The Wonderful World of Fungi

Part XII – Division Basidomycota – Introduction

By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
AAAC Collection Station—Birmingham, Alabama

It has been a while since I wrote anything about fungi on this blog so I thought perhaps I should return to these remarkable organisms in another blogpost. As you may have guessed if you are a reader of some of my posts, I really think fungi are an incredible group of organisms. I find all of the fungal groups interesting and just down right fun to look at and study. What follows is a brief review of fungal taxonomy followed by a brief introduction to the remarkable group of fungi known as the basidiomycetes.

Amanita caesarea

Amanita caesarea

Fungal taxonomy follows very specific rules regarding how a given fungus is named and how it is related to other fungi. All true fungi are placed in the Kingdom Fungi. The fungi are then subdivided into smaller and smaller taxa based upon presumed relationships among the members. The largest subgroup is called a division, which is then subdivided into classes, classes are subdivided into orders, then into families, then into genera, and finally into species. The fungi grouped into a division have some characteristic(s) that all members of that division share, but which are not found in other fungi. (Note: Division is an equivalent term to Phylum). A fungus that produces sexual reproductive spores called basidiospores on the outside of a structure called a basidium is placed in the Division Basidiomycota, common name basidiomycetes. The Basidiomycota together with the Ascomycota (see a previous blog about this group of fungi) form the subkingdom Dikarya. The basidiomycetes are a diverse group of fungi in regards to the fruiting bodies they produce that house the basidia (plural of basidium). The fruiting bodies vary greatly in size, shape, color, and substrata on which they grow, as well as modes of spore dispersal. The dominant means of spore dispersal is via air currents and thus we find basidiospores in our daily pollen/mold spore collections. The fruiting bodies are short-lived and make up only a minor part of the fungal body. The majority of the body of the fungus is the mycelium or thallus, which is not normally seen since it is in the substratum on which the fungus is growing. Basidiomycetes have a unique mycelium (it is dikaryotic and has clamp connections) compared to that of other fungi. This unique mycelium will be addressed in greater detail in a subsequent blog.

The basidiomycetes are the most conspicuous fungi because of the above ground fruiting bodies that they produce, some of which attain a very large size. Almost all of the fungi people refer to as mushrooms are members of this group. These fungi are important for numerous reasons: edible and poisonous mushrooms, plant and animal pathogens, ecological roles they play in nature

Most of the commonly eaten species of mushrooms are basidiomycetes and there are many more species which are edible but are not routinely consumed by the general population since they are not cultivated species. There are also numerous poisonous species of mushrooms in this division of fungi. Most cases of fungal poisoning result from ingestion of species that are classified as basidiomycetes.

There are some well-known plant pathogens in this group of fungi that cause significant crop damage. The rust and smut fungi are basidiomycetes and these fungi cause widespread and extensive plant damage each year. There are also basidiomycetes that cause disease in animals and human. For example, the systemic mycosis cryptococcosis is caused by Cryptococcus neoformans, which is the yeast form of the heterobasidiomycete, Filobasidiella neoformans. Schizophyllum commune causes diseases ranging from asthma to brain tumors in humans.

Members of this fungal division also play several important roles in nature: components of numerous symbiotic associations, formation of lichens, (although there are relatively few who are involved in lichenization), formation of mycorrhizae, and decomposition of cellulose and lignin. Lichens are symbiotic associations between fungi and algae and the majority of lichens have an ascomycete as the fungal symbiont. However, there are a few basidiomycetes that are lichenized.

Some basidiomycetes develop symbiotic associations with certain plant roots, relationships referred to as mycorrhizae. Mycorrhizae are important to the plants which possess them because they are crucial in the uptake of minerals by the plant roots and the overall health of the plants. The fungi that form mycorrhizae with orchids are typically basidiomycetes.

The basidiomycetes are the principal decomposers of cellulose and lignin, components making up plant cell walls. Without these fungal decomposers we would quickly be overrun by a buildup of plant matter in the environment.