Mushrooms: Friend Or Foe?

Mushrooms are a type of fungus that feeds on organic matter. While most mushrooms are saprotrophs, feeding on dead organic matter, some are parasitic, obtaining nutrients from living organisms. Parasitic fungi invade their hosts, penetrating their defences and drawing sustenance from their cytoplasm, which can cause disease or even death. Interestingly, some parasitic mushrooms are edible and even medicinal, such as lion's mane mushrooms and elm oyster mushrooms. Poplar mushrooms, another edible variety, are semi-parasitic, causing heart rot in trees. While parasitic fungi may be detrimental in certain contexts, they play an important role in diverse ecosystems by providing nutrients for new plant growth.

Parasitic mushrooms are the fruiting bodies of parasitic fungi

Saprotrophic fungi, which feed on dead organic matter, are the largest group of fungi, while parasitic fungi are the second-largest group. This means that most fungi are not parasites. However, parasitic fungi play an important role in natural ecosystems, and some even provide medicinal benefits. For example, Chaga is a parasitic fungus that grows on birch trees in cold climates. It has medicinal compounds that are formed through its relationship with its host tree. Chaga is commonly ground into a powder and used to make tea.

Another example of a parasitic fungus is Hypomyces lactifluorum, which infects the mushrooms of other fungi, such as Russula brevipes and Lactarius piperatus. It alters the host mushroom's colour, texture, shape, and taste, transforming them into edible mushrooms known as lobster mushrooms. Lion's mane mushrooms are another type of parasitic mushroom that grows on dead trees and logs, as well as from wounds on living trees. They are classified as both saprotrophic and parasitic and are known for their medicinal properties.

Caterpillar fungus, or yartsa ganbu, is a unique parasitic fungus that infects a specific species of caterpillar, kills them, and then uses the nutrients to produce a small mushroom that grows out of the caterpillar's head. This fungus is highly valued for its health benefits and is considered a delicacy. While parasitic mushrooms can provide benefits, it is important to note that they can also be detrimental to their hosts and may cause significant damage.

Parasitic fungi invade and feed on other living organisms

Parasitism is a close relationship between two species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm. Parasites can be single-celled organisms like protozoans or multi-cellular organisms like plants and animals. Fungi, including mushrooms, can also be parasites.

An example of a parasitic fungus is Chaga, which grows on birch trees in cold climates. Chaga is slow-growing, taking 3 to 5 years to form and up to 20 years to weaken its host. It spreads its spores to new trees through wind dispersal and enters the tree through wounds in the bark. Chaga is known for its medicinal compounds, which can only form through its relationship with a host birch tree.

Another example is Cordyceps militaris, which invades living insect pupae and draws nutrients from them to enable its growth and reproduction. Certain fungi, like Septobasidium, form highly specialized parasitic relationships with insects. In this case, the fungus sinks haustoria into the bodies of scale insects that feed on trees, feeding on them without killing them. However, the parasitized insects become sterile, and the uninfected members of the colony ensure the perpetuation of the insect species and the spread of the fungus.

Parasitic fungi can be detrimental to their hosts

Parasitism is a kind of symbiosis, a close and persistent long-term biological interaction between a parasite and its host. Parasites are often much smaller than their hosts, and they do not kill them. However, they can reduce host fitness and harm the host by exploiting hosts for resources necessary for their survival, such as food and shelter. Parasitic fungi are one such example. They invade and feed on other living organisms, including plants, animals, insects, and other fungi. They extract nutrients from the organism's living tissue, often causing damage and sometimes even killing the host.

Parasitic fungi manipulate their own genes of cytokinin biosynthesis and metabolism, and the corresponding genes of the host plant, to infect their hosts. They can also produce cytokinins in large quantities, which are associated with the occurrence of diseases that manifest themselves in the form of tumors, "green islands", and raids. For example, corn cobs infected with the smut fungus Ustilago maydis contained much higher amounts of cis-zeatin and lower levels of cytokinin glucosides than healthy ones.

Chytridiomycetes species are host-specific parasites, only attacking certain groups, species, or strains of microalgae. They can survive without their host if they have enough organic substrates to live on. Chaga, for instance, is a parasitic fungus that grows on birch trees in cold climates. It takes 3 to 5 years to form and up to 20 years to weaken the host until the tree eventually disintegrates from the inside out and dies.

