Introduction to Fungi, Third Edition

NEMATOPHAGOUS FUNGI
675
We owe much of our knowledge of nematophagous
fungi to the numerous publications
by Charles Drechsler. These are cited in many of
the general accounts given by Duddington
(1955, 1957), Barron (1977, 1981), Dowe (1987),
Gray (1987), Nordbring-Hertz (1988) and Dix
and Webster (1995). A superb film has been
produced by Nordbring-Hertz et al. (1995). Cooke
and Godfrey (1964) have provided a key to
identification.
25.1.1 Predatory fungi belonging to
Ascomycota
Predatory nematophagous fungi are easy to
study by means of the sprinkle-plate technique.
A small pinch of soil is added to a Petri dish
containing tapwater agar or dilute cornmeal
agar, and the dish is incubated for a few weeks at
room temperature. Saprotrophic nematodes
present in the soil will crawl out over the agar
surface, feeding on bacteria. If predatory fungi
are present in the soil, they develop structures
for trapping nematodes, and the trapped dead or
dying animals are easy to see with a dissecting
microscope. Conidia will be produced around
colonized nematodes, and if these are transferred
to standard media such as cornmeal agar,
mycelial colonies will grow. Whilst a few
zygomycetes belonging to the order Zoopagales
trap nematodes by secreting an adhesive from
undifferentiated hyphae or short hyphal
branches upon contact with nematodes
(Drechsler, 1962; Saikawa & Morikawa, 1985),
and two basidiomycete genera also trap nematodes
(see Fig. 25.7), the most striking predatory
species are conidial forms of Ascomycota.
Detailed species descriptions may be found in
the works by van Oorschot (1985) and Rubner
(1996).
The taxonomy of predatory ascomycetes has
traditionally been based on the morphology of
conidia, especially the number of septa, and the
degree of clustering of conidia on the conidiophore.
However, DNA-based phylogenetic
approaches have confirmed long-held suspicions
that these criteria are artificial, and that more
natural taxa can be obtained by grouping
predatory ascomycetes according to the type of
trap they form. These results have been summarized
and discussed by Scholler et al. (1999). All
known trap-forming ascomycetes described so
far belong to the family Orbiliaceae (Hagedorn &
Scholler, 1999), with some species shown to
produce an apothecium referrable to Orbilia
itself (Pfister, 1997). This family is still of
uncertain affinity (incertae sedis) within the
Ascomycota but may belong to the Pezizales.
The following trapping structures have been
described.
Adhesive knobs and lateral branches.
Single-celled globose knobs, covered by a sticky
secretion and spaced at intervals along a hypha,
form the morphologically simplest trapping
organs. These knobs are borne directly on the
hypha or on short lateral branches in such a way
that a nematode may become attached to several
knobs (Fig. 25.1). A different type of sticky knob
is produced on thin stalks (see Fig. 25.5b).
Sometimes a nematode attached to an adhesive
knob may pull it off the subtending hypha by its
violent movement, but respite is only temporary
because penetration of the host may still occur
from such detached knobs. Barron (1977) has
suggested that detachable knobs may provide an
effective means of dispersal. In some species,
lateral knob-bearing branches may develop
in sufficient proximity to each other for anastomosis
to take place, and this results in the
formation of a primitive two-dimensional sticky
network. Scholler et al. (1999) have proposed
two genera to accommodate fungi with
knob-like traps, namely Dactylellina (formerly
Monacrosporium, Arthrobotrys and Dactylella spp.)
for species with stalked knobs, and Gamsylella
(formerly Dactylella spp.) for forms with unstalked
knobs and/or primitive nets.
Adhesive nets. One of the most common
types of trap is a three-dimensional adhesive
network formed by anastomosis of the recurved
hyphal tips of a lateral branch system. The
network is lifted above the general level of the
mycelium. The entire surface of the network is
covered by an adhesive, as shown by scanning
electron microscopy (Fig. 25.2; Nordbring-Hertz,
1972), and a nematode which thrusts its body
into the network is quickly immobilized.
Scholler et al. (1999) have grouped fungi