Recent advances in MOLECULAR BIOLOGY have stimulated a renewed interest in fungal classification. Although these studies are not yet complete, there is some consensus; the kingdom Fungi contains 3 major divisions: Chytridiomycota, Zygomycota and Dikaryomycota.
Chytridiomycota are largely aquatic or semiaquatic and usually have swimming stages in their life cycles. Zygomycota are nonmotile, mostly terrestrial and have relatively simple life cycles lacking extended diploid or dikaryotic stages. Dikaryomycota are nonmotile, mostly terrestrial and usually have extended diploid (paired chromosomes) or dikaryotic stages (paired nuclei chromosomes) in their life cycles. The great majority of fungi belong to either Ascomycotina or Basidiomycotina, subdivisions of Dikaryomycota.
Mineral nutrition of fungi can resemble that of plants, whereby all necessary minerals (nitrogen, phosphorus, sulphur, iron, etc), can be assimilated in simple inorganic forms. On the other hand, most fungi can also assimilate minerals combined in organic molecules.
One of the most characteristic features of fungi is their ability to digest complex sources of nutrition outside their cells and then absorb the resulting product. This process is carried out by extracellular enzymes specialized for the digestion of one or a few specific substances. Extracellular enzymes are produced by fungi to digest cellulose, starch, pectin, wood, hair, skin and numerous other substances. Each fungus produces a characteristic set of these enzymes but none produces all.
Although many fungi are saprotrophs (grow on dead organic materials) many others are parasites and derive their nutrition from other living organisms.
Fungal mutualisms are numerous and widespread. The most well-known of these are LICHENS and MYCORRHIZAE. Lichens are the result of a mutualistic association between fungi and algae. The resulting "dual organism" has the ability to live in environments too hostile to support the growth of either the alga or the fungus alone. Mycorrhizae are mutualistic associations between fungi (assimilates mineral nutrients) and the roots of plants (a sugar source.)
Parasitic fungi are numerous and attack members of most major groups of organisms. Of greatest importance to humans are those that cause PLANT DISEASE. Among these diseases are WHEAT stem rust (Puccinia graminis), which reduced Canadian crops in the first half of the 20th century; other cereal rusts (Puccinia), which take a continuing toll; white pine blister rust (Cronartium ribicola), which nearly eliminated the eastern white pine; cereal smuts (Tilletia, Ustilago); potato late blight (Phytophthora infestans); sunflower downy mildew (Plasmopara halstedii); onion downy mildew (Peronospora destructor); CHESTNUT blight (Cryphonectria parasitica), which virtually destroyed the American chestnut; and apple scab (Venturia inaequalis), which often causes heavy losses. Another important aspect of fungal activity is the production of serious toxins in moldy grain.
Although many fungal diseases cause losses to agriculture and forestry, it must be recognized that most wild plants have fungal parasites and yet are able to grow and reproduce without substantial interference from them. Recent studies show that healthy tissues of many plants yield numerous fungi, occasionally over 100 from a single-plant species. These fungi do no visible harm to the plant and may even be beneficial.
Devastating losses occur only when large areas are planted to one, often genetically uniform, crop. In natural grasslands, many GRASS species usually occur in mixed stands. This variety buffers the association against violent change, and rusts, although present, do little harm.
In managed forests, decay can be very costly, and trees are cut while relatively young to reduce loss from decay. This may have unwanted consequences because slash may stimulate growth of decay fungi, and remaining trees may become damaged and provide sites for infection. With fungi that are adapted to all trees and various climates, the problem is complex.
The breakdown of grasslands, litter in forests and other biomes is as important as tree decay, although less spectacular. In each habitat there is an elaborate sequence of fungi, BACTERIA and minute animals which completely reduce tissues to plant nutrients. Fungi are especially important in severe climates (eg, arctic deserts) that have minimal bacterial activity.
Relationship with Humans
These fungi are more than just pests; many produce dangerous mycotoxins that threaten the health of anyone that eats them. Mycotoxins can diffuse through foods and it is important to discard the contaminated substance, not just remove the mold. Research on mycotoxins is an important activity in government and university laboratories in Canada and many other countries.
Molds can also contaminate indoor air, either through their spores or by production of toxic substances. Recent studies have linked indoor mold contamination to serious health problems of the occupants. It is now believed that houses with moldy interiors are as great a threat to the health of children as parental smoking.
Although plant disease and mold contamination are serious problems, the fungi should not be dismissed as entirely villainous. Many are useful and indispensable. Most notable among these are the YEASTS, single-celled fungi belonging to Dikaryomycota. Yeast fermentations are responsible for breadmaking and the production of alcohol.
Some fungi produce antibiotics such as penicillin, one of the great success stories of the 20th century, produced by species belonging to Ascomycotina (Penicillium). There is a worldwide search in progress currently for new fungal products that can be used safely to combat diseases, INSECT PESTS, WEEDS and other threats to our comfort and security.
MUSHROOMS belong to Basidiomycotina and are widely known for their edibility. The cultivation of edible mushrooms is an important activity in Canada. In Ontario, mushrooms are considered to be one of the most important vegetable crops.
Author D.B.O. SAVILE AND DAVID MALLOCH
Links to Other Sites
Canadian Biodiversity Information Facility
A searchable information source about biological species such as plants, animals, and fungi found in Canada. A Government of Canada website.
This digital online database features images of many unicellular and multicelluar species. Includes pictures of human cells, tissues, and organs. A University of Ottawa website.
Flora of North America
The FNA website features information on the names, taxonomic relationships, continent-wide distributions, and morphological characteristics of all plants native and naturalized found in North America north of Mexico.
The website for FLORA Ontario Integrated Botanical Information System. Search the database for information about specific plant species.
Tree of Life
Explore the diversity of Earth's life forms at the Tree of Life website. Also includes beautiful photographs, an extensive glossary of biological terms, and "Treehouses" for younger readers.
White Nose Syndrome in Ontario Bats
Watch a Government of Ontario video clip about the occurance of White Nose Syndrome in Ontario bat populations. From YouTube.
This site features a map illustrating the occurances of white-nose syndrome in North American bat populations. From the US Geological Survey.
The Arctic Ecosystem: Lichens - Nunavut, Canada
A brief video depicting various lichens found in the Arctic. From the Canadian Tourism channel on YouTube.
Shawnadithit grew anxious waiting for her uncle, Longnon, to return to camp at the junction of Badger Brook and the Exploits River, deep in the wilds of Newfoundland...