untitled

of clamp connections (Fig. 2.20) and the development

of asci in the Ascomycota. In both cases the nuclear

distribution is maintained by a hypha growing back

upon itself and fusing with the cell behind it. And in

both cases there is a dikaryotic phase, although it

occurs only temporarily in the ascus initials. These lines

of evidence reinforce the view that the Ascomycota and

Basidiomycota are closely related.

Ecology and significance

The Phylum Basidiomycota exhibits the most extreme

variation of any group of fungi. Some of the yeasts

on plant surfaces are basidiomycetous, although they

are far outnumbered by the ascomycetous yeasts.

A common example is the salmon-colored yeast,

Sporobolomyces roseus, which grows on senescent leaf sur-

faces. It divides by budding but also has a sexual stage,

in which single yeast cells produce a sterigma and release

a basidiospore, apparently by the same mechanism as

in mushrooms. S. roseusis a common inhabitant of

leaf surfaces, and the spore population in the air can

be high enough to cause respiratory allergies. But a few

basidiomycetous yeasts are much more significant

and can be life-threatening pathogens of humans,

especially of people with impaired immune defenses.

The most notable examples are Blastomyces dermatitidis,

Cryptococcus neoformansand Histoplasma capsulatum,

all of which can produce sexual stages when strains

of different mating types are crossed in laboratory

conditions, but the sexual stages are seldom found in

nature. The sources of inoculum that initiate infection

in the lungs are common environmental samples,

such as birds’ droppings and even the bark of eucalypt

trees (Chapter 16). The important lesson from this is

that even the apparently insignificant saprotrophic

fungi of plant surfaces and other materials can have a

significant impact on human health.

Some of the most economically important plant

pathogens are members of the Basidiomycota. They

include the many species and genera of rust fungi

(Uredinales) that are specialized to infect particular

types of plant. One of the best-known examples is

Puccinia graminis, which causes black stem rust of

wheat. These fungi can have complex life cycles, with

up to four different types of spore being produced at

different stages of the life cycle (Chapter 14). Another

important group of pathogens, the smut fungi

(Ustilaginales), are unusual because they grow only as

yeasts in culture but as mycelia in their hosts. Again,

these are discussed in Chapter 14.

The most familiar fungi in the Basidiomycota

belong to the class Hymenomycetes, and include

almost all the larger fungi, such as the mushrooms (com-

monly known as toadstools), woody brackets, puffballs,

earth-stars, splash cups, and jelly-like structures. Most

of these “higher” Basidiomycota degrade polymers

such as cellulose, hemicelluloses and lignin. They are

found in composts (e.g. Coprinus), leaf litter (e.g.

Mycena), the thatch of dead leaf sheaths in old grass-

lands (e.g. Marasmius oreades, which produces “fairy

rings”; Chapter 11) and in woody substrates, where

they play a major role in wood decay (Chapter 11).

Some of these fungi cause major tree diseases; e.g.

Armillaria spp. in hardwood stands (Chapter 5) and

Heterobasidion annosumin conifers (Chapter 12).

Many other mushroom-producing fungi form

mycorrhizal associations with forest trees, although

DIVERSITY OF FUNGI 33

Hyphal tip with two nuclei

Clamp branch forms

Branch attaches to hypha;

nuclei divide

Clamp branch sealed

off by septum

Wall dissolves, nucleus migrates

Clamp

Clamp

Fig. 2.20The role of clamp connections in maintaining a regular dikaryon.

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