Some other fungi of the Entomophthorales display
a dispersal strategy similar to that described above.
One example is Conidiobolus coronatus, a common
saprotrophic fungus in soil. Another example is
Basidiobolus ranarum, which is found in the gut, the
faeces or the intestines of insect-eating reptiles or
amphibians (e.g. frogs, from which the species name
ranarumis derived). This fungus has recently been
transferred to the Chytridiomycota, based on SSU
rDNA sequencing. A third fungus, Pilobolus (Fig.
2.11b) in the Order Mucorales, shows a parallel mode
of dispersal. This fungus is common in animal dung
and produces a large sporangium which is shot free from
the dung, but the sporangium subsequently breaks
down to release many individual spores. We return to
this topic when we discuss spore dispersal, in Chapter
10.Zygomycota as pathogens of humansSeveral common members of the Mucorales, especially
Rhizopus arrhizus, Absidia corymbiferaand Rhizomucor
pusillus(a thermophilic species) can cause serious,
life-threatening infections of humans. Similar infec-
tions can occasionally be caused by Conidiobolus
(Entomophthorales) and Basidiobolus ranarum. Col-
lectively, these diseases are termed zygomycosis
(infections caused by members of the Zygomycota). In
the vast majority of cases the infection is associated
with predisposing factors. For example, diabetics
with ketoacidosis are among the most vulnerable to
infection, which often develops in the rhino-cerebral
area (the nasal passages, progressing into the brain).
But many other predisposing factors can be involved,
including severe malnourishment in children, severe
burns, cancer, lymphoma, and immunodeficiency or
immunosuppression.
These fungal infections can develop rapidly by
spread through the arteries and invasion of the sur-
rounding tissues. They often require surgical excision
of the affected areas. Even so, the prognosis is poor –
the overall mortality rate is about 50% and can rise to
85% in patients with the rhino-cerebral form of the
disease. The zygomycoses fall clearly into the category
of opportunistic infections, because they are associated
with impaired host defenses. They are among the
most difficult infections to control because of their rapid,
invasive growth.AscomycotaThe phylum Ascomycota contains about 75% of all
the fungi that have been described to date. It is not
only the most important phylum, but also the mostdiverse, and many of the relationships within this
group have yet to be resolved by modern molecular
methods. The one feature that characterizes all mem-
bers of this phylum is the ascus– a cell in which two
compatible haploid nuclei of different mating types
come together and fuse to form a diploid nucleus,
followed by meiosis to produce haploid sexual spores,
termed ascospores. In many species the meiotic divi-
sion is followed by a single round of mitosis, leading
to the production of eight ascospores within each
ascus (Fig. 2.13). In the more advanced members of
the group, many asci are produced within a fruiting
body, termed an ascocarp. This can take various
forms – a flask-shaped peritheciumwith a pore at its
tip, a cup-shaped apothecium, a closed structure that
breaks down at maturity, termed a cleistothecium, or
a pseudotheciumwhich is usually embedded in a pad
of tissue, termed a stroma. In other cases, the asci are
produced singly and are not enclosed in a fruiting
body – for example, this is true for the budding
yeast Saccharomyces cerevisiae(which produces only
4 ascospores in a naked ascus) and the fission yeast
Schizosaccharomyces octosporus (eight-spored asci).
Several of these points are illustrated in Figs. 2.13 –
2.15.
The vast majority of Ascomycota also produce asex-
ual spores by mitosis. These mitospores(as opposed
to meiospores, derived by meiosis) are extremely com-
mon and function in dispersal. In fact, a substantial
number of Ascomycota are known only in the form of
their mitosporic (asexual) stages. Their sexual stages
remain to be discovered and in some cases might have
been abandoned, but SSU rDNA sequence analysis
can be used to link them to the Ascomycota. To give
just one example, the extremely common fungus,
Aspergillus fumigatus, has never been shown to produce
a sexual stage. This fungus is becoming increasingly
common as a major cause of death in hospital
patients undergoing deep surgical procedures, and it
also causes the serious respiratory disease termed
aspergillosis (Chapters 8 and 16). In cases such as
this it is important to know the sources of genetic
variation – is there a sexual stage that we have yet to
discover?
Like many members of the Ascomycota, N. crassa, has
a sexual stage, leading to the production of asci con-
taining ascospores. N. crassais heterothallic, requiring
strains of different mating types (termed Aand a) for
sexual development. The female sex organ, termed an
ascogonium, is often a coiled, multinucleate hypha with
a receptive trichogyne. It is fertilized either by contact
with a spermatium (a small uninucleate cell that can-
not germinate) or by a conidium. Then the ascogonium
produces an ascogenous hyphathat will eventually give
rise to the asci. The process by which this happens is
shown in Fig. 2.16.DIVERSITY OF FUNGI 27
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