untitled

alternative means of genetic recombination, such as

a “parasexual cycle” that has been described in

(mitosporic) genera such as Penicilliumand Aspergillus

(Chapter 9).

The methods of conidial production

The mitosporic fungi, like the asexual stages of known

Ascomycota, produce conidia in various ways, but

never by cytoplasmic cleavage in a sporangium. Some

of the many methods of conidium production are

shown in Fig. 2.33. It has been proposed that the

main types of conidium development can be either

thallic (essentially by a fragmentation process invol-

ving septation, as in the fungus Geotrichum candidum;

Fig. 2.33g) or blastic, involving swelling of the con-

idium initials before they are separated by septa (e.g.

Cladosporium(Fig. 2.33a) and Alternaria(Fig. 2.33b) ).

Most types of conidium development would fall

into the “blastic” type. For example, the conidia of

Aureobasidium pullulans, now named by its sexual

(teleomorph) stage, Sydowia polyspora, produce conidia

by budding at or near the septa (Fig. 2.33h). The

conidia of Humicolaspp. are produced by a ballooning

of the tip of a conidiophore branch (Fig. 2.33d).

The conidia of many fungi are produced by extrud-

ing spores from flask-shaped cells termed phialides

(Aspergillus, Penicillium, Phialophora; Figs. 2.33c,e,f ).

Other variations are seen in the ways that conidiophores

are arranged. For example, Penicilliumspp. typically have

brush-like conidiophores, terminating in phialides,

and the successive conidia accumulate in chains

(Fig. 2.33e). Aspergillusspp. typically have a conidio-

phore with phialides arranged on a swollen vesicle,

and again they produce chains of conidia (Fig. 2.33c).

The conidiophores can be aggregated into a stalk

termed a synnemaor coremium(e.g. Pesotum, Fig.

2.33i) or the conidia can arise from a pad of tissue

(an acervulusas in Gloeosporium, Fig. 2.33k) or they

can be produced in a flask-shaped pycnidium(e.g.

Phomopsis, Fig. 2.33j).

Ecology and significance

As will be clear from the comments above, mitosporic

fungi are extremely common and produce abundant

conidia which are dispersed by wind, rain or animals.

Many of these fungi are common saprotrophs, with

important roles in nutrient recycling. Several have

conidia that are large enough 20– 40 μm to impact

in the nostrils, causing hay-fever symptoms (e.g. Alter-

naria). Others have conidia that are small enough

(about 4 μm diameter) to escape impaction in the

upper respiratory tract, and reach the alveoli where they

can cause acute allergic responses (Chapter 10) or in

some cases (e.g. Aspergillus fumigatus) establish long-term

Mitosporic fungi

Finally, among the true fungi, we consider the

large number of ascomycetous fungi that produce

conidia, but whose sexual stages are absent, rare or

unknown. In the past, these fungi have variously

been termed “Deuteromycota” (Deuteromycotina) or

“Fungi Imperfecti” but the more appropriate term is

mitosporic fungi, indicating that the spores are pro-

duced only by mitotic nuclear division. The majority

of these fungi are likely to be assigned to genera of

the Ascomycota by gene sequence comparisons. But at

present, the genera and species with no known sexual

stage are given provisional generic names, termed

“form genera.” When a sexual stage is discovered the

fungus must be renamed, and described according

to the features of its sexual reproductive stage – the

so-called “perfect state.” (Other terms used include

anamorphfor the asexual stage, and teleomorphfor

the sexual stage)

Some very common and important fungi are known

only by their asexual stages. Examples include fungi of

the form-genera Alternaria (although one species has

a sexual stage, termed Lewia infectoria), Aspergillus

(although two species have a sexual stage: Fenellia

flavipesfor Aspergillus flavipes, and Emericella nidulans

for Aspergillus nidulans), Cladosporium, Humicola,

Penicillium, Phialophora, etc.

The interesting question is: why have so many

fungi largely or completely abandoned sexual reproduc-

tion? The answer might be that they have developed

DIVERSITY OF FUNGI 39

Fig. 2.32Amanita muscaria, the “fly agaric” (about 12 cm

diameter). The cap is bright red, with white scales. The

stalk (stipe) bears a ring (annulus) just below the cap, and

at the base of the stalk is a cup-shaped volva (not clearly

visible, but see Fig. 13.6). This species is a common mycor-

rhizal associate, especially of birch (Betulaspp.) and pines

(Pinusspp.). It is extremely poisonous.

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