Introduction to Fungi, Third Edition

BLASTOCLADIALES
161
a thick-walled dark-brown sculptured resting
sporangium within the terminal sac. The resting
sporangium releases posteriorly uniflagellate
swarmers which, after swimming, germinate
to form sexual thalli of two kinds. About half
of the sexual thalli are colourless (‘female’),
and about half are orange-coloured (‘male’).
However, in contrast to the anisogamy of
Eu-Allomyces, inBlastocladiella there is no distinction
in size between the gametes. The orange
and colourless gametes pair to produce zygotes,
which germinate directly to produce asexual
thalli. In other species (e.g. B. cystogena) the life
cycle is of the Cystogenes type, i.e. there are no
gametothalli.
In yet other species there is no clear evidence
of sexual fusion. In B. emersonii (Fig. 6.23), the
resting sporangial thallus contains a single
globose, dark reddish brown resting sporangium
with a dimpled wall. Meiosis occurs during
development of the resting sporangium (Olson
& Reichle, 1978). After a resting period, the wall
cracks open and one to four papillae protrude
from which swarmers are released. The swarmers
germinate to form two types of thallus bearing
thin-walled zoosporangia. About 98% of the
swarmers give rise to thalli bearing colourless
sporangia (Fig. 6.23a), and about 2% to thalli
with sporangia coloured orange due to the
presence of g-carotene. The colourless thalli
develop rapidly and are ready to discharge
zoospores within 24 h. These have about twice
the DNA content as the swarmers released from
resting sporangia (Horgen et al., 1985). Thus
young colourless thalli are at first haploid, but
release diploid zoospores. The manner in which
the diploid state of the colourless thalli or of the
resting sporangia is brought about is not known.
The life cycle of B. emersonii thus corresponds
to that of the sub-genus Brachyallomyces.
Blastocladiella emersonii has a number of other
unusual features. If zoospore suspensions are
pipetted onto yeast peptone glucose (YPG)
agar, the majority of thalli which develop will
be of the thin-walled colourless type. On the
same medium containing 10 mM bicarbonate,
resting sporangial thalli develop. The addition
of 40 80 mM KCl, NaCl or NH 4 Cl, or exposure
of cultures to ultra-violet light, will similarly
induce the formation of resting sporangia
(Horgen & Griffin, 1969). Thus, by means of
simple manipulation of the environment it is
possible to switch the metabolic activities of
the fungus into one of two morphogenetic
Fig 6.23 Blastocladiella emersonii. (a) Thin-walled thallus releasing zoospores. (b) Three-day-old thallus with immature resting
sporangium. (c) Thallus with germinating resting sporangium showing the cracked wall and four exit tubes. (d) Zoospores from
thin-walled thallus.