Introduction to Fungi, Third Edition

CLASSIFICATION
247
mutants and the ease with which it can be grown
and cross-mated in culture. The dissection of
ascospores from its asci by micromanipulation
has enabled tetrad analysis to be performed.
Research on this fungus led to the important
one-gene one-enzyme concept. Budding yeast
(Saccharomyces cerevisiae) and a fission yeast
(Schizosaccharomyces pombe) were amongst the
first eukaryotes for which the entire genome
was sequenced. Studies on S. cerevisiae were basic
to the understanding of the biochemistry of
anaerobic respiration whilst studies of S. pombe
have provided key facts by which to interpret the
fundamental process of cell division, which in
turn has a bearing on the understanding of the
apparently uncontrolled growth of cancerous
cells. The economic significance of fermentation
processes involving ascomycetes and their conidial
relatives is immense. Examples include alcoholic
fermentations by yeasts as the basis of the
wine and brewing industries, antibacterial antibiotics
such as penicillin from Penicillium chrysogenum
and cephalosporin from Acremonium spp.,
and organic acids such as citric acid from
Aspergillus niger. The immunosuppressant drug
cyclosporin, which reduces the tissue rejection
response and thus facilitates organ transplants,
is a metabolite of Tolypocladium inflatum. Some
ascomycetes are important in food production as
in bread-making by yeast, cheese ripening by
Penicillium roqueforti and P. camemberti and
the fermentation of soybeans and wheat
by Aspergillus, yeasts and bacteria to produce
soy sauce. The mycoprotein Quorn is produced
from mycelial biomass of Fusarium venenatum.
Examples of the direct use of ascocarps as food or
food flavourings are morels (Morchella spp.) and
truffles (Tuber spp.).
However, food spoilage may result from
ascomycete contamination. A well-known example
is contamination of cereal grains and grass
by sclerotia of the ergot fungus Claviceps
purpurea, which can cause severe, sometimes
fatal, neurological, muscular and circulatory
diseases such as gangrene or abortion in cattle
and man. Studies on the alkaloid toxins
contained in ergot sclerotia led to the discovery
of drugs useful in obstetrics and the treatment
of migraine, and in the identification of the
hallucinogen lysergic acid. Another potentially
serious mycotoxin is aflatoxin produced in
groundnuts, cereals and other foodstuffs
infected by Aspergillus flavus. Aflatoxins are
highly carcinogenic in poultry and mammals,
including man. Other mycotoxins include zearalenone
from Gibberella zeae, which causes
infertility in cattle and pigs, and trichothecenes
from Trichothecium roseum and Fusarium spp.,
which cause aleukia in farm animals and
man. A family of plant growth hormones,
the gibberellins, now produced commercially,
were discovered in an investigation of Bakanae
(foolish seedling) disease of rice.
It is not surprising that such a large group
as the Ascomycota should contain numerous
pathogens of plants and animals. Lifestyles
are similarly varied, including biotrophic, hemibiotrophic
and necrotrophic associations. Many
ascomycote pathogens are of considerable economic
importance.
8.10 Classification
It is impractical to attempt a detailed classification
of ascomycetes which could include
around 55 orders and 291 families (Kirk
et al., 2001). We shall adopt the simplified
classification outlined by M. E. Barr (2001) and
Kurtzman and Sugiyama (2001). Based on a
wealth of microscopic data, and especially the
results of several phylogenetic analyses, five
major groups (classes) of Ascomycota have
been proposed, namely Archiascomycetes
(Chapter 9), Hemiascomycetes (Chapter 10),
Plectomycetes (Chapter 11), Hymenoascomycetes
(Chapters 12 16) and Loculoascomycetes
(Chapter 17). The latter three are sometimes
called ‘higher ascomycetes’ or Euascomycetes.
The class Hymenoascomycetes contains ascomycetes
producing asci in a hymenium, i.e. in a
fertile layer around which the ascocarp develops.
This is in contrast to the Loculoascomycetes,
where the asci develop in a pre-formed stroma.
Since the Hymenoascomycetes are a very large
and diverse group, we have subdivided them in
this book.