Introduction to Fungi, Third Edition

8
Ascomycota (ascomycetes)
8.1 Introduction
The phylum Ascomycota (colloquially called
ascomycetes) is by far the largest group of
fungi, estimated to include more than 32 000
described species in 3400 genera (Kirk et al.,
2001). It is assumed that the majority of
ascomycetes has yet to be discovered, and the
total number of species may well be higher by
a factor of 10 20 or even more (see Hawksworth,
2001). The name is derived from the Greek words
askos (a leather bottle, bag or bladder) and mykes
(a fungus), so ascomycetes are sac fungi. The
characteristic feature of the group is that the
sexually produced spores, the ascospores
(see p. 25), are contained within a sac, the
ascus. In most ascomycetes the ascus contains
eight ascospores and is turgid, ejecting its spores
by a squirt mechanism.
There is a very wide range of lifestyles.
Some ascomycetes are saprotrophs, others
necrotrophic or biotrophic parasites of plants
and animals, including humans. Examples of
biotrophic parasites are the Erysiphales, the
cause of many powdery mildew diseases of
plants (Chapter 13), the Taphrinales (p. 251)
causing a range of plant diseases associated
with growth abnormalities, and the
Laboulbeniales, relatively harmless ectoparasites
of beetles and some other insects (Blackwell,
1994; Weir & Blackwell, 2001). Many ascomycetes
grow as endophytes in symptomless associations
with plants. Some are mutualistic symbionts, for
example the lichens (Chapter 16) which make up
about 40% of the described species of ascomycetes.
Lichens are dual organisms consisting of
a fungus (usually an ascomycete) and a photosynthetic
alga or cyanobacterium living in close
association. This type of association has evolved
independently in several unrelated groups of
ascomycetes and indeed it has been claimed that
several major fungal lineages are derived from
lichen-symbiotic ancestors (Lutzoni et al., 2001),
although this hypothesis is under dispute (Liu &
Hall, 2004; see Fig. 8.17). Symbiotic mycorrhizal
relationships also exist between true truffles
(e.g. Tuber spp.) or false truffles (e.g. Elaphomyces
spp.) and trees such as oak and beech (see pp. 423
and 313). The range of habitats is wide, as would
be expected of such a large and diverse group of
fungi. Ascomycetes grow in soil, are common on
the above-ground parts of plants, and are also
found in freshwater and in the sea.
Most ascomycetes are recognized by their
fruit bodies or ascocarps, i.e. the structures
which surround the asci. These will be described
more fully later (see Fig. 8.16).
8.2 Vegetative structures
Ascomycetes may grow either as yeasts, i.e.
unicells multiplying by budding or fission,
or as mycelia consisting of septate hyphae
(Fig. 8.1a). Some fungi may switch from the
yeast to the filamentous state or vice versa,
i.e. they are dimorphic. A good example of a
dimorphic fungus is Candida (see Fig. 8.1b).