The ascomycetes are fungi (phylum Ascomycota or Ascomycotina) that produce sexual spores in a specialized cell called an ascus. These diverse fungi, with more than thirty thousand species, can be found in almost every ecosystem worldwide. One of the most famous members of the ascomycetes is the truffle.
Ascomycetes, one of the four phyla of the fungus kingdom, by definition possess an ascus, a single cell inside of which sexual spores are produced.
The reproductive process has been well documented and occurs when the dikaryotic mycelium (the mass of hyphae forming the body) undergoes changes that precede the formation of the ascus. Dikaryotic is the genetic state in which two haploid nuclei are present in the cell. One nucleus is donated by each parent.
The first change occurs when the end cell of a hyphal strand begins to form a small bend. The cell divides into three cells; the outer two cells are haploid, and the middle cell is dikaryotic. The middle cell then elongates, and the nuclei migrate into its center.
The two haploid nuclei then fuse to form a single diploid nucleus, which undergoes mitosis and meiosis to form eight haploid nuclei. Cell walls form around the nuclei producing eight haploid ascospores. The ascospores are then liberated from the ascus.
The ascus wall determines the kind of dispersal of the spores. Some asci have a thin, single-layer wall, which breaks down to liberate spores. Unitunicate asci have a multilayer cell wall with a pore at the end of the ascus. Spore release is active through the pore.
|beautifully colored ascomycete|
Bitunicate asci have multilayer cell walls, and release of spores is by the separation of the layers of the cell wall, with the inner layer inflating to several times its normal size and then lifting off of the ascus, allowing the spores to be released.
The spores are released into the environment, where they germinate and produce haploid hyphae. The haploid hyphae fuse with compatible haploid hyphae, forming dikaryotic hyphae, and the process begins to repeat itself.
There are five different ways in which asci are formed in nature. First, asci can be produced by exposure to the environment, as are the asci of yeasts or the ascus of the peach leaf-curl pathogen Taphrina deformans. With these fungi, the ascospores are released by the breakdown of the ascus wall.
The other four ways of production of asci all take place inside structures made from mycelium, called ascocarps. These structures range from totally closed to open, like a cup. The totally closed ascocarps are called cleistothecia. Within these, the asci are scattered, and the spores are released by breakdown and decomposition of the fungal tissue.
Ascomycetes as Pathogens
|Ascomycetes as Pathogens|
It produces a mass of mycelium, called a sclerotium, which is hard and has a density similar to that of a seed. Because of this, the sclerotia are often found in the threshed grain. Sclerotia contain an accumulation of alkaloids and other secondary metabolites.
When the sclerotia are ground into flower and baked into bread, many of these secondary metabolites are passed into the bread. During the Middle Ages this fungus was responsible for a human disease called St. Anthony’s Fire. Today, this fungus is used for the natural production of a coagulant which is used in medicine.
Another group of plant pathogens are the powdery mildews. There are several hundred species of these fungi, which produce a powdery spore mass on the outer surfaces of plant leaves. If a leaf is infected before it has expanded, it will remain small and puckered and may drop from the plant.
The powdery mildews are superficial and send hyphae through the leaf cuticle into the epidermis. The fungus then grows over the surface of leaf, giving it a powdery appearance. During the winter, the fungus produces cleistothecia on the surface of the leaf. Powdery mildew can occur on most plant species and can be very damaging to crop and ornamental plants.
Truffle is a generic term for fungi that form mycorrhizae (a symbiotic association) with the roots of various trees. The fungus grows into the roots and helps the plant tolerate stress, providing the plant with increased absorption of phosphorus from the soil.
In return, the plant gives the fungus metabolites that it needs for growth. These fungi then produce fruiting bodies, either in the soil or upon the surface of the soil. The truffles are produced in the soil up to a depth of 1 foot.
Truffles can be located in the soil using a trained sowor dog that is able to sniff out the volatile chemicals that are produced. However, it is important to note that edible fungi such as truffles can often be mistaken for highly toxic fungi and should never be gathered or eaten without expert identification.
Truffles are used as condiments and are able to impart unique aromas and sensations to food. They are shaped like small balls, varying in size from that of a pea to that of a golf ball. Truffles are only produced in nature and therefore fetch high prices.
The price of truffles depends on the species, size, and freshness. Prices of the famous French Black Périgord truffle (Tuber melanosporum) can reach into thousands of dollars per kilogram (2.2 pounds). In the United States, the Oregon white truffle is quite appealing and can be found in the Pacific North-west.
Morels (Morchella) are another choice edible ascomycete. Shaped like a little hat sitting upon a stalk, they are brown in color and have an appealing aroma. They add flavor to any food and are a favorite of many wild animals.
Some members of the ascomycetes are used for genetic studies. Such is the case for Neurospora, a common fungus found growing on soil and organic matter and one of the first organisms to be found in an area after a fire. A swith all ascomycetes, there are two compatible mating types, which makes for easy genetic study.
Using spore characteristics of shape, color, and texture, it is possible to see how mitosis and meiosis occur in the ascus by determining the placement of the spores. The fungus is readily mutated, which further enhances genetic study.