based on 18SrDNA sequences. Only branches with bootstrap support values above 50 are shown. Note that the nematode-trapping fungi form a monophyletic clade among an unresolved cluster of apothecial ascomycetes. The phylogenetic pattern within the clade of nem-atode-trapping fungi is concordant with the morphology of the traps (The tree is redrawn from Ahren et al. (1998). The pictures are reproduced from Norbring-Hertz et al. (1995), courtesy of Birgit Nordbring-Hertz and IWF, Göttingen)

involves both the activity of hydrolytic enzymes solubilizing components of the cuticle and the activity of a mechanical pressure. Concomitant with penetration, the nematodes become paralyzed (immobilized). Once inside the nematode, the penetration tube swells to form an infection bulb from which trophic hyphae develops that colonizes the dead nematode. The infection bulb can be considered as an intermediate morphological structure, between the highly differentiated trap cells and the trophic hyphae which develops from the bulb. Upon maturation of the bulb the dense bodies are degraded. At the same time, the endoplasmic reticulum proliferates extensively. When the nematode cavity becomes invaded by the trophic hyphae, the internal tissues of the nematode are rapidly degraded. Some of the nematode content is converted to lipid droplets, which can be metabolized to support growth of new vegetative mycelium that develops outside the nematode. The infection process is usually completed in 48-60 h (Dijksterhuis et al. 1994).

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