What! The oyster mushrooms and chicken leg mushrooms you usually eat actually "eat meat"?

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 pure fungi, mushrooms and plants are essentially different.

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.

When it comes to plants that prey on insects, everyone 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.

Venus flytraps (above) and pitcher plants (below) can trap insects and other small animals to supplement their own nutrition by forming specific predatory organs.

01

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 a wide range of life (fresh water, sea water, and land everywhere).

They are very small, with a body length generally ranging from 0.5 to 3 mm. They mainly live in the soil or parasitize on animals and plants.

Nematodes have extremely 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 wireworms that parasitize insects, and the roundworms and pinworms we are familiar with can all cause destructive harm to the host through rapid reproduction in a short period of time.

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 worms is the most common and most abundant source of meat in the living environment of mushrooms.

Therefore, most of the predatory fungi observed by mycologists in the early days fed on nematodes.

Moreover, most of these fungi belong to ascomycetes or basidiomycetes, and their sexual generations often form fruiting bodies that we call "mushrooms" (in layman's terms, the organs through which fungi reproduce sexually).

The oyster mushrooms (Pleurotus ostreatus), cloud mushrooms (Stropharia officinalis), and chicken leg mushrooms (Coprinus comatus) that we often eat are all typical nematode-feeding fungi.

Therefore, scientists gave this type of fungus a vivid general name - "Carnivorous mushroom".

It may be hard for you to imagine that mushrooms such as Pleurotus ostreatus (A), Coprinus comatus (B), and Stropharia rugosa (C), which we usually eat as vegetarian dishes, can actually be eaten as meat. Image source: provided by the author

02

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:

1. Predator Pie

Predatory fungi are like Shaolin masters in the world of martial arts, fighting head-on with real guns and swords.

They hunt by relying on predators differentiated from nutrient hyphae, and their strategies for winning are 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.

2. Endoparasites

Endoparasitic fungi are like the Wudang masters in the world of martial arts, using softness to overcome hardness and using little force to achieve great results.

They mainly kill and eat insects by producing three types of spores: cystic spores, sticky spores and engulfing spores, and then allowing the spores to 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 Cordyceps sinensis that everyone is familiar with is actually the larvae of Lepidoptera insects that are parasitized after eating the spores of the Cordyceps fungus.

The swallowed 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 body and develops new spores (B). Image source: Reference 4

3. Egg parasitism

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 and usually parasitize on host plants silently.

Only when the time is right will they colonize the cysts, oocysts or eggs of the nematodes. They can use the nutrients in the worm's body 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.

Conidia of Paecilomyces lilacinus germinate on the surface of root-knot nematode eggs (A), while fungi of the genus Chlamydomonas can parasitize by forming appressorium on nematode eggs (B). Image source: Reference 5

4. Toxin-producing group

Toxin-producing fungi are comparable to the poison masters such as Ouyang Feng, Ding Chunqiu and Lan Fenghuang in Jin Yong's martial arts novels. They can kill or inhibit the life activities of nematodes by secreting toxins.

The toxins secreted by these fungi mainly kill insects by paralyzing nerves, destroying body walls, inhibiting functional enzymes, etc. The oyster mushrooms (Pleurotus ostreatus) that we usually eat are masters of toxin production.

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

03

"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...

1. Three-dimensional bacteria network

The so-called three-dimensional fungal net is actually a mesh trap with a three-dimensional structure formed by the chain interaction of the nutritional hyphae of fungi.

The reason for the formation of such a structure is that the nematodes have no visual organs. The fungus uses this trap to wait for the nematodes to come to the door.

Once the nematodes enter the fungal web, the sticky hyphae on the web will trap them firmly, and then slowly digest, decompose and absorb them.

2. Sticky Ball

A sticky ball is a spherical or nearly spherical sticky cell that usually develops from the vegetative hyphae of fungi.

The reason why it is sticky is that it is surrounded by a layer of fibrous extracellular polymers. After the nematodes are attached by the sticky substance on the surface, the hyphae can invade and digest the nematodes.

