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Taxonomy and relationships

The Ascomycota is considered to be a monophyletic

group (all members sharing a common ancestry) that

dates back to the coal age, at least 300 million years

ago. This group is closely related to the Basidiomycota

(the phylum containing mushrooms, etc.) as a sister

group that shares similar features.

Currently, the Ascomycota is divided into three

subgroups, although the status of some of the early-

diverging members is still unclear:

1 Archaeascomycetes, including the fission yeast,

Schizosaccharomyces pombe, and some primitive

plant pathogens such as Taphrina spp. (Fig. 2.17).

2 Hemiascomycetes – the true yeasts such as

Saccharomyces cerevisiae, used to produce alcohol

and for bread-making. These fungi do not produce

ascocarps.

3 Euascomycetes, comprising the majority of species

that produce hyphae and ascocarps.

In addition to the main distinguishing feature, the ascus,

the Ascomycota are characterized by cell walls composed

primarily of chitin and glucans, by hyphae that have

cross walls(septa; singular septum) at regular intervals

but with a central pore that allows the passage of

nuclei and other cellular organelles, and by the pro-

duction of non-motile asexual spores(mitospores,

termed conidia) that are produced in various ways, but

never by cytoplasmic cleavage in a sporangium.

The life cycle of Ascomycota

We will use Neurospora crassaas an example to illustrate

significant features of the life cycle of Ascomycota

(Fig. 2.15). Neurospora grows as a branched network of

hyphae with perforated septa, and produces aerial

hyphae that terminate in branched chains of conidia

(asexual spores). These are formed by a bud-like pro-

cess involving swelling and repeated septation. When

mature, the conidia develop a pink coloration and are

easily removed from the hyphae by air currents. This

cycle of sporulation, dispersal and spore germination

occurs repeatedly when substrates are readily available.

There are several variations in sexual reproductive sys-

tems of the Ascomycota, which we need mention only

briefly. For example, N. crassaand N. sitophilaare hetero-

thallic (out-crossing) but some species (Neurospora

tetrasperma, Podospora anserina) produce only 4 binu-

cleate ascospores in each ascus, and are homothallic.

A single ascospore can give rise a colony that will

produce the sexual stage.

Ecology and significance

Because the Ascomycota is a very large and important

phylum, many aspects of its biology are covered

in the later chapters of this book. The group as a

whole includes many economically important plant

pathogens, such as the powdery mildew fungi of

many crop plants (Chapter 14), the vascular wilt fungi

(Chapter 14), Ophiostoma ulmi and O. novo-ulmi,

which cause the devastating Dutch elm disease that

swept repeatedly across Europe and the USA in the last

century (Chapter 10), the equally devastating chestnut

blight disease in the USA, caused by Cryphonectria par-

asitica(Chapter 9) and several toxigenic fungi, such as

Claviceps purpurea(ergot of cereals; Chapter 7).

The Ascomycota also includes several pathogens of

humans, domesticated animals and livestock. For

example, the ubiquitous dermatophytic (ringworm)

fungi are estimated to infect about half of the total

human population, especially in the tropics and

subtropics, but also in developed countries, where

infections such as “athlete’s foot” and “nail fungus”

are common. The ascomycetous yeast, Candida albicans,

is a common commensal organism in the gut and

on other mucosal membranes of humans, causing

irritation to people who wear dentures and to

women during menstruation or pregnancy. And,

some ascomycetous fungi (Blastomyces dermatitidis,

Histoplasma capsulatum) can cause life-threatening

30 CHAPTER 2

Fig. 2.17Taphrina populina, a plant-parasitic member

of the Archaeascomycota which causes yellow blister

of poplar leaves (Populusspp.). The fungus grows as a

mycelium in the plant leaves, causing leaf distortion, and

it produces naked asci which project from the surface of

the leaves. Within the asci the ascospores often bud, so

that the asci become filled with a mixture of ascospores

and budding yeast cells (particularly evident in the top

ascus). (See further images on the website for this book

(see Online resources).)

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