Nematode trapping fungi are ubiquitous throughout soil environments. They are a heterogeneous group characterized by the type of trap they produce, of which the most common are adhesive networks, sticky knobs, constricting rings and non-constricting rings.
Trap formation is thought to provide an example of convergent evolution: the analysis of 18S rDNA sequences suggests that nematode parasitism may have emerged on more than one occasion, once with adhesive rings and again with constriction rings. Adhesive knobs are the most morphologically simple trap. Dispersed along the hyphae and coated by a sticky secretion, nematodes can become attached to the adhesive knob or violently pull it off, in which case it can still be penetrated once the knob germinates, and is also thought to aid in dispersal of the fungus. Adhesive nets are formed by anastomosis of recurved hyphal tips in a branching network; each lateral branch secretes an adhesive substance and is raised above the mycelia. Fungi that produce this type of trap often belong to the genus Arthrobotrys, of which A. oligospora has been most extensively studied. Non-constricting rings are formed by anastomosis of a recurved branch with itself to form a three celled adhesive ring in which a nematode can become ensnared. Constriction ring development is similar, although they are non-adhesive and differ greatly in the mechanism by which they trap nematodes. When a nematode ruptures the lumen surface of a constriction ring, the ring inflates and violently constricts the nematode within 0.1 seconds. The pressure that the nematode exerts on the ring causes the activation of G-proteins, this increases the presence of Ca2+ within the cytoplasm, causing the activation of calmodulin which opens the water channels, and thereby inflates the ring trapping the nematode.
Other traps utilize toxins, such as those produced by the basidiomycete Pleurotus, which it uses to immobilize its nematode prey. Other nematode destroying fungi - besides trap forming groups - are the endoparasitic fungi and the egg and cyst parasites. Endoparasites are regarded as obligate parasites since mycelia development takes place within the nematode and only the reproductive-conidiophores are externalised. Fungi that parasitise eggs and cysts are taxonomically diverse, but are usually saprotrophic soil fungi found in plant roots. Some fungi produce zoosporic traps: motile spores – requiring moist soil - that are propelled by flagella that attach to the nematode cuticle or orifice where they encyst and penetrate the host.
Arthrobotrys oligospora (x 100)
Adhesive traps produced by A. oligospora.
Pythium spp. (x 100)
Image of constriction ring, or non-constriction ring (they look identical), produced by Pythium spp.
Pythium spp. (x 400)
Image of constrictoin ring, or non-constriction ring, produced by Pythium spp. The three-celled ring structure is clearly discernible.
Constricted nematode (x 100)
Nematode bearing two constriction rings giving a 'sausage-dog' appearance.
Constricted nematode (x 100)
Nematode constricted by Pythium spp.