Lobster mushrooms are another example of edible parasitic fungi that feed on the mushrooms of another fungus. Most parasitic fungi are tiny and do not produce mushrooms, but a few species do. Lion's mane mushrooms, for instance, are classified as both saprotrophic and parasitic, as they are often found growing on dead trees and logs, as well as on wounds of living trees. They are one of the few potent medicinal mushrooms that are safe to consume in large quantities.

Parasitic fungi can manipulate their host's behaviour

Parasitic fungi are the fruiting bodies of parasitic organisms that extract nutrients from their living hosts to survive. They invade and feed on other living organisms, including plants, animals, insects, and other fungi. While most fungi are not parasites, some have evolved to manipulate their host's behaviour to increase their transmission to uninfected individuals, thereby enhancing the transmission of their genes into the next generation. This phenomenon, known as the "extended phenotype," was first proposed by Richard Dawkins in 1982. It suggests that the behaviour observed in infected animals is due not only to their genes but also to the genes of the parasites infecting them.

One well-studied example of parasitic fungi manipulating host behaviour is Ophiocordyceps unilateralis, commonly known as the zombie-ant fungus. This fungus primarily infects ants from the tribe Camponotini, including carpenter ants. The infection process begins with the attachment of O. unilateralis spores onto the ant's exoskeleton, followed by infiltration. Once inside the host, the fungus secretes various metabolites that have significant behavioural consequences. It manipulates the ant's behaviour, causing it to leave its canopy nest and descend to the forest floor, which provides an ideal environment for fungal growth due to its temperature and humidity.

The presence of large fungal cell populations beside the host's brain and within its muscles is crucial for behavioural manipulation. Studies have shown that O. unilateralis secretes different metabolites depending on the host tissue it encounters and whether the host is alive or dead. This adaptive behaviour ensures the repeated reproduction of the fungus. Ultimately, the fungus feeds on the ant's brain, killing it, and then sprouts its fruiting bodies from the cuticle, releasing spores to infect other ants.

Another example of parasitic manipulation is observed in worms that induce crickets and other terrestrial insects to commit suicide in water. This behaviour enables the parasite to exit into an aquatic environment favourable for its reproduction. Additionally, ants that consume caterpillar secretions containing dopamine exhibit reduced mobility, which may benefit the parasite by decreasing the host's motivation to move. These manipulations of the host's navigational system and motivation to move showcase the complex strategies parasites have evolved to enhance their transmission and survival.

Parasitic fungi can be edible and medicinal

Parasitic fungi are the fruiting bodies of fungi that extract nutrients from their living hosts to survive. They invade and feed on living organisms, including plants, animals, insects, and other fungi. While most parasitic fungi are tiny and do not produce mushrooms, a few species do. Some of these mushroom-bearing parasitic fungi are edible and even medicinal.

Lion's mane mushrooms, for instance, are classified as both saprotrophic and parasitic. They grow on dead trees and logs and from wounds on living trees. They are one of the few potent medicinal mushrooms that are safe to consume in large quantities. Elm oysters are another example of edible and medicinal mushrooms that can be grown at home. They have a meaty texture, a delicate aroma, and a mild, nutty flavor. They are packed with nutrients and active compounds with antioxidant, anti-inflammatory, and antitumor properties.

Chaga is another parasitic fungus with medicinal properties. It is commonly found growing on birch trees in cold climates and has a rigid, black structure. Chaga has a mildly fruity flavor with vanilla undertones and is often ground into a powder for tea. Many of its beneficial medicinal compounds require a relationship with a host birch tree, and cultivated chaga lacks these properties.

Lobster mushrooms are edible mushrooms that are formed when the parasitic fungus Hypomyces lactifluorum feeds on the mushrooms of another fungus. This parasite typically infects the white fruiting bodies of Russula brevipes and Lactarius piperatus, altering their color, texture, shape, and taste. The resulting mushroom has a firm, dense texture, a seafood-like flavor, and a strong aroma.

While parasitic fungi can be a source of food and medicine, it is important to note that they can also be detrimental to their hosts and may pose threats to human health and the ecosystem.

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