All fungi are heterotrophic. Most are saprobes, but the higher fungi display a diversity of other relationships. Several species of Ascomycetes and several Basidiomycetes combine with green algae or even cyanobacteria to form lichens. Lichens are extremely widespread in nature and can tolerate severe environments. They are even found close to the south pole, one of the drier, colder sites in the world. They are also abundant in deserts and alpine areas. "Reindeer mosses", the mainstay of reindeer andother large grazers in the arctic, are fruticose lichens in the genus Cladonia. While tolerant of severe environments, lichens are very sensitive to air pollution. Lichens have almost disappeared from the Los Angeles basin and there is evidence that the level of pollution has crossed the injury threshold of the native lichens in the San Bernardino Mts. and southern Sierra Nevada.

Certain fungi play a crucial role in the mineral nutrition of higher plants. These fungi form associations with plant roots and are called mycorrhizae. The fungus partner obtains carbohydrates from the plant while the plant partner receives an enlarged "root" system that makes available critical mineral nutrients.

(B) Evolutionary relationships and distinguishing characters

The higher fungi appear to be related to one another. Each has cell walls made of chitin. The most primitive of the three divisions, Zygomycota, is coenocytic. In Ascomycota, porous septations (cell walls) occur, but the pores are large enough that nuclei can actually pass through. The septations of the basidiomycetes have pores generally too small for this to occur.

All three groups have few histones associated with DNA in their chromosomes, have no flagellated or ciliated stages, and contain no centrioles. Additionally, the mitotic spindle forms inside the nuclear membrane, which does not break down during nuclear division. This suite of characteristics distinguish these fungi.

(C) Sexual Reproduction

Sexual reproduction usually involves the fusion of two distinct hyphae. In the two more advanced divisions the 2 nuclei do not fuse immediately; rather, a filament with more than one nucleus per cell from different individuals is formed. This is heterokaryosis. True diploidy occurs when the nuclei fuse. Meiosis usually occurs immediately afterwards resulting in the formation of haploid spores. The resulting haploid spores germinate to form new mycelia. Members of all three groups often produce special structures to aid spore dispersal. For example, a mushroom is an example of a "fruiting" body (composed of many heterokaryotic cells) from which spores disperse in some basidiomycetes.

(D) Examples

Some important or familiar genera include: Rhizopus, the common bread mold, a Zygomycete; Claviceps purpurea, ergot, a plant parasite that causes severe illness or death in humans, an ascomycete; Penicillium, an ascomycete genus within which some fermenting species form cheeses, while others are the source of the antibiotic penicillin. Yeasts (Saccharomyces) are unicellular ascomycetes which ferment sugars in the production of alcoholic beers and wines, as well as bread; truffles and morels, reproductive structures of various ascomycete species, are prized as gourmet foods. Amanita is a familiar basidiomycete that kills amateur mushroom collectors each year after the collectors eat the fruiting body, unaware that it is highly toxic.

During this lab focus on the appearance and structure of the hyphae, as this is the basic form of the organism. Examine carefully the reproductive structures. Because fungi are usually filamentous and grow in soil, wood, and detrital materials, much of our familiarity with them is limited to their reproductive structures.

Throughout this exercise be sure to refer to the illustrations in Rust, pp 15-20.