John W. Taylor, Joey Spatafora, and Mary Berbee
The Sac Fungi
====== Archaeascomycetes (fission yeast, Pneumocystis, Taphrina, etc.)
<<===| === Hemiascomycetes (baker's yeast, Candida, etc.)
=== Euascomycetes (morel, truffle, Penicillium, Neurospora, etc.)
Containing clade(s): Fungi
The Ascomycota, or sac fungi, is monophyletic and accounts for
approximately 75% of all described fungi. It includes most of the
fungi that combine with algae to form lichens, and the majority of
fungi that lack morphological evidence of sexual reproduction. Among
the Ascomycota are some famous fungi: Saccharomyces cerevisiae, the
yeast of commerce and foundation of the baking and brewing industries
(not to mention molecular developmental biology), Penicillium
chrysogenum, producer of penicillin, Morchella esculentum, the edible
morel, and Neurospora crassa, the "one-gene-one-enzyme" organism.
There are also some infamous Ascomycota, a few of the worst being:
Aspergillus flavus, producer of aflatoxin, the fungal contaminant of
nuts and stored grain that is both a toxin and the most potent known
natural carcinogen, Candida albicans, cause of thrush, diaper rash
and vaginitis, and Cryphonectria parasitica, responsible for the
demise of 4 billion chestnut trees in the eastern USA (Alexopoulos et
al., 1996). Asexual Ascomycota, such as Penicillium or Candida
species, used to be classified separately in the Deuteromycota
because sexual characters were necessary for Ascomycota
classification. However, the comparison of nucleic acid sequence, as
well as nonsexual phenotypic characters, have permitted the
integration of asexual fungi into the Ascomycota (Taylor,
The shared derived character that defines the Ascomycota is the
ascus. It is within the ascus that nuclear fusion and meiosis take
place. In the ascus, one round of mitosis typically follows meiosis
to leave eight nuclei, and eventually eight ascospores. Ascospores
are formed within the ascus by an enveloping membrane system, which
packages each nucleus with its adjacent cytoplasm and provides the
site for ascospore wall formation. These membranes apparently are
derived from the ascus plasma membrane in the Euascomycetes and the
nuclear membrane in the Hemiascomycetes (Wu and Kimbrough, 1992;
In hyphal Ascomycota (left), the
terminal hyphal segments develop into 8-spored
asci. In yeasts (right) a single cell simply
becomes the ascus, often with just 4 spores.
At the time they are released from the ascus, the thick-walled
haploid ascospores are resistant to adverse environments. But, given
the right conditions, they will germinate to form a new haploid
The body of Ascomycota is shared by other fungi and consists of a
typical eukaryotic cell surrounded by a wall. The body can be a
single cell, as in yeasts, or a long tubular filament divided into
cellular segments, which is called a hypha (plural, hyphae). Both
yeasts and hyphae have cell walls made of varying proportions of
chitin and beta glucans (Wessels, 1994).
Like other fungi, Ascomycota are heterotrophs and obtain nutrients
from dead or living organisms (Griffin, 1994; Carroll and Wicklow,
1992). If water is present, as saprotrophs they can consume almost
any carbonaceous substrate, including jet fuel (Amorphotheca resinae)
and wall paint (Aureobasidium pullulans), and play their biggest role
in recycling dead plant material. As biotrophs, they may form
symbioses with algae (lichens), plant roots (mycorrhizae) or the
leaves and stems of plants (endophytes). Other Ascomycota
(Ceratocystis and Ophiostoma) form symbiotic associations with an
array of arthropods, where they can line beetle galleries and provide
nutrition for the developing larvae. In return, the beetles maintain
a pure culture of the fungus and transport it to newly established
galleries. As parasites, ascomycetes account for most of the animal
and plant pathogens including Pneumocystis carinii, responsible for
pneumonia of humans with compromised immune systems and Ophiostoma
ulmi, the Dutch elm disease fungus that is responsible for the demise
of elm trees in North America and Europe (Agrios,
Ascomycota can be found on all continents and many genera and
species display a cosmopolitan distribution (Candida albicans or
Aspergillus flavus). Others are found on more than one continent
(Ophiostoma ulmi, or Cryphonectria parasitica), but many are known
from only one narrowly restricted location. For example, the White
Piedmont Truffle (Tuber magnatum) is known from only one provence of
From a human perspective, the most unusual aspect of
all fungi is that they have more than one reproductive option. The
textbook Ascomycota can make spores sexually (ascospores or
meiospores) and asexually (condia or mitospores). Following
meiosis,the ascospores take shape inside the ascus when new cell
walls surround each nucleus as can be seen in the electron micrograph
above (Wu and Kimbrough, 1992). Conidia contain mitotic nuclei, and
their cell wall is simply a modified hyphal or yeast wall.
