Introduction to Fungi, Third Edition

ENTOMOPHTHORALES
207
Basidiobolus microsporus, which grows in
deserts in California, has a method of asexual
reproduction not found in B. ranarum. Primary
conidia can germinate directly or by repetition
as in B. ranarum, but capilliconidia have not
been found. However, under relatively dry
conditions primary conidia may produce large
numbers of exogenous obclavate spores (microspores)
each attached by a separate pedicel to
the wall of the primary conidium. They have
been interpreted as modified sporangiospores
(Benjamin, 1962).
In culture it has been found that light,
especially blue light of wavelength 440 480 nm,
stimulates conidial development and discharge
in B. ranarum. The effect of light is to stimulate
aerial growth from hyphal bodies within the
medium, and the aerial hyphae which develop
in the light become modified as conidiophores
(Callaghan, 1969a,b).
Sexual reproduction in B. ranarum
Zygospores are formed following conjugation.
The fungus is homothallic and development can
be seen on certain agar media (e.g. Czapek-Dox
agar) within 4 5 days in cultures derived from a
single conidium. Zygospore development appears
to occur most readily in the dark, and under these
conditions the hyphal bodies become bicellular
prior to developing into zygospores (Callaghan,
1969b). On either side of a septum, beak-like
projections develop, and the single nucleus
within each hyphal segment migrates into the
tip and divides there. One daughter nucleus is cut
off by a septum in the terminal cell of the beak
and later disintegrates, whereas the second
nucleus migrates back into the parent cell.
Following this, one of the parent cells enlarges
to several times the volume of the adjacent cell
and a pore is formed connecting the two cells
through the original septum separating them.
A nucleus from the smaller cell passes through
the pore and lies close to the nucleus of the larger
cell. Nuclear fusion may occur directly or after
a further division. The enlarged parent cell forms
the zygospore which has a thick wall when
mature (Fig. 7.38). Meiosis occurs within the
mature zygospore to give four haploid nuclei, of
which three usually degenerate. The mature
zygospores of some isolates of B. ranarum have
thick undulating walls of variable thickness,
but in others the wall may be smooth. On germination
the zygospore forms a germ tube
or a conidiophore terminated by a ballistosporic
conidium. Capilliconidia may also develop from
germinating zygospores (Dykstra & Bradley-Kerr,
1994). The complicated and unusual life cycle
of B. ranarum is illustrated in Fig. 7.37.
Basidiobolus ranarum is an atypical zygomycete
in that its mycelium becomes divided into uninucleate
segments. The nucleus is also unusually
large, up to 25 mm, and this fact has led to several
investigations of its cytology (e.g. Robinow, 1963;
Tanaka, 1970; Sun & Bowen, 1972). The number
of chromosomes has been estimated to be as high
as 900, and the nucleus may be polyploid.
Pathogenicity of B. ranarum
Basidiobolus is probably not harmful to most
insects and mites, although it has been isolated
as a mass infection of mosquitoes, from termites
and from larvae of Galleria (Krejzová, 1978). It was
earlier thought to be harmless to reptiles and
amphibians and there is no evidence of intestinal
lesions in them. However, an epizootic cutaneous
infection caused by B. ranarum has been reported
from the dwarf African clawed frog, Hymenochirus
curtipes (Groff et al., 1991). There are many reports
of the isolation of B. ranarum from man and
domestic animals such as horses (see Gugnani,
1999; Ribes et al., 2000). Although several specific
names have been applied to isolates pathogenic
to humans and other mammals, the consensus is
that they should be regarded as synonyms
of B. ranarum (McGinnis, 1980). This view is
supported by ribosomal DNA analysis (Nelson
et al., 1990). Isolates from humans, unsurprisingly,
are capable of growing at 37°C (Cochrane
et al., 1989). Human disease caused by B. ranarum
is more common in tropical and subtropical
regions than in temperate zones. Infection is
associated with subcutaneous swellings of
affected areas of the lower limbs but rare intestinal
infections are also known. It is assumed that
the inoculum is usually soil-borne, and the use
of fallen leaves in place of toilet paper has
sometimes been implicated as the cause of