Ascomycetes
Over 60,000 species of ascomycetes (sac fungi) have been described by mycologists to date. Ascomycetes consist of some of the most devastating plant pathogens (Campbell, 622). For example, Cryphonectria parasitica, is the agent of destruction for an estimated 4 billion chestnut trees in the eastern United States (Tree of Life, http://tolweb.org/tree?group=fungi). Conversely, various sac fungi are important saprobes of plant material. Approximately half form mutualistic associations with algae producing lichens. Others form mycorrhizae with plants. Some live between the mesophyll cells in leaves and give off toxic compounds to help protect plant tissues from insects (Campbell, 622).
The production of sexual spores in saclike asci is the defining feature of the phylum Ascomycota. The majority of sac fungi bear their sexual stages in macroscopic fruiting bodies (ascocarps) that contain the spore-forming asci (Campbell, 622). Ascomycetes take their name from the sac like mother cell (ascus) that produces sexual spores (ascospores). Different species vary in the number of spores in each sac, but most usually contain eight. The spores of ascomycetes are extremely small and measured in micrometers (Barron , 47). When the ascospores are released from the ascus they are resistant to unfavorable environmental conditions. Although, when the conditions are just right (temperature, moisture, etc.) the spore(s) will germinate to form a new fungus. Dispersal of the ascospores is accomplished by one of the following methods: shot by water pressure from the ascus, wind, splashing or running water, and / or animals (Tree of Life, http://tolweb.org/tree?group=fungi).
The fruitbodies of sac fungi (ascoma) come in the following forms: cleistothecium, perithecium and apothecium (Barron, 47).
Usually less than a millimeter in diameter, the cleistothecium fruitbody is difficult to spot without the aid of a hand lens. Thousands of sacs (asci) along with their spores are produced inside the cleistothecium. Eventually, the asci are released into the environment as the outside wall of the cleistothecium is broken down. A common example of fungi that produce cleistothecium fruitbodies are powdery mildews. These fungi appear as dark dots on the underside of many ornamental plants in gardens. In excess of 30,000 different flowering plants are known to be attacked by powdery mildews (Barron ,48).
The perithecium fruitbody is a
tiny flask-shaped structure that typically measures less than 1 mm tall.
Perithecia are frequently found implanted in great numbers in a large fungal
mass referred to as a stroma. A common example of a fungus in which the stroma
can be seen is Xylaria polymorpha, better known as “Dead Man’s Fingers”.
Breaking the stroma of Xylaria in two reveals numerous tiny chambers
lining the outside edge. Each chamber is a perithecium. Typically only the
neck of the perithecia protrudes from the stroma giving the surface a pimpled
appearance. The tips of the necks contain pores where spores are either shout
out or ooze out like toothpaste (Barron , 48).
The largest and most variable
fruitbody type is the apothecium. Their shapes range from the classical cup and
saucer forms to the more irregular forms of morels and saddle fungi. The key
feature that defines this type of fruitbody is the production of asci in a layer
over the upper or outside facing surface of the fruitbody called the hymenium.
The hymenium of various species is capable of producing millions of asci over
its surface. Once the asci become ripe the spores that they produce are
violently discharged into the air above the fruitbody, which in turn are carried
over long distances by the wind to new sites for growth. For the most part the
spores that are shot into the air are invisible to the naked eye. However, from
time to time millions of asci may discharge at once producing a puff of “smoke”
as they release hundreds of millions of spores into the air. This phenomenon is
commonly referred to as “puffing”. You can at times coax a cup fungus to “puff”
for you by gently breathing on it (Barron, 48).
Ascomycetes can also reproduce asexually. They do so by producing enormous numbers of asexual spores externally at the tips of specialized hyphae called conidiophores. These naked spores are called conidia, taken from the Greek for “dust” (Campbell, 622).
Works Cited
Barron, George. Mushrooms of Northeast
North America. Edmonton: Lone Pine,
1999.
Campbell, Neil A. Biology. 4th ed. Menlo Park: The Benjamin/Cummings
Publishing Company, 1996.
Tree of Life Web Project. 2006. The University of Arizona College of Agriculture and
Life Sciences and The University of Arizona Library. 21 April 2006.
<http://tolweb.org/tree/>