You treat oyster mushrooms as vegetables, but oyster mushrooms love to eat meat

Produced by: Science Popularization China

Author: Li Bo (Shaanxi Institute of Biological Agriculture)

Producer: China Science Expo

In the eyes of many people, mushrooms are accustomed to being regarded as plants. After all, in wet markets and supermarkets, they are regarded as commodities similar to vegetables.

However, as genuine fungi, mushrooms are essentially different from plants. Their cells do not contain chlorophyll and cannot obtain nutrients through photosynthesis, so they must survive through parasitism or saprophytic means.

However, there are also some tough guys among fungi who are not satisfied with living under the roof of others and eating "leftovers". They can make a living by preying on animals with their own abilities.

Speaking of plants that prey on insects, you may be familiar with Venus flytraps and pitcher plants. They rely on their special organs to prey on insects and other small animals. So how do fungi prey on animals?

Today, I will show you what special skills these meat-loving mushrooms have.

Figure 1 Venus flytraps (left) and pitcher plants (right) can trap insects and other small animals to supplement their own nutrition by forming specific predatory organs

(Photo source: Veer Gallery)

The discovery of carnivorous mushrooms

More than a hundred years ago (1884), German scholar Zopf accidentally discovered that a fungus called Arthrobotrys oligospora can prey on nematodes.

The so-called nematodes are actually a type of pseudocoelom, which is a relatively primitive group in the animal kingdom. There are many species (nearly 30,000 species) and they live in a wide range (fresh water, sea water, and land). They are very small, with a body length of generally 0.5-3 mm, and they mainly live in the soil or parasitize in animals and plants.

Nematodes have a strong reproductive capacity and can reproduce a large number of individuals in a short period of time in a suitable environment. For example, root-knot nematodes and pine wood nematodes that parasitize plants, iron wire nematodes that parasitize insects, and the familiar roundworms and pinworms can all cause destructive harm to the host through rapid reproduction in a short period of time.

Figure 2 Caenorhabditis elegans is a model animal for studying nematodes. Like other nematodes, it has no visual organs and mainly relies on touch and smell to feed.

(Image source: Reference 3)

From the above characteristics of nematodes, it can be seen that this type of worm is the most common and abundant source of meat in the living environment of mushrooms. Therefore, most of the predatory fungi observed by mycologists in the early days feed on nematodes. Moreover, most of these fungi belong to ascomycetes or basidiomycetes, and their sexual generations often form fruiting bodies that we call "mushrooms" (commonly speaking, the organs for sexual reproduction of fungi).

The mushrooms we usually eat, such as Pleurotus ostreatus, Stropharia rugosus, Coprinus comatus, etc., are all typical nematode-eating fungi. Therefore, scientists have given this type of fungi a vivid general name - "carnivorous mushrooms".

Figure 3 You may find it difficult to imagine Pleurotus ostreatus (A), Coprinus comatus (B), and Stropharia rugosa (C)

These mushrooms that we usually eat as vegetarian dishes can actually be eaten as meat.

(Image source: provided by the author)

The Four Major Sects of Carnivorous Mushrooms

In the treacherous world of nature, more than 700 species of carnivorous mushrooms have been discovered so far. Although there are many types, they can be roughly classified into "four major schools" based on their living habits and predation characteristics:

Predator

Predatory fungi are like Shaolin masters in the martial arts world, taking the path of real swords and guns, and direct confrontation. They rely on the differentiation of vegetative hyphae to form predators to hunt, and the routines of winning are also varied.

But in the end, traps are still used to catch nematodes. Despite the diversity of traps, research shows that they have mainly formed two evolutionary lineages: one is fungi that produce contractile rings; the other is fungi that form sticky traps , including three-dimensional fungal webs, sticky balls, and sticky branches.

Endoparasites

Endoparasitic fungi are like Wudang masters in the martial arts world, using softness to overcome hardness and using little force to overcome great power. They mainly kill and eat insects by producing three types of spores: cystic spores, sticky spores and swallowing spores, and then letting the spores attach to the surface of the insect body or invade the body to form a parasitic relationship.

