What is a Mushroom?

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by Getty Images / Martin Wahlborg

Mushrooms are the fruiting bodies of mycelium, a network of threadlike cells that is the vegetative body of the fungus. I think of mushrooms as temples of sex: ornate and highly organized structures that emerge from (and of) mycelium to create and disperse spores– the analog of our eggs and sperm. Mushrooms arise only when conditions are conducive and when the mycelium recognizes sexual reproduction as a priority for the devotion of energy and resources. Once their spores have been dispersed, the fruiting body withers. Mycelium can persist in its substrate as long as it has adequate resources and is not attacked, eaten, or otherwise destroyed. Depending on the circumstances and species, this can be as short as months or as long as millennia. Meanwhile, most types of mushrooms are evanescent–some emerge, sporulate, and decay within hours. More last a number of days to weeks, and some live for months, years, or even decades.

Taxonomy and Classification of Mushrooms

Fungi are a large and diverse group of organisms that are classified separately from plants, animals, bacteria, and protists, forming their own kingdom. Mushroom-forming fungi exist in two of the seven broad divisions (phyla) of fungi: the Ascomycetes and the Basidiomycetes. Ascomycota, the sac fungi, is a very large and diverse group that includes some mushrooms; however, most Ascomycetes take forms other than mushrooms. The group includes many unicellular yeasts, including Saccharomyces cerevisiae, the species we have to thank for bread, beer, and wine. Many are molds such as Penicillium, Aspergillus, and Fusarium.

While humans have succeeded in cultivating some of the edible and medicinal Ascomycete mushrooms such as morels (Morchella spp.), caterpillar fungus (Cordyceps militaris), and some truffles (Tuber spp.), most of these require advanced techniques, so they will not be the focus of this book. Although many of the techniques presented here can be applied to the cultivation of their mycelium, advanced techniques are required for fruiting them. The phylum Basidiomycota, the club fungi, includes the great majority of mushrooms, such as all the mushrooms with gills, pores, or teeth, and most of the jelly and coral fungi. Nearly all the mushrooms that are grown by humans are in this group, so we will focus on their life cycle and biology.

A Generalized Basidiomycete Mushroom Life Cycle

Spores (center and 12 o’clock)are miniscule propagules. Like sperm and eggs, spores are haploid, containing only half a set of genes (and they lack an embryo, which seeds have). Unlike sperm and eggs, spores can begin to grow without being fertilized by one another.

Germination (12:30)occurs when a spore finds itself in conditions that will support its growth: i.e., the right water, food, air, and temperature. The spore then pushes out a threadlike cell called a hypha (plural: hyphae), which grows longer and branches, forming permeable cross-walls (septa) to control the flow of cell contents. A mass of hyphae is referred to as mycelium. As this mycelium grows, it exudes pheromones for communication with other fungi. These pheromones, similar to those of humans and other animals, call out chemically to potential mates. When a compatible mate is sensed– the pheromone fitting lock-in-key into the receptor–attraction (1o’clock)is established, and the two mycelia grow toward each other.

Fusion (2 o’clock)occurs when the two mycelia meet and become one mycelium. Their nuclei migrate (3 o’clock)into the mycelium of each other, until each cell contains one nucleus from each parent spore. This dikaryotic (two-nuclei) phase (3:30), which is fleeting in our species (the brief moment when the sperm and egg fuse), forms most of the mushroom life cycle. In cultivation, this genetically unique, mated mycelium is referred to as a strain.

Myceliation (vegetative growth, 4 o’clock)continues as the mycelium exudes enzymes to break down molecules in its substrate (the material it lives in and eats) and absorbs the components it needs to build more hyphae and grow. Its immune system produces extracellular antibiotics, antioxidants, and other chemicals to protect itself from viruses, bacteria, other fungi,mycophagous (fungus-eating) invertebrates, and oxidative stress.

Once sufficient energy is accumulated, and when environmental conditions are right, the mycelium is finally capable of producing mushrooms. They begin as dense hyphal knots (4:30), which then differentiate into primordia (5 o’clock), also called pins or pinheads by some cultivators. The mushrooms develop (6 o’clock)as the mycelium pumps the primordia full of nutrient-rich cell fluids, causing stalks to elongate and caps to expand (center).

As the fertile undersurface (gills, teeth, tubes, etc.) matures (6:30-8 o’clock), waves of the sexual cells called basidia (9 o’clock)develop on its surface. Within each basidium, the two nuclei fuse.Their genes combine, and they divide (10 o’clock) into four newnuclei (10:30), each with a unique half-set of genes. These nucleimigrate to the outside of the mature basidia (11 o’clock). Dosedwith just enough nutrients to fuel the initial germination, these newspores are liberated by ballistospory (11:30). Once in the open air,they are carried on the breeze, hopefully landing on fertile substratenear compatible mates. Because relatively few spores will live to complete their life cycle, each mushroom may produce millions to trillions of spores.

Queering the Fungal Queendom

I’ve just described how most mushrooms live and reproduce, but there are many exceptions. While most species’ spores must breed with another spore (heterothallic) as shown in the illustration, about 10% of mushroom species’ spores are self-fertile (homothallic). Often, more than one factor determines mating type (what we think of as “sex”), meaning that many mushroom species have more than two mating types. The extreme example is the split gill fungus Schizophyllumcommune. It has 23,328 mating types, each compatible with 99.98% of the others. This has given it remarkable adaptability.

While aerial spore dispersal (ballistospory) is the norm, some mushrooms have evolved internal spore production and alternative spore dispersal strategies that involve animals, raindrops, or other agents. These are called the gasteroid fungi, because they make their spores inside stomach-like structures. Wait. Do you know of any other beings that make their reproductive units in stomach-like structures? I think we should call these species the uteroid fungi. Several uteroid mushrooms are cultivated, including some truffles and stinkhorns.


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Excerpted from DIY Mushroom Cultivation, by Willoughby Arvalo. Published by New Society Publishers © 2019 by Willoughby Arvalo. All rights reserved.