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diseases of humans whose immune system is deficient

or impaired. The “Moulds of Man” are covered in

Chapter 16.

In a different context, some of the Ascomycota form

mycorrhizal associations with forest trees (e.g. Tuber

spp – the truffles) and an estimated 96% of the

13,500 known species of lichens have Ascomycota as

the fungal partner. Very few of these lichenized fungi

grow in a free-living state. In terms of numbers, the

ascomycetous lichen fungi probably represent the

largest single component of biodiversity in the entire

fungal kingdom (Chapter 13). Many ascomycetous

yeasts (Saccharomyces, Candida oleophila, etc.) com-

monly grow as saprotrophs on leaf and fruit surfaces,

and some are now being marketed as biological con-

trol agents to prevent fruit rots (Chapters 11, 12).

Many other ascomycetous fungi degrade cellulose

and other structural polymers; e.g. Chaetomiumin soil

and composts, Xylariaand Hypoxylon as agents of

wood decay, Sordariaand Ascobolusin herbivore dung,

and Lulworthia on wood in estuarine environments

(Chapter 11).

Basidiomycota

According to Kirk et al. (2001), the Basidiomycota con-

tains about 30,000 described species, which is 37% of

the described species of true fungi. Although the most

familiar examples of this group are the mushrooms and

toadstools, there is an enormous diversity of species,

including basidiomycetous yeasts, many important

plant pathogens, and some serious human pathogens.

The one single feature that characterizes the group is

the basidiumin which meiosis occurs, leading to the

production of sexual spores (basidiospores) that usu-

ally are produced externallyon short stalks termed

sterigmata (Fig. 2.19).

Several other features are found in the

Basidiomycota. The hyphae often have a complex

dolipore septum(Chapter 3) that prevents nuclei

from moving between different hyphal compart-

ments. The cell wall typically is composed of chitin and

glucans (but chitin and mannans in the yeast forms).

The nuclei typically are haploid, but throughout most

of the life of a colony each hyphal compartment

contains two nuclei, representing two different mating

compatibility groups. A mycelium of this type is termed

a dikaryon. The significance of this will be explained

shortly.

Taxonomy and relationships

The Basidiomycota is a monophyletic group (all of its

members having a common ancestry) and is a sister

group to the Ascomycota. In other words, both of

these groups have a common ancestor. Three major

sub-groups are recognized within the Phylum

Basidiomycota:

1 Urediniomycetes, including the rust fungi

(Uredinales) which are economically significant

plant pathogens of many crops and wild plants.

2 Ustilaginomycetes, including the smut fungi

(Ustilaginales) some of which again are significant

plant pathogens, and gain their name from their

black, sooty spores.

3 Hymenomycetes, including mushrooms, puffballs,

and jelly fungi.

However, the phylogenetic relationships between and

within these three groups are still unclear.

Significant features in the life-cycle of Basidiomycota

We will begin by considering the generalized life-cycle

of a typical mushroom (Fig. 2.18). Basidiospores, each

containing a single haploid nucleus, germinate and

grow into hyphal colonies that have a single nucleus

in each hyphal compartment. This phase is termed a

monokaryon(meaning that it has one nuclear type in

each hyphal compartment). The monokaryon can pro-

duce small oidiathat either act as fertilizing elements

or they can germinate to produce further monokaryotic

colonies. The next stage of development occurs when

the hyphae of two monokaryons of different mating

compatibility groups fuse with one another, or when

an oidium of one mating compatibility group attracts

a hypha of a different mating compatibility group. This

causes a strong “homing response” but the chemical

attractants causing it have not been identified. Once

the two compatible strains have fused by the process

termed plasmogamy, all subsequent growth occurs by

dikaryotichyphae (with two nuclei – one of each com-

patibility group – in each hyphal compartment).

The fungus can grow for many weeks, months or even

years in this form, producing an extensive network of

dikaryotic hyphae, but in response to environmental

signals (Chapter 5) it will produce a mushroom or other

type of fruitbody. All the tissues of the mushroom are

composed of dikaryotic hyphae, but at a relatively

late stage in development of the mushroom the gills

become lined with (dikaryotic) basidia. Within each

basidium the two nuclei fuse to form a diploid

nucleus (karyogamy) which subsequently undergoes

meiosis. Sterigmata then develop from the top of each

basidium, and the four haploid nuclei migrate into the

developing basidiospores (Fig. 2.19). Sometimes only two

spores are produced, instead of four – a common

example being the cultivated mushroom, Agaricus bis-

porus. This is because, after meiosis in the basidium,

2 nuclei enter each spore, and the germinating

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