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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.