Recognition of the unique characteristics of fungi led mycologists to establish a separate kingdom, Kingdom Fungi, in the late 1960s. More recently, some mycologists have noted that some organisms, such as slime molds, downy mildews, and water molds, have characteristics that place them in the Kingdom Protista rather than the fungi.
Unlike true fungi, some slime molds have a mobile, multinucleate feeding stage similar to amoebas. Downy mildews and water molds produce motile cells for part of their life cycle, have hyphal walls that lack chitin, and make an egg cell and sperm nuclei. Some scientists have proposed that downy mildews and water molds deserve to be classified in a separate kingdom, called Kingdom Stramenopila.
Fungi are classified primarily by the type of spores and fruiting bodies produced. Many mycologists divide the Kingdom Fungi into four main phyla: Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota. A fifth phylum, Deuteromycota, is used by some taxonomists for fungi that typically produce only asexual spores.
The phylum Chytridiomycota, commonly called Chytrids, includes approximately 800 species that are found in aquatic (freshwater and marine) or moist habitats. Chytrids are among the smallest and simplest fungi. Most have a central body with small tubelike extensions, while others produce a small network of hyphae.
Chytrids develop a structure called a sporangium that has motile spores equipped with a posterior flagellum, a long, whiplike tail that aids in locomotion. Chytrids grow as saprobes in damp soils and water, or as parasites of plants, animals, algae, protists, and other fungi. Some do not require oxygen and live only in the guts of herbivores, where they break down material containing cellulose and other compounds. Because chytrid spores are motile, some mycologists have classified them in the Kingdom Protista.
The Zygomycota include approximately 900 terrestrial species, including many important decomposers, mycorrhizal fungi, and parasites of spiders and insects. One of the most common zygomycetes is black bread mold, often found on bread, fruit, and other food products. The fungus looks like a fuzzy growth with tiny black dots at the tips of the fuzz. The black dots are sporangia growing at the ends of special hyphae. The sporangia produce asexual, nonswimming spores called sporangiospores. Zygomycetes reproduce sexually by forming thick-walled zygospores.
The largest group of fungi, with around 50,000 known species, is the Ascomycota, or sac fungi. This group includes yeasts, lichens, morels, cup fungi, truffles, and a number of plant parasites such as powdery mildews. Named for the sexual spores produced inside saclike cells called asci (singular, ascus), Ascomycota also may produce very fine, almost powdery asexual spores called conidia. Certain Ascomycota such as cup fungi produce fruiting bodies with sexual spores on their upper surface, while others, including the truffles, produce sexual spores inside tuber-like fruiting bodies that develop underground.
Ascomycetes are used to produce Camembert and Roquefort cheeses. The slight grittiness in these cheeses is due to conidia being crushed between the teeth. The mold ergot, which infects the flowers of rye and other grains, produces toxins that can poison humans and other animals that eat the infected grain. The yeast Candida albicans is a common pathogen of humans, causing such ailments as oral thrush and vaginal yeast infections. In people with weakened immune systems, this yeast may spread widely throughout the body and become life threatening.
The Basidiomycota, also known as club fungi, include around 25,000 species of mushrooms, puffballs, bird’s nest fungi, jelly fungi, rusts, smuts, and shelf and bracket fungi. This division contains important plant parasites, mutualists, and saprobes, including decay fungi that cause brown rot and white rot of wood. These fungi are named for their specialized, club-shaped reproductive cells, called basidia, which form spores called basidiospores. Basidia may line gills or tubes on the underside of fleshy fruiting bodies, which consists of a stalk and cap—the familiar components of most mushrooms. Certain Basidiomycota produce spores inside tuber-like underground fruiting bodies, called “false truffles.”
Many basidiomycetes are saprobes, which play a vital role in the decomposition of litter, wood, and dung. A number of mushrooms are good to eat, such as boletes and chantarelles, both of which are highly prized for their distinct flavor. Other mushrooms are well known for their poisonous qualities, including the death cap (Amanita phalloides). Some, such as the liberty cap (Psilocybe semilanceata) and the fly agaric (Amanita muscaria), are well known for their hallucinogenic properties. Smuts—such as Ustilago, which attacks corn, and stinking smut (Tilletia), which attacks wheat—are common basidiomycetes that invade flowering plants, especially cereal grasses, and cause serious economic loss. Rusts, such as Puccinia, which attacks wheat, invade plant cells of agricultural crops and forest trees, causing millions of dollars in losses each year.
The Deuteromycota, or imperfect fungi, comprise about 25,000 species, many of which do not have a defined sexual cycle. They typically reproduce asexually by spores called conidia on specialized hyphae called conidiophores. The deuteromycetes include many molds, some of which are important to humans. Penicillium, the mold used to develop the first antibiotic, is sometimes classified in the Deuteromycota. On the other side of the ledger, the deuteromycetes also include organisms such as ringworm that are serious animal and plant pathogens.
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