The so-called spores are actually reproductive cells that can directly or indirectly develop into new individuals after separating from their parents. The familiar Cordyceps sinensis is actually formed by the larvae of Lepidoptera insects eating the spores of Cordyceps fungi and then being parasitized.

Figure 4 The ingested spores enter the body through the nematode's mouthparts (A), attach to the nematode's mouth or intestinal wall to form a parasite and eventually kill the nematode. The fungus then grows sporangium from the worm's body and develops new spores (B)

(Image source: Reference 4)

Egg parasites

Compared with the "fierce and direct" predators and the "internal force to defeat the enemy" endoparasitic fungi, the egg-parasitic fungi are the evil demons in the world of martial arts. They are not good at martial arts but only pick on the weak and bully the elderly, children, women and children to be domineering.

Moreover, most of these fungi are opportunists, usually parasitizing on host plants silently, and only colonize on nematode cysts, oocysts or eggs when the time is right. They can use the nutrients in the worm to grow and produce a variety of enzymes to inhibit the hatching of eggs. Paecilomyces lilacinus, which is currently commonly used to control root-knot nematodes, is a typical representative of this school.

Figure 5 Conidia of Paecilomyces lilacinus germinate on the surface of root-knot nematode eggs (A)

Fungi of the genus Chlamydosporium can parasitize nematode eggs by forming appressorium (B)

(Image source: Reference 5)

Toxic

Toxin-producing fungi are comparable to the masters of poisoning in Jin Yong's martial arts novels, such as Ouyang Feng, Ding Chunqiu, and Lan Fenghuang. They can kill or inhibit the life activities of nematodes by secreting toxins. The toxins secreted by these fungi mainly achieve insecticidal functions by paralyzing nerves, destroying body walls, and inhibiting functional enzymes. The oyster mushrooms (Pleurotus ostreatus) that we usually eat are masters of toxin production.

Figure 6 A. Nematodes crawling in a petri dish with Pleurotus ostreatus hyphae;

B. The nematodes encounter the toxin on the hyphae (Droplet);

C. After about 1 minute, the nematode is paralyzed by the toxin;

D. After 72 hours, the nematodes were gradually decomposed and eaten by the hyphae of Pleurotus ostreatus

(Image source: Reference 3)

"Weapon Master" - Carnivorous Mushroom

Sun Wukong wields the golden cudgel and Guan Yu has the crescent-shaped sword.

In China, the ranking of gods depends on magic weapons, and generals kill enemies with swords. No matter who you are, if you go out into the martial arts world without a weapon, you will probably die early.

After introducing the origins of carnivorous mushrooms, we have to talk about the insect-catching weapons they have evolved...

Three-dimensional bacteria network

The so-called three-dimensional fungal web (as shown in Figure 7A) is actually a three-dimensional mesh trap formed by the nutritional hyphae of fungi. The reason for the formation of such a structure is mainly to take advantage of the fact that nematodes have no visual organs. Fungi use this trap to wait for the nematodes to come to the door. Once the nematodes enter the fungal web, the sticky hyphae on the fungal web will firmly trap them, and then slowly digest, decompose and absorb them.

Sticky Ball

The sticky ball (as shown in Figure 7B) is a spherical or nearly spherical sticky cell that is usually developed from the vegetative hyphae of fungi. The reason why it is sticky is that it is surrounded by a layer of fibrous extracellular polymers on the surface. After the nematodes are attached by the sticky substances on the surface, the hyphae can invade and digest the nematodes. In addition to the spherical sticky predators, some fungi also use hyphae to form sticky branched structures (as shown in Figure 7D), which have the same function.

Non-contracting ring

The non-contractile ring (as shown in Figure 7C) is similar in shape to the lasso used by humans for hunting, except that it cannot contract. It is a ring of three or four cells formed by the fusion of thin branches on the hyphae. When the nematode enters the ring, although it cannot contract, the sticky substance attached to the ring will trap it, and the nematode will eventually be eaten.