In addition to spherical sticky predators, some fungi also use hyphae to form sticky branching structures that have the same function.

3. Non-contracting ring

The non-contractile ring is similar in shape to the lasso used by humans for hunting, but it cannot be contracted.

It is a ring of 3 or 4 cells formed by cell fusion of fine 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 not escape the fate of being eaten.

Three-dimensional bacterial web (A), sticky sphere (B), non-contractile bacterial ring (C) and sticky branch (D).

4. Shrink ring

Compared with the above three weapons, the contractile ring is a more sophisticated predatory organ.

It is usually composed of 3 cells. When the nematode drills into the ring, the cells will expand rapidly and be captured. It works like a trigger trap designed by humans.

The contractile ring shrinks automatically because the nematode exerts pressure on the cells lining the ring.

Under the transmission of cell signals, the three cells on the contractile ring will expand rapidly in less than 0.1s to lock the nematode, and then the hyphae will gradually penetrate and digest the entire worm body.

The mode of action of the contractile fungus ring and its effectiveness in capturing nematodes Source: Wikipedia

5. Spiny 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 small balls will scratch the bodies of nematodes passing by, causing them to lose the ability to move and then allow the hyphae to prey on them.

The thorn-like balls developed from the hyphae of Coprinus comatus. Image source: Wikipedia

6. Spores

In addition to forming predatory organs, some fungi can also produce spores that use 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, some with hooks, some with points, some with thorns, some with blades...

These shapes allow the spores to attach firmly to the bug's gullet.

Once the spores enter the body, they begin to take root and sprout inside the bug until they pierce the intestines and spread throughout the body, eventually killing the bug 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.

7. Toxins

In addition to predatory organs and spores, it is more economical and efficient for fungi to use toxins formed by their own metabolism to paralyze and kill nematodes.

Studies have found that the nematicidal compounds produced by carnivorous mushrooms include nearly 100 species in 10 major categories, including quinones, alkaloids, terpenes, peptides, furans, etc., among which there are many metabolites with broad-spectrum insecticidal effects.

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.

04

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. Their main components are lignin and cellulose. These two high-molecular polymers are often connected to each other to form a lignin-carbohydrate complex, which has a strong structure and is difficult to decompose.

Moreover, even after being decomposed by fungi with great difficulty, these dead branches and leaves can only provide carbon source nutrients.

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.

05

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

During the long process of evolution, faced with a harsh living environment, in order to meet their own nutritional needs, 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 forces carnivorous mushrooms to evolve these efficient and precise killing instincts.

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

References:

[1]. Li Tianfei et al. A brief history of research on nematode-feeding fungi. Research and Application of Entomogenous Fungi in China (Volume 4), China Agricultural Science and Technology Press, 1997

[2]. Liu Xingzhong et al. Research progress on nematode-feeding fungi. Proceedings of the Third National Symposium on Entomogenic Fungi, 1991.

[3]. Ching-Han Lee, et al. Sensory cilia as the Achilles heel of nematodes when attacked by carnivorous mushrooms. PNAS, 2020, 117(11)

[4]. FH Wood. Nematode Trapping Fungi from a Tussock Grassland Soil in New Zealand. New Zealand Journal of Botany, 2012, 11(2)

[5]. Ma Ni et al. Research progress in genome sequencing and analysis of nematode-feeding fungi. Microbiology Bulletin, 2018, 45 (4)

[6]. Zhang Ying et al. Overview of research on nematode-feeding fungi resources. Journal of Mycology, 2011, 30 (6)

[7]. Higgins ML, et al. Fungal morphogenesis: ring formation and closure by Arthrobotrys dactyloides. Science, 1967, 155(3760)

[8]. Luo H, et al. Coprinus comatus damages nematode cuticles mechanically with spiny balls and produces potent toxins to immobilize nematodes. Appl Environ Microbiol, 2007, 73

[9]. Dong Yanjin et al. Research progress on fungal nematicidal metabolites. Mycosystems, 2001, 20 (2)

Produced by | Science Popularization China

Author: Li Bo, Shaanxi Institute of Biological Agriculture

Producer｜China Science Expo

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