Introduction to Fungi, Third Edition

172 ZYGOMYCOTA
The columella is curved from its inception.
Cleavage planes separate the nuclei within the
sporangium, and finally the spores are cleaved
out. They may be uninucleate or multinucleate
according to the species, e.g. M. hiemalis and
Absidia glauca have predominantly uninucleate
spores (Storck & Morrill, 1977) whilst M. mucedo,
P. blakesleeanus, Rhizopus stolonifer and Syzygites
megalocarpus have multinucleate spores
(Hammill & Secor, 1983). The number of spores
formed is very variable. On nutrient-poor media
minute sporangia containing very few spores
may be formed, but in P. blakesleeanus the number
of spores may be as high as 50 000 100 000 in a
single sporangium of normal size.
The ultrastructure of developing sporangia
of Gilbertella has been studied by Bracker
(1968a) and is essentially similar in multisporous
columellate sporangia of other Mucorales,
e.g. M. mucedo (Hammill, 1981) and Zygorhynchus
heterogamus (Edelman & Klomparens, 1994). One
difference is that in M. mucedo mitotic division
continues during sporangial development, which
has not been reported from the other species
studied. The cleavage of the sporangial cytoplasm
to form spores is accomplished by the
fusion of membranous cleavage vesicles lined by
electron-opaque granules. The vesicles are at first
globose but coalesce and become flattened to
form cleavage furrows. A three-dimensional network
of cleavage furrows envelops the individual
spore protoplasts, radiating outwards until they
fuse with the sporangial plasma membrane.
After cleavage, the flattened membrane which
bounded the cleavage vesicle persists as the
plasma membrane of the sporangiospore,
whereas the electron-opaque granules make up
part of the spore wall (Fig. 7.7). The columella
is delimited from the rest of the sporangium by
a process similar to that which cuts out the
spores. Edelman and Klomparens (1994) noted
that in Z. heterogamus the wall of the sporangium
contains chitin, but the walls of the sporangiospores
and columella do not. They have suggested
that the columella may be a source of
chitinase which causes enzymatic degradation of
the mature sporangial wall whilst not affecting
the walls of the spores or the columella itself.
Fig 7.7 Zygorhynchus heterogamus. Diagrammatic interpretation of ultrastructural transition from cleavage furrows isolating spore
protoplasts (a) to post-cleavage spores (b). (a) shows the cleavage furrow membrane (1), an electron-translucent layer (2), a layer of
fusing electron-opaque granules (3), an electron-transparent zone (5) and the electron-translucent matrix of the cleavage furrow
(6). (b) shows the spore plasma membrane which was initially the cleavage furrow membrane (1), an electron-translucent layer (2),
the electron-opaque layer which originated from the fusing granules of the cleavage furrows (3), an additional electron-translucent
layer which had no corresponding layer within the cleavage furrows (4), a uniform zone of electron transparency, similarly placed in
the cleavage furrows (5), and a non-uniform granular matrix separating the spores (6).Redrawn with permission from Edelmann and
Klomparens (1994), Mycologia. ßThe Mycological Society of America.