Figure 7 Three-dimensional bacterial net (A), sticky ball (B), non-contractile bacterial ring (C) and sticky branch (D)

(Image source: References 5, 6)

Shrink Ring

Compared with the above three weapons, the contractile ring is a more sophisticated predatory organ. It is usually composed of three cells (Figure 8A). When the nematode drills into the ring, it will be captured due to the rapid expansion of the cells (Figure 8B, C). Its function is like a trigger trap designed by humans.

The contractile ring automatically contracts because the nematode exerts pressure on the cells on the inner wall of the ring. Under the transmission of cell signals, the three cells on the contractile ring will expand rapidly in less than 0.1 seconds to lock the nematode, and then the hyphae will gradually penetrate and digest the entire worm body.

Figure 8 The mode of action of the contraction ring and its effect in capturing nematodes

(Image source: Wikipedia)

Echinoid globules

The thorn-like balls are a new predatory weapon discovered in Coprinus comatus in recent years. The fungus uses its own hyphae to develop into small balls covered with thorns similar to caltrops. These balls will scratch the bodies of nematodes passing by, causing the nematodes to lose their mobility and then allow the hyphae to prey on them.

Figure 9 The spiny spheres formed by the hyphae of Coprinus comatus

(Image source: Reference 6)

spore

In addition to forming predatory organs, some fungi can also produce their own spores as sugar-coated bullets to attract insects. These spores are delicious food for insects (not just nematodes). But they are like the eighteen kinds of weapons mentioned in storytelling, with hooks, points, thorns, blades... These shapes can make the spores firmly attached to the insect's esophagus. Once the spores enter the mouth, they begin to take root and sprout in the insect's body until they pierce the intestines and spread throughout the body, eventually killing the insect and digesting it.

In addition, some fungi can even produce swimming spores, which, like tracking missiles, can autonomously seek out attack targets and attach themselves to the bodies or eggs of insects to parasitize them.

toxin

In addition to predatory organs and spores, it is more economical and efficient for fungi to paralyze and kill nematodes by using toxins formed by their own metabolism. Studies have found that the nematicidal compounds produced by carnivorous mushrooms involve nearly 100 species in 10 major categories, including quinones, alkaloids, terpenes, peptides, and furans, many of which have broad-spectrum insecticidal metabolites.

But don't worry, although these compounds are toxic to insects, they have no effect on humans. That's why we have been able to eat Pleurotus eryngii mushrooms as food for thousands of years.

Why do mushrooms eat meat?

In the eyes of the world, mushrooms, like plants, are "lower" organisms that cannot "move". Why do they go to so much trouble to prey on animals that are more "higher" than themselves?

In fact, the answer is very simple - because there is not enough nutrition!

As we all know, mushrooms mainly grow on rotten wood or dead leaves, whose main components are lignin and cellulose. These two high-molecular polymers are often connected to form a lignin-carbohydrate complex, which has a strong structure and is difficult to decompose. Moreover, even if they are decomposed by fungi with great difficulty, these dead branches and leaves can only provide carbon source nutrition.

There are a large number of nematodes widely distributed in soil, fresh water and plants. These primitive, fleshy, protein-rich little bugs are a rare source of nitrogen nutrients for fungi.

Fossil records show that carnivorous mushrooms originated in the Mesozoic Era about 100 million years ago. The way of survival by obtaining nitrogen sources from animals has greatly expanded the habitat range of fungi, which has important evolutionary significance for adapting to a wider living environment.

To be or not to be? That is the question!

In the long process of evolution, faced with a difficult living environment, in order to meet their own nutritional needs, the fungi, unwilling to be dull, can only try to obtain the necessary nutritional supplements from other life forms. It is this simple and eternal goal that has forced carnivorous mushrooms to evolve these efficient and precise killing instincts.

Carnivorous Mushroom: Please! I just want to eat my fill!

Editor: Sun Chenyu

References:

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(Note: Latin text should be italicized.)