How did beetles get their hard shells?

Author: Ji Qiaoqiao (Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences)

The article comes from the Science Academy official account (ID: kexuedayuan)

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Beetles (Coleoptera) are a general term for insects in the order Coleoptera. They are distinguished from other insects by the "hard armor" on the outside of their bodies. As the largest order in the class Insecta, there are currently 183 families of beetles in the world, with more than 400,000 species described, accounting for 25% of all species in the world. The "hard armor" is the beetle's exoskeleton, which can withstand a certain amount of pressure and protect its internal tissues.

How hard is it?

Figure 1 A stone weighing more than 15.2kg can break the chest (without any external force other than the weight itself) (Source: Youku video)

Figure 2 The wiggling tentacles remain intact even after being run over by a car (Source: Nature)

Recently, scientists have discovered that the elytra of a beetle called Phloeodes diabolicus can withstand a maximum pressure of 149N (about 15.2 kg, about 39,000 times its own weight) and can survive being run over by a car! Through research on it, scientists have created carbon fiber composite fasteners that are stronger than standard aviation fasteners.

Figure 3 The structure of the fusion site of the elytra of the iron beetle (Source: Reference [2])

So, the question is, what is the composition of such a hard shell?

How did beetles get their hard shells?

The substance that supports the basic form of the human body is calcium skeleton. We can supplement the calcium needed by the body by eating more milk, soy products, kelp, dried shrimps and sunbathing. If insects also have calcium skeletons, it is a bit difficult to imagine how they need to obtain this trace element from nature. After all, they spend most of their time looking for food to supplement the nutrients that maintain the basic functions of the body. If they spend a lot of time looking for calcium, they really can't survive.

The main component of the beetle's internal and external skeleton is chitin (also called chitin), which is a large structural polysaccharide modified by glucose chains. It is widely found in fungal cell walls and certain hard structures of invertebrates and fish. When beetles are laid in the form of eggs, they already have a small amount of polysaccharides to maintain their hatching and form chitin. In the later stage, the nutrition needed by the body is mainly obtained through active feeding.

Chitin is similar to cellulose and keratin (cellulose is the tissue that supports plant morphology and material transport; keratin is the main component of animal nails). It is a structural polymer made of smaller monomers or monosaccharides to form strong fibers. When tissues are secreted outside or inside cells, these tissues will form chemical bonds and connect to each other, increasing the strength of the entire structure.

How is chitin synthesized in beetles?

The process of polysaccharide synthesis of chitin is mainly participated in by multiple enzymes including trehalase (TRE) and chitin synthase (CHS), which is mainly divided into the following steps:

(PS: The following content is added to pretend to be very professional. It is enough to know that polysaccharides form chitin under the catalysis of various enzymes. You can try to understand it. Students who really don’t understand can jump directly to the next part ╮(￣▽ ￣)╭)

1) Synthesis of trehalose:

Glucose-6-phosphate combines with uridine diphosphate (UPD)-glucose under the catalysis of trehalose synthase (TPS) to form trehalose-6-phosphate, which is then further dephosphorylated to form trehalose and phosphate under the action of trehalose phosphorylase (TPP).

2) Synthesis of glucose:

The generated trehalose is further hydrolyzed by trehalose hydrolase (TRE) to produce glucose. TRE can be divided into soluble trehalase (TRE1) and membrane-bound trehalase (TRE2). The former is free in the cytoplasm and is responsible for the decomposition of endogenous (such as circulatory system and digestive system) trehalose; the latter is an extracellular enzyme that binds to mitochondria in muscles and is responsible for the absorption and assimilation of exogenous trehalose. One molecule of trehalose can be hydrolyzed into two molecules of glucose, which are used in various physiological and life activities, including various aspects of insect stress resistance adaptation, which are also achieved by controlling the content of trehalose.

Figure 4 Synthesis pathway of chitin (Source: adapted from reference [3])

3) Synthesis of chitin:

Glucose can only be synthesized into UDP-N-acetylglucosamine after passing through hexokinase (HK), glucose-6-phosphate isomerase (G6PI), fructose-6-phosphate aminotransferase (GFAT), glucosamine-6-phosphate-N-acetyl aminotransferase (GNPNA), phosphoacetamido glucose mutase (PGM) and UDP-N-acetylglucosamine pyrophosphorylase (UAP), and finally chitin is formed by CHS catalysis. CHS is also divided into CHS1 and CHS2. The former is mainly responsible for the synthesis of chitin in the insect cuticle and trachea, while the latter is only responsible for the synthesis of chitin on the midgut peritrophic membrane (close to the inner wall of the midgut, which can wrap food, protect the midgut epithelial cells, and facilitate the absorption and digestion of food).

What is the use of understanding chitin?

After understanding the synthesis pathway of chitin, we can better explain some phenomena that occur during the development of the beetle's hard shell, and we can also intervene in the beetles by acting on the chitin synthesis pathway.

For example, when TRE is inhibited, trehalose cannot be hydrolyzed into glucose, metabolism is blocked, and chitin synthesis in most insects is reduced, resulting in wing malformation, difficulty in molting, weight loss, growth retardation, and even death. Therefore, when crops or forests are attacked by insects, we can consider controlling the disaster by inhibiting TRE (the same is true for inhibiting multiple pathogenic microorganisms); or if the insects we raise show the above symptoms, we will know the cause of the disease and prescribe the right medicine.

In addition, chitin is also widely used in our lives: 1) It has the function of repairing cells, anti-oxidation ability, prevents cell aging, promotes cell renewal, etc., and is widely used in the beauty and skin care industry; 2) It has natural antibacterial effects and can be combined with pure cotton and other fiber products for the production of baby clothing and underwear; 3) Its products are hard, non-toxic, and have good biocompatibility, and can be used for medical treatment materials, such as artificial livers, artificial kidneys, artificial bones, artificial blood vessels, etc.; 4) It is also used in many places in industry, such as the purification of tap water and wastewater, adhesives for dyes and fabrics, etc.

References:

[1] https://zh.wikipedia.org/wiki/%E9%9E%98%E7%BF%85%E7%9B%AE

[2] Rivera J, Hosseini M et al. Toughening mechanisms of the elytra of the diabolical ironclad beetle. Nature https://doi.org/10.1038/s41586-020-2813-8 (2020).

[3] Tang Bin, Zhang Lu, Xiong Xuping, Wang Huijuan, Wang Shigui. Research progress on trehalose metabolism and its regulation of chitin synthesis in insects[J]. Chinese Agricultural Science, 2018, 51(04): 697-707.