Fungus

All members of the kingdom Fungi are commonly known by the same name, fungus. Fungi have some characteristics in common with both PLANTS and ANIMALS, yet most biologists consider them to be an independent group. Plants, animals and fungi are thought to have a common ancestor, probably a simple eukaryotic (cells having distinct nuclei) organism highly unlike its modern descendants. Recent evidence suggests that fungi and animals are more closely related to each other than either is to plants. In common with animals, all fungi lack chlorophyll and cannot carry out photosynthesis.

Structure
Fungi may occur as independent single cells or they may be multicellular. The fundamental structure of multicellular fungi is the hypha (tubular tissue) usually subdivided by septa (cross-walls). Hyphae not partitioned by septa are termed coenocytic. Although coenocytic hyphae may constitute a single elongated cell, they actually contain many nuclei and function more like multicellular structures.

Classification
Although only about 60 000 species of fungi have been named by mycologists, it has been estimated that the actual number is closer to 1.5 million. Some mycologists consider this estimate to be low. If this figure is accepted, then 94% of all existing fungi have yet to be named. Large-scale destruction of natural habitats may preclude the discovery of most of these.

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.

Habitat
Fungi grow in almost all environments from tropic to high arctic: in soil, freshwater, seawater and in association with many other organisms. Almost every plant and animal, including humans, has at least one fungal parasite. Species attacking humans on the skin or in the lungs are relatively few but are difficult to eradicate. Fungi are highly specialized in their nutrient requirements as well as in their patterns of mating and dispersal. Each species is unique for one or more of these features, making it ideally suited for living in narrowly defined environments. The great numbers of other organisms (living and dead), differences in moisture levels, temperature and other factors account for the great diversity of fungi.

Dispersal
Fungi are dispersed in a variety of ways. It is usually said that most fungi are dispersed by air currents, although studies of airborne fungi using spore traps usually report a limited number of species. Insects, mites and other arthropods are probably more important to fungal dispersal than generally supposed. Other aids to fungal dispersal include water splash and currents, gut passage through invertebrates and vertebrates and motility.

Diet
The nutrition of fungi varies greatly but can be subdivided into 2 components, carbon (or energy) and mineral. Carbon nutrition is not substantially different from that of animals and usually involves the orderly oxidation of hexoses (6-carbon sugars) such as glucose. This most often occurs aerobically by the normal process of respiration but may also be carried out anaerobically (ie, by fermentation).

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.

Associations
Mutalistic and parasitic associations involving fungi are many and often very important.

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.

Biological Importance
Although not strictly parasitism, the decay of living trees is important in natural FORESTS. Decay often occurs in the nonliving heartwood portion of forest trees and may weaken them to the point that they fall. Decay occurs chiefly in overmature or wind-thrown trees; fungi speed the recycling of wood and bark into usable nutrients.

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
The fungi best known to city dwellers are conveniently, if inexactly, termed MOLDS. They include the variously coloured growths that occur in damp environments on fruit, bread, cheese, leather and other organic substances, and involve members of several groups of fungi.

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.

See alsoBIOLOGY and CROP RESEARCH.