Introduction to Fungi, Third Edition

204 ZYGOMYCOTA
Fig 7.34 Basidiobolus ranarum. (a) Gut-stage cell from
fresh frog dung. (b) Germination of gut-stage cells
producing a coarse septate mycelium. (c) Hyphal apex
with the terminal cells full of cytoplasm.The prominent
nucleus in the apical cell is arrowed. (d) Apex of an older
hypha in which only the two terminal cells contain
cytoplasm; those behind are empty. (e) Branches arising
beneath the septa of three subterminal cells. Scale bar:
(a,b) ¼ 25 mm, (c) ¼ 50 mm, (d,e) ¼ 100 mm.
behind the septum delimiting the apical
segment, after mitotic division of the nucleus
(Fig. 7.34e). The conidiophores, which develop in
a few days, are phototropic and resemble the
sporangiophores of Pilobolus but bear a colourless
pear-shaped to globose ballistosporic conidium
(Fig. 7.35a). O’Donnell (1979) interpreted the
conidium as a monosporous sporangiolum but
since it can, under certain conditions, cleave to
form endospores it may also be regarded as a
modified sporangium. The conidium is uninucleate.
A conical columella projects into it. Beneath
is a swollen sub-conidial vesicle containing liquid
under turgor pressure. This is probably generated
by a single large vacuole which fills most of
the sub-conidial vesicle at maturity. A line of
weakness can be detected as a slight constriction
around the base of the vesicle, and when this
ruptures the conidium and vesicle fly forward for
a distance of 1 2 cm. The elastic upper portion
of the vesicle contracts and the vacuolar sap
within it squirts out backwards, so that it
behaves as a minute rocket (Ingold, 1971).
During their flight the conidium and the rocket
motor (i.e. the vesicle) may be separated or the
two parts remain attached to each other until
landing (Figs. 7.35c,d).
Conidium germination in B. ranarum
Primary ballistosporic conidia can germinate in a
number of different ways depending on external
conditions (Zahari & Shipton, 1988; Waters &
Callaghan, 1999).
(1) By direct germination, producing one to
several germ tubes from which the vegetative
mycelium develops (Fig. 7.35e). Germination of
this type requires a nutrient concentration above
that of 0.1% malt extract agar.