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complex fruiting body, such as a mushroom. This
enables the mass redistribution of materials necessary
to fuel the development of the fruiting structure.
From even this brief account, it is clear that septa
play several important roles in fungal biology. They can
isolate compartments or they can allow the free pas-
sage of organelles through the septal pores, and they
can be degraded to allow the mass translocation of
nutrients and cytoplasmic components to sites of
future development.

The fungal nucleus

Fungal nuclei are usually small (1–2μm diameter) but
exceptionally can be up to 20 –25μm diameter (e.g.
Basidiobolus ranarum, Chytridiomycota; Chapter 4).
They are surrounded by a double nuclear membrane
with pores, as in all eukaryotes. However, fungi are
notable for several peculiar features of their nuclei and
nuclear division (Heath 1978). First, the nuclear mem-
brane and the nucleolus remain intact during most
stages of mitosis, whereas in most other organisms the
nuclear membrane breaks down at an early stage dur-
ing nuclear division. A possible reason for this is that
the retention of a nuclear membrane might help to
prevent dispersion of the nuclear contents in hyphal
compartments that contain several nuclei and rapidly
streaming protoplasm. Second, in fungi there is no clear
metaphase plate; instead the chromosomes seem to be
randomly dispersed, and at anaphase the daughter
chromatids pull apart along two tracks, on spindle
fibres of different lengths. A third point of difference
is that fungi have various types of spindle-pole
bodies(also called microtubule-organizing centers).
They are responsible for microtubule assembly during
nuclear division. In Ascomycota and mitosporic
fungi the spindle-pole bodies are disc-shaped, whereas
in Basidiomycota they are often composed of two
globular ends connected by a bridge. However, the
significance of these differences is unclear. All fungi need
a microtubule-organizing center to ensure that the
chromosomes separate correctly during nuclear division.
The vast majority of fungi are haploidwith chro-
mosome numbers ranging from about 6 to 20. For
example there are six chromosomes in Schizophyllum
commune (Basidiomycota), seven in Neurospora
crassa (Acomycota), eight in Emericella (Aspergillus)
nidulans(Ascomycota), 16 in Saccharomyces cerevisiae
(Ascomycota), and 20 in Ustilago maydis(Basidio-
mycota). A few fungi are naturally diploid (e.g. the yeast
Candida albicans, and members of the Oomycota).
And some fungi can alternate between haploid and
diploid generations – e.g. S. cerevisiae, and Allomyces
spp. (Chytridiomycota) – or exist as polyploid series
(Allomycesand several Phytophthoraspp.).


Many aspects of fungal genetics and fungal
genomes are discussed in Chapter 9, but here we need
to address one of the most remarkable features: fungi
are the only major group of eukaryotic organisms
that are haploid. Almost all other eukaryotes are
diploid, including plants, animals, Oomycota, and
the many single-celled organisms loosely termed “pro-
tists.” The likely reason for the haploid nature of
fungi is that it confers a specific advantage. On the one
hand, as we have seen, fungal hyphae usually contain
several nuclei in each hyphal compartment, and there
is continuity of protoplasmic flow between the com-
partments. If a recessive mutation occurs in one of the
nuclei, then the fungus will still behave as wild-type,
but the recessive mutation will be retained, thereby
storing potential variability – the hallmark of diploid
organisms. On the other hand, a mutated nucleus can
be exposed to selection pressure periodically, when the
fungus produces uninucleate spores or when a branch
arises and a single “founder” nucleus enters it. So, in at
least some respects, haploid fungi with multinucleate
compartments have many of the advantages associated
with both a haploid and a diploid lifestyle. We return
to this subject in Chapter 9.

Cytoplasmic organelles

Many of the cellular components of fungi are func-
tionally similar to those of other eukaryotes, but differ
in some important respects. The major differences are
discussed below.

The plasma membrane

As in all eukaryotes, the fungal plasma membrane
consists of a phospholipid bilayer with associated
transmembrane proteins, many of which are involved
directly or indirectly in nutrient uptake. The membrane
also can anchor some enzymes. In fact, the two main
wall-synthetic enzymes, chitin synthase and glucan
synthase, are integral membrane proteins; they
become anchored in the membrane in such a way that
they produce polysaccharide chains from the outer
membrane face (Chapter 4). The plasma membrane
has a third important role, in relaying signals from the
external environment to the cell interior – the process
termed signal transduction, discussed in Chapter 4.
The fungal plasma membrane is unique in one
important respect – it typically contains ergosterolas
the main membrane sterol, in contrast to animals,
which have cholesterol, and plants which have phyto-
sterols such as ββ-sitosterol. The Oomycota also have
plant-like sterols. But some plant-pathogenic fungi
such as Pythiumand Phytophthoraspp. are unable to

58 CHAPTER 3
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