Introduction to Fungi, Third Edition

25
Anamorphic fungi (nematophagous and
aquatic forms)
Throughout this book we have attempted to
consider fungi showing predominantly or purely
asexual reproduction together with their known
or suspected teleomorphs. However, certain
groups of taxonomically diverse fungi colonizing
the same specialized habitats or substrates
are best understood in their ecological context,
especially if they show strikingly similar adaptations
and morphology despite their different
evolutionary histories. Two cases illustrating
such convergent evolution among anamorphic
fungi are the nematophagous habit and the
aquatic habitat, which we shall consider in turn
in this chapter.
25.1 Nematophagous fungi
Nematodes are a very varied group of invertebrates.
They are particularly common as freeliving
saprotrophic species in the soil, around
plant roots, on dung and in all kinds of decomposing
plant matter, as well as in freshwater and
marine habitats. Most saprotrophic nematodes
feed on bacteria, although fungal hyphae may
also be consumed. Other species parasitize
animals, releasing their eggs or motile stages
into the environment when their hosts defaecate.
Plant-parasitic species chiefly attack roots as freeliving
or sedentary organisms. Sedentary species
form adult stages inside plant root tissues where
they cause the economically important root knot
diseases (Meloidogyne spp.) or root cyst diseases
(Heterodera spp. and Globodera spp.). Plant-parasitic
nematodes are readily recognized because their
mouth parts are modified as stylets with which
they penetrate plant tissues. Gravid females of
cyst nematodes enlarge, and their bodies become
converted into a hardened cyst containing the
eggs.
Fungi have evolved a range of mechanisms
to attack nematodes, which can be grouped
into three broad categories described below.
A summary of genera and their taxonomic
relationships is presented in Table 25.1.
Nematophagous fungi are common in most
types of soil which are rich in organic matter,
and they have been found in arctic, temperate
and tropical climates (see Dix & Webster, 1995).
There is no strong evidence of host selectivity.
Predatory nematophagous fungi produce
a sizeable mycelium in the soil, with trapping
devices formed at intervals along the length
of the hyphae. The varied and occasionally
spectacular trapping mechanisms have captured
the imagination of generations of mycology
students. The most common traps are sticky
knobs, adhesive networks, non-constricting
rings or constricting rings (see pp. 675 680).
Penetration of captured nematodes is followed
by the growth of trophic hyphae throughout the
nematode body, and digestion of its contents.
Because most predatory nematophagous fungi