Introduction to Fungi, Third Edition

NEMATOPHAGOUS FUNGI
681
Fig 25.7 Nematode-trapping Basidiomycota. (a) Growth of Pleurotus hyphae towards buccal cavities of two nematodes killed
at za distance by secreted toxins. (b d) Nematoctonus state of Hohenbuehelia. (b) Hourglass-shaped cell producing an adhesive.
(c) Production of an awl-shaped conidium from a hypha bearing clamp connections. (d) Conidia attached to the cuticle of a dead
nematode by a sticky knob at the tip of a bent neck. Infection has given rise to trophic hyphae. (b,c) to same scale.
are obligate parasites without a free-living
saprotrophic phase. In some species, the conidia
attach themselves to the cuticle of the host and,
on germination, penetrate into the body cavity,
eventually filling it with hyphae. Drechmeria
coniospora (Figs. 25.8a c) has conical conidia
which, at maturity, bear a globose, adhesive
knob at the narrow end. Such conidia readily
adhere to the cuticle of a nematode which
brushes past them, and they become preferentially
attached to the buccal end of nematodes
(Jansson et al., 1985). In some other forms, infection
follows ingestion of a conidium. When the
crescent-shaped conidia of Harposporium anguillulae
are ingested, the pointed end of the spore
becomes lodged in the wall of the oesophagus
and, upon germination, penetrates the body
cavity from within (Aschner & Kohn, 1958). The
conidiophores emerging from the dead host bear
sessile, subglobose phialides (Figs. 25.8d,e). The
mycelium within the host may form chlamydospores
which presumably survive in the soil when
the body of the nematode decays.
Little is known about the taxonomic position
of endoparasitic fungi. Several genera contain
members parasitizing soil invertebrates other
than nematodes. For instance, the teleomorph of
Harposporium, Podocrella (syn. Atricordyceps), is
related to the insect-parasitic genus Cordyceps in
the Clavicipitaceae (see p. 360). There are also
numerous examples of lower fungi adopting
an endoparasitic mode of life, such as Catenaria
anguillulae (Chytridiomycota) and Myzocytium
spp. (Oomycota) which infect nematodes by
means of zoospores, or Haptoglossa which
produces the spectacular gun cells shown in
Fig. 3.9.
Although endoparasitic fungi can sometimes
be observed on soil-sprinkle plates, a more
efficient technique for their detection is the
Baermann funnel. A conical funnel is fitted with
rubber tubing at its base, and the opening is
sealed with a clamp. The funnel is lined with
tissue paper or filter paper; the soil sample is
filled into the funnel and submerged in water.
Nematodes will migrate through the filter paper