Fungus

I

Introduction

Fungus, any member of a diverse group of organisms that—unlike plants and animals—obtain food by absorbing nutrients from an external source. The fossil record suggests that fungi were present 550 million years ago and may have evolved even earlier. Today thousands of different types of fungi grow on and absorb food from substances such as soil, wood, decaying organic matter, or living plants and other organisms. They range from tiny, single-celled organisms invisible to the naked eye to some of the largest living multicellular organisms. In Michigan for example, the underground portion of an individual Armillaria mushroom, a type of fungus, extends more than 12 hectares (30 acres). Other fungi are among the longest-lived organisms on Earth—some lichens, a living partnership of a fungus and an alga, are thought to be more than 4,500 years old.

A large and widely distributed group of organisms, fungi perform activities essential to the functioning of all natural ecosystems. They are among the foremost decomposers of organic matter, breaking down plant and animal remains and wastes into their chemical components. As such, fungi play a critical role in the recycling of minerals and carbon. Fungi’s value to humankind is inestimable. Certain types of fungi, including several types of mold, have proven extremely valuable in the synthesis of antibiotics and hormones used in medicine and of enzymes used in certain manufacturing processes. Some fungi, such as mushrooms and truffles, are considered tasty delicacies that enhance a wide variety of recipes. Not all fungi are beneficial—some damage agricultural crops, cause disease in animals and humans, and form poisonous toxins in food.

Common fungi include mushrooms, puffballs, truffles, yeasts, and most mildews, as well as various plant and animal pathogens (disease agents), such as plant rusts and smuts. Some experts estimate that there are 1.5 million fungus species, of which approximately 100,000 have been identified. The unique characteristics of fungi led scientists to classify these important organisms into a separate kingdom, Kingdom Fungi (also known as Mycetae). Certain fungus-like organisms, such as downy mildews, water molds (also known as oomycetes), and slime molds, once classified as fungi, are now placed in the Kingdom Protista.

II

Unique Feeding Method

Fungi lack chlorophyll, the green pigment that enables plants to make their own food. Consequently, fungi cannot synthesize their own food the way plants do. In order to feed, fungi release digestive enzymes that break down food outside their bodies. The fungus then absorbs the dissolved food through its cell walls.

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Depending as they do on outside sources for food, fungi have developed various living arrangements that enhance their opportunities for food absorption. Some fungi live as parasites, feeding on living plants, animals, and even other fungi. Certain fungus parasites injure plants and animals, causing millions of dollars of damage to farm animals, crops, and trees each year. For example, the fungus Ophiostoma ulmi, which causes Dutch elm disease, has killed tens of millions of elm trees around the world.

Fungi that obtain their food by breaking down dead organisms or substances that contain organic compounds, such as starch and cellulose, are called saprobes or saprophytes. While they are invaluable decomposers of organic material, saprobes can also cause food spoilage and destroy wood products. During the American Revolution (1775-1783), more British ships were destroyed by wood-digesting saprobes than by enemy attack. Some saprobes even grow in aviation fuels, where they breakdown the fuels, destroying their usefulness.

Some fungi also form highly specialized relationships with other organisms (see Symbiosis). For example, the roots of most plants develop a mutually beneficial association with fungi to form mycorrhizae. Mycorrhizae greatly increase the nutrient-absorbing capacity of the plant root—the fungus absorbs minerals from the soil and exchanges them for organic nutrients synthesized by the plant. Fungi also form mutualistic associations with various animals. For example, leaf-cutting ants cut pieces of leaves and bring them into their underground nests, where they feed them to certain fungi. These fungi primarily live in ant nests, and the ants eat nothing but the fungi. Some termites and wood-boring beetles use fungi to break down the cellulose in wood, making the wood easier for the insects to digest. Plant parasites such as rusts invade plant cells via specialized structures called haustoria that absorb nutrients from the cell.

III

Fungi Structure

With the exception of one-celled species, most fungi are composed of threadlike tubular filaments called hyphae. Each individual hypha is surrounded by a fairly rigid wall usually made of chitin—the same material that forms the exoskeletons of insects. Hyphae that are partitioned by dividing cross walls are called septate hyphae, and hyphae without cross walls are called nonseptate, or coenocytic, hyphae. Fungal cells contain cytoplasm, which is a mixture of internal fluids and nutrients. Cytoplasm flows freely within the hyphae, providing nutrients wherever they are needed.

Hyphae grow by elongation at the tips and by branching to form an interwoven mat known as the mycelium. As the mycelium develops, it may produce large fruiting bodies or other structures that contain reproductive spores. Fruiting bodies are often the most visible structure of a fungus, usually growing above the soil or other surfaces so that the spores can be dispersed by air currents or other mechanisms. In contrast, the mycelium is usually hidden beneath the surface of the plant, animal, or other material it is decomposing. For example, a mushroom mycelium is typically buried beneath the soil surface, while its fruiting body, the familiar umbrella-shaped structure, sprouts from the ground.

IV

Reproduction

The wide variety of fungi demonstrate many reproductive methods. In general, most fungi reproduce by making tiny spores. Fungi typically produce large numbers of spores. A giant puffball, for example, produces an estimated 7 trillion spores.

Fungi typically follow a reproductive cycle that involves the production of sexual spores. These spores contain one or more nuclei and are usually haploid—that is, their nuclei contain one set of chromosomes. When environmental conditions are favorable, the spores germinate and develop into a mycelium that produces fruiting bodies with enormous numbers of sexual spores, which repeat the reproductive cycle. Some fungi produce asexual spores directly from hyphae, which then germinate to produce additional mycelium. The mycelium spreads rapidly, aiding the fungus in dispersal and colonization.

In the reproductive cycle of mushrooms, the mycelium contain hyphae of two mating types, sometimes called plus and minus strains, with no obvious anatomical differences distinguishing them. If plus and minus strains of hyphae fuse, sexual reproduction begins. Initially the nuclei of the two hyphae remain separate, producing an intermediate stage called the dikaryon, meaning “two nuclei.” The dikaryon stage can last from weeks to years, depending upon the species. The two nuclei in the dikaryon eventually fuse to produce a diploid cell—that is, a cell that contains one nucleus with two sets of chromosomes. This cell immediately undergoes meiosis, a type of nuclear cell division that produces offspring with half the genetic material as the parents. Meiosis usually produces four genetically unique haploid spores and the reproductive cycle begins again. This population of genetically different spores has a better chance of surviving environmental changes, such as disease or temperature changes, that may wipe out an entire population of genetically identical spores.

Many fungi can reproduce by the fragmentation of their hyphae. Each fragment develops into a new individual. Yeast, a small, single-celled fungus, reproduces by budding, in which a bump forms on the yeast cell, eventually partitioning from the cell and growing into a new yeast cell.

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