Ascospores may or may not be shot by water pressure from the ascus
and although wind is the primary dispersal agent once the spores have
been released from the ascus, Ascomycota also use splashing or
running water or animals to disperse their spores (Ingold, 1965).
Conidial diversity reaches its climax with the ascomycetes, with
forms ranging from single spores hardly different from
hyphae(Geotrichum candidum) to elaborate heads of ornamented condida
(Aspergillus niger) and beyond (Cole and Kendrick, 1981).
Ascomycota are either single-celled (yeasts) or filamentous
(hyphal) or both (dimorphic). Yeasts grow by budding or fission and
hyphae grow apically and branch laterally. Most yeasts and
filamentous Ascomycota are haploid, but some species, Saccharomyces
cerevisiae for example, can also be diploid. Mitospores may simply
reproduce the parent, or may also act as gametes to fertilize a
compatible partner. Some Ascomycota must outbreed (heterothallic),
others can also self, and some can only self (homothallic)
(Alexopoulos et al. 1996).
Genetic regulation of sex expression and mating is well-understood
in some model Ascomycota such as yeast, where there are two sexes and
mating is coordinated by oligopeptide pheromones (Marsh, 1991; Glass
and Lorimer, 1991). In hyphal species, cytoplasmic fusion may not be
immediately followed by nuclear fusion, leading to a short dikaryotic
phase. The dikaryotic hyphae may be protected and nourished by
differentiated haploid hyphae which form a fruiting body (the ascoma;
plural ascomata). Ascomata may be closed (cleistothecium), open by a
narrow orifice (perithecium), or broadly open like a cup
(apothecium). Ascospores are released from the ascoma and germinate
to form a new haploid mycelium.
The Ascomycota is a sister group to the Basidiomycota. This
relationship is supported by the presence in members of both phyla of
cross-walls (septa) that divide the hypahe into segments, and pairs
of unfused nuclei in these segments after mating and before nuclear
fusion (dikaryons). Further support comes from the apparent homology
between structures that coordinate simultaneous mitosis of the two
dikaryotic nucli (Ascomycota croziers and Basidiomycota
Sexual Ascomycota all have asci. Comparison of nuclear small
subunit ribosomal RNA gene sequence demonstrates a monophyletic
Ascomycota, although support for the basal branch is not strong
(Berbee and Taylor, 1993; Bruns et al., 1992). Early diverging
Ascomycota have been grouped into the Archaeascomycetes, although
support for the monophyly of this group is not strong (Nishida and
Sugiyama, 1994). The placement of Neolecta among the
Archaeascomycetes is surprising because of the presence of an ascoma,
a feature not found in other Archaeascomycetes or in any
Hemiascomycetes (Landvik et al. 1992). However, there is no reason
that the Hemiascomycetes could not have lost ascomata as hyphal
growth became suppressed in favor of yeasts. The Hemiascomycetes form
a well-supported monophyletic taxon, as do the Euascomycetes (Gargas
et al., 1995). Asexual fungi sharing morphological or molecular
characters of sexual Ascomycota are classified in the Ascomycota;
examples include Candida albicans (Hemiascomycetes) and Pencillium
By comparing nucleic acid sequences, the timing of Ascomycota
evolution has been estimated (Berbee and Taylor, 1993). The
Archaeascomycetes, Hemiascomycetes and Euascomycetes all became
established in the coal age, a bit more than 300 million years ago.
