Introduction to Fungi, Third Edition

ENTOMOPHTHORALES
203
An illustrated account of entomopathogenic
species has been provided by Samson et al.
(1988) and a key to genera by Humber (1997).
The major entomopathogenic genera are Batkoa,
Conidiobolus, Entomophaga, Entomophthora, Erynia,
Furia, Massospora, Neozygites, Pandora and
Zoophthora. Some of these insect pathogens hold
promise for the control of insect pests, not least
because many of them can be grown in culture,
albeit on complex media containing ingredients
such as sugars, egg yolk, yeast extract and milk
(Wolf, 1981; Papierok & Hajek, 1997). The cells of
Entomophthorales are uninucleate or coenocytic
with chitinous walls, or they may exist in
the bodies of insects as wall-less protoplasts.
The absence of a wall presumably reduces the
elicitation of immune responses in their hosts
(Dunphy & Nolan, 1982). Asexual reproduction in
most genera is by means of forcibly discharged
conidia, and on germination such conidia may
develop a variety of secondary conidia. Sexual
reproduction is by isogamous or anisogamous
conjugation between uni- or multinucleate
gametangia, to give a thick-walled zygospore.
Azygospores may also be formed without conjugation,
but it is likely that nuclear fusion and
reduction division occur during their development
(McCabe et al., 1984).
Fossil evidence indicates that, as insect
pathogens, members of the group were extant
at least 25 million years ago. A well-preserved
specimen of a winged termite probably infected
with a species of Entomophthora has been found
embedded in amber dated around the
Oligocene Miocene border in the Dominican
Republic (Poinar & Thomas, 1982).
According to Benny et al. (2001), the order
Entomophthorales consists of six families including
the Basidiobolaceae. If this family is
excluded, the remaining Entomophthorales
appear to be monophyletic by DNA-based analysis
(Jensen et al., 1998). In many phylogenetic
schemes, Basidiobolus ranarum seems to be more
closely related to Chytridiales and Neomastigales
than to Entomophthorales (see Figs. 1.26, 7.1),
and Cavalier-Smith (1998) has placed it in a
separate order, the Basidiobolales. In the current
context, we sacrifice these taxonomic details in
favour of a better understanding of the
Zygomycota as a whole, and therefore retain
Basidiobolus in the Entomophthorales.
Important criteria in the classification of
Entomophthorales are the branched or unbranched
nature of the conidiophores, whether
the conidia are uninucleate or multinucleate,
whether the wall of the conidium is single
(unitunicate) or separates into two layers
(bitunicate), and the presence or absence of
secondary conidia and their morphology
(Humber, 1989).
7.5.1 Basidiobolaceae: Basidiobolus
Basidiobolus is the only genus in the
Basidiobolaceae. The best-known species is B.
ranarum, which has a worldwide distribution. It
fruits on the dung of frogs, toads, lizards, some
insectivorous fish and mammals such as bats. It
has also been found on the dung of kangaroos
and wallabies (Speare & Thomas, 1985). If a frog
is captured and placed in a jar with a little water,
it will defaecate in due course and its dung can
be filtered off. If the damp filter paper is placed
in the lid of an inverted Petri dish containing a
suitable agar medium (e.g. 1% peptone agar,
potato-dextrose agar, or cornmeal agar), conidia
of B. ranarum will be shot upwards from the dung
onto the agar surface, and within a few days
coarsely septate colonies will become visible on
the agar (Weber & Webster, 1998a). The presence
of Basidiobolus and other ballistosporic fungi such
as Conidiobolus in surface soil and litter can
also be disclosed by the ‘canopy’ technique.
A suspension of soil is filtered and the filter
paper, bearing a thin layer of soil, is placed in the
lid of a Petri dish facing downwards over a
suitable agar medium. The dish is illuminated
from below and this encourages the discharge of
conidia onto the agar (Smith & Callaghan, 1987;
Callaghan, 2004).
In agar cultures of Basidiobolus, the cytoplasm
in the mycelium moves towards the hyphal apex
so that only a few terminal segments contain
cytoplasm and a single large, prominent nucleus
whilst the older segments are empty, being
isolated by retraction septa (Fig. 7.34d). The
cytoplasm-filled mycelial segments are termed
hyphal bodies. Branching occurs immediately