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DIFFERENTIATION AND DEVELOPMENT 97

Sporangiospores

Sporangiospores are usually formed within a thin-
walled sporangium, like the well-known sporangia
of Mucorand other Zygomycota (Fig. 5.16). Within
a sporangium the nuclei undergo repeated mitotic
divisions, then the cytoplasm is cleaved around the
individual nuclei by the alignment and fusion of
membranes. This is followed by production of a wall
around each of the spores in the Zygomycota, or by
the development of flagella in the wall-less spores of
Chytridiomycota and Oomycota. Finally, the spores
are released by controlled lysis of all or part of the
sporangium wall.
The process of cytoplasmic cleavage to produce
sporangiospores seems to occur in several ways.
In Gilbertella persicaria(Zygomycota; Fig. 5.16) and
Phytophthora cinnamomi(Oomycota) a large number of
cleavage vesicles are produced and these migrate
around the nuclei so that they are aligned; then they
fuse with one another so that their membranes
become the plasma membranes of the spores. How-
ever, in Saprolegniaand Achlya(Oomycota) a large
central vacuole develops in the sporangium and forms
radiating arms between the nuclei, then the arms
fuse with the plasma membrane of the sporangium
to delimit the spores. The cytoskeleton is intimately
involved in these processes, as Heath & Harold (1992)
showed by the use of an actin-specific fluorescent
stain (phalloidin conjugated to rhodamine). The
cleavage planes in these sporangia of Saprolegnia and
Achlyawere associated with sheet-like arrays of actin
which only appeared at the beginning of cleavage and
disappeared when cleavage was complete.
In a study of cleavage in the zoosporic fungus
Phytophthora cinnamomi(Hyde et al. 1991) the flagellar
axonemes (microtubular shafts) were found to develop
in a different class of vesicle – the flagellar vacuoles near
each nucleus, rather than the normal cleavage vesicles.
Then the flagellar vacuoles fused with the cleavage ves-
icles so that the flagella were located on the outside of

elegans(related to A. mellea). In this fungus, the rhizo-
morph apex will only grow if it remains hyaline, and
this means that the partial pressure of oxygen at the
surface of the apex must be 0.03 or less (compared with
about 0.21 in air). Above this level, the apex rapidly
becomes melanized, stopping its growth. Yet growth of
the apex is strongly oxygen-dependent, and the fun-
gus seems to resolve this dilemma by a combination
of factors. A high respiration rate is maintained at the
apex, supported partly by diffusion of oxygen along the
central channel, while the surface of the apex is cov-
ered by a water film which limits the rate of oxygen
diffusion: at 20°C, oxygen diffuses about 10,000 times
more slowly through water than through air. The
dependence on a water film ensures that rhizomorphs
grow naturally at a specific depth in soil, depending
on the soil type and the climate. If a tip grows too
close to the soil surface then the width of the water
film is reduced and oxygen diffuses to the tip more
rapidly, causing melanization. These tips near to the
soil surface then break down to produce “breathing
pores” connected to the central channel. Conversely,
if the apex grows too deeply into moist soil then the
water film increases and the rate of growth becomes
oxygen-limited. Thus, the peculiar organization of a
rhizomorph helps to regulate growth to specific zones
in the soil, and these zones are where tree roots occur,
maximizing the opportunities for infection.

Asexual reproduction

The enormous diversity of asexual spores of fungi is
discussed in Chapter 10. Here we focus on the devel-
opmental processes leading to spore formation, and
in this respect there are two fundamentally different
patterns. Sporangiosporesare formed by cleavage of
the protoplasm within a multinucleate sporangium
(Chytridiomycota, Oomycota, and Zygomycota).
Conidia develop directly from hyphae or special
hyphal cells (Ascomycota, mitosporic fungi, and some
Basidiomycota) but never within a sporangium.

Fig. 5.15Diagram of a rhizomorph of
Armillariella.

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