Fossils of these early Ascomycota are not going to be easy to
recognize, because they probably lacked ascoma and their spores were
not distinctive. Fungal-like fossils claimed to be older than 1.0 to
1.2 billion years are probably artifactual. The earliest ascomycete
fossil ascomata and spores are controversial because their age of
deposition significantly predates molecular estimates of their time
of origin. The fruiting bodies may be zygomycetous, and the spores
may have washed into older sediments, or the molecular estimates may
Archaeascomycetes is a class recently discovered from comparison
of nucleic acid sequences and contains species previously thought to
be Hemiascomycetes. Some species, such as the fission yeast,
Schizosaccharomyces pombe, are unicellular, but others grow as hyphae
as well as single cells (for example, Taphrina species). The genera
are distantly related to each other, possibly remnants of an early
radiation of Ascomycota. Archaeascomycetes lack ascomata (Nishida and
The Hemiascomycetes comprises the yeasts and is home to the most
famous fungus, Saccharomyces cerevisiae, better known as the baker's
yeast. Although most members are primarily unicellular, the basal
taxa make abundant hyphae. Hemiascomycetes lack ascomata (Barnett et
Euascomycetes contain well over 90% of Ascomycota, and the species
are hyphal, with almost all of the sexually reproducing forms
possessing ascomata. Most of the recent molecular phylogenetic
effort has been directed at this class (e.g., Berbee and Taylor
1992a, b; Spatafora and Blackwell, 1993, Spatafora,
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Press, San Diego.
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Mycology. John Wiley and Sons, New York. 868p.
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and identification. Cambridge University Press, Cambridge.
Berbee, M. L., and J. W. Taylor. 1992a. Convergence in ascospore
discharge mechanism among Pyrenomycete fungi based
on 18S ribosomal RNA gene sequence. Mol. Phylog.
Berbee, M. L., and J. W. Taylor. 1992b. Two ascomycete classes
based on fruiting-body characters and ribosomal DNA
sequence. Mol. Biol. Evol. 9:278-284.
Berbee, M. L., and J. W. Taylor. 1993. Dating the evolutionary
radiations of the true fungi. Can. J. Bot. 71:1114-1127.
Bruns, T. D., R. Vilgalys, S. M. Barns, D. Gonzalez, D. S. Hibbett,
D. J. Lane, L. Simon, S. Stickel, T. M. Szaro, W. G.
Weisburg, and M. L. Sogin. 1992. Evolutionary relationships
within the fungi: analyses of nuclear small subunit
rRNA sequences. Mol. Phylog. Evol. 1:231-241.
Carroll, G.C. and D. T. Wicklow, 1992. The Fungal Community:
Its Organization and Role in the Ecosystem. Marcel
Dekker, Inc., New York.
Cole, G. T., and B. Kendrick. 1981. Biology of conidial fungi.
Academic Press, New York.
Gargas, A., P. T. DePriest, M. Grube, and A. Tehler. 1995.
Multiple origins of lichen symbioses in fungi suggested
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Glass, N. L., and I. A. J. Lorimer. 1991. Ascomycete mating
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Landvik, S., O. E. Eriksson, A. Gargas, and P. Gustafsson. 1993.
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Spatafora, J., and M. Blackwell. 1993. Molecular systematics
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Taylor, J. W., B. Bowman, M. L. Berbee, and T. J. White. 1993.
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Many thanks to Dave Carmean, Soren Rosendahl for scanning photos and
David Maddison for page design advice.
Department of Plant and Microbial Biology, 111 Koshland Hall,
University of California, Berkeley, CA 94720-3120, USA
Department of Botany and Plant Pathology, 2082 Cordley Hall, Oregon
State University, Corvallis, OR 97330-2902, USA
Department of Botany, University of British Columbia, Vancouver, BC
V6T 2B1, CANADA
Correspondence regarding this page should be directed to John
Page copyright © 1996 John Taylor, Joey Spatafora, Mary
First online 11 March 1996
Last saved 27 May 1996
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