Bombyx mori, commonly known as thedomestic silk moth, is amoth species belonging to thefamilyBombycidae. It is the closest relative ofBombyx mandarina, the wildsilk moth.Silkworms are thelarvae of silk moths. The silkworm is of particular economic value, being a primary producer of silk. The silkworm's preferred food are theleaves ofwhite mulberry, though they may eat other species of mulberry, and even leaves of other plants like theOsage orange. Domestic silk moths are entirely dependent on humans for reproduction, as a result of millennia of selective breeding. Wild silk moths, which are other species ofBombyx, are not as commercially viable in the production of silk.
Sericulture, the practice of breeding silkworms for the production of raw silk, has existed for at least 5,000 years in China,[1] whence it spread to India, Korea, Nepal, Japan, and then the West. The conventional process of sericulture kills the silkworm in the pupal stage.[2] The domestic silk moth was domesticated from the wild silk mothBombyx mandarina, which has a range from northern India to northern China, Korea, Japan, and the far eastern regions of Russia. The domestic silk moth derives from Chinese rather than Japanese or Korean stock.[3][4]
Silk moths were unlikely to have been domestically bred before theNeolithic period. Before then, the tools to manufacture quantities of silk thread had not been developed. The domesticatedBombyx mori and the wildBombyx mandarina can still breed and sometimes produce hybrids.[5]: 342 It is unknown ifB. mori can hybridize with otherBombyx species. Compared to most members in the genusBombyx, domestic silk moths have lost theircoloration as well as their ability to fly.[6]
Mulberry silkworms can be divided into three major categories based on seasonal brood frequency. Univoltine silkworms produce only one brood a season, and they are generally found in and around Europe. Univoltine eggs must hibernate through the winter, ultimately cross-fertilizing in spring. Bivoltine varieties are normally found in East Asia, and their accelerated breeding process is made possible by slightly warmer climates. In addition, there are polyvoltine silkworms found only in the tropics. Their eggs typically hatch within 9 to 12 days, meaning there can be up to eight generations of larvae throughout the year.[7]
Eggs take about 14 days to hatch into larvae, which eat continuously. They have a preference forwhite mulberry, having an attraction to the mulberry odorantcis-jasmone. They are notmonophagous, since they can eat other species ofMorus, as well as some otherMoraceae, mostlyOsage orange. They are covered with tiny black hairs. When the color of their heads turns darker, it indicates they are about tomolt. After molting, the larval phase of the silkworms emerge white, naked, and with little horns on their backs.
After they have molted four times, their bodies become slightly yellow, and the skin becomes tighter. The larvae then prepare to enter the pupal phase of their life cycle, and enclose themselves in a cocoon made up of raw silk produced by thesalivary glands. The final molt from larva to pupa takes place within the cocoon, which provides a layer of protection during the vulnerable, almost motionless pupal state. Many otherLepidoptera produce cocoons, but only a few — the Bombycidae, in particular the genusBombyx, and theSaturniidae, in particular the genusAntheraea — have been exploited for fabric production.
The cocoon is made of a thread of raw silk from 300 to about 900 metres (1,000 to about 3,000 ft) long. The fibers are fine and lustrous, about 10 μm in diameter. The number of cocoons required to produce a final yield of silk is approximately 4,000–7,000 per kilogram (2,000–3,000/lb); at least 32 million kilograms (70 million pounds) of raw silk are produced each year, requiring nearly 10 billion cocoons.[8][better source needed]
If the animal survives through the pupal phase of its life cycle, it releasesproteolytic enzymes to make a hole in the cocoon so it can emerge as an adult moth. These enzymes are destructive to the silk and can cause the silk fibers to break down from over a mile in length to segments of random length, which reduces the value of the silkthreads, although these damaged silk cocoons are still used as "stuffing" available in China and elsewhere in the production ofduvets, jackets, and other purposes. To prevent this, silkworm cocoons are boiled in water. The heat kills the silkworms, and the water makes the cocoons easier to unravel. Often, the silkworm is eaten.
As the process of harvesting the silk from the cocoon kills the pupa, sericulture has been criticized by animal welfare and rights activists.Mahatma Gandhi was critical of silk production based on theahimsa philosophy "not to hurt any living thing". This led to Gandhi's promotion of cotton spinning machines, an example of which can be seen at the Gandhi Institute,[9] and an extension of this principle has led to the modern production practice known asAhimsa silk, which iswild silk (from wild and semiwild silk moths) made from the cocoons of moths that are allowed to emerge before the silk is harvested.
The moth is the adult phase of the silk worm's life cycle. Silk moths have a wingspan of 3–5 cm (1–2 in) and a white, hairy body. Females are about two to three times bulkier than males (due to carrying many eggs). All adultBombycidae moths have reduced mouthparts and do not feed.
The wings of the silk moth develop from larvalimaginal disks.[10] The moth is not capable of functional flight, in contrast to the wildB. mandarina and otherBombyx species, whose males fly to meet females. Some may emerge with the ability to lift off and stay airborne, but sustained flight cannot be achieved as their bodies are too big and heavy for their small wings.
2- thoracic legs.Adult silk moth
The legs of the silk moth develop from the silkworm's larval (thoracic) legs. Developmental genes like Distalless andextradenticle have been used to mark leg development. In addition, removing specific segments of the thoracic legs at different ages of the larva resulted in the adult silk moth not developing the corresponding adult leg segments.[10]
A study of an egg of a silkworm from Hooke'sMicrographia, 16651679 study of the silkworm metamorphosis byMaria Sibylla Merian, it depicts the fruit and leaves of amulberry tree and the eggs and larvae of the silkworm moth.
Due to its small size and ease of culture, the silkworm has become amodel organism in the study of lepidopteran and general arthropod biology. Fundamental findings on genetics, pheromones, hormones, brain structures, and physiology have been made with the silkworm.[11][citation needed] One example of this was the molecular identification of the first known pheromone,bombykol, which required extracts from 500,000 individuals, due to the small quantities of pheromone produced by any individual silkworm.[citation needed]
Many research works have focused on the genetics of silkworms and the possibility of genetic engineering. Many hundreds of strains are maintained, and over 400Mendelian mutations have been described.[12] Another source suggests 1,000 inbred domesticated strains are kept worldwide.[13] One useful development for the silk industry is silkworms that can feed on food other than mulberry leaves, including an artificial diet.[12] Research on the genome also raises the possibility of genetically engineering silkworms to produce proteins, including pharmacological drugs, in the place of silk proteins.Bombyx mori females are also one of the few organisms with homologous chromosomes held together only by thesynaptonemal complex (and not crossovers) duringmeiosis.[14] In the oocytes ofB. mori, meiosis is completelyachiasmate (lacking crossovers).[15] Even thoughsynaptonemal complexes are formed during thepachytene stage of meiosis inB. mori, crossing-overhomologous recombination does not occur between the pairedchromosomes.[16]
Researchers atTufts developed scaffolds made of spongy silk that feel and look similar to human tissue. They are implanted during reconstructive surgery to support or restructure damaged ligaments, tendons, and other tissue. They also created implants made of silk and drug compounds which can be implanted under the skin for steady and gradual time release of medications.[19]
Researchers at theMIT Media Lab experimented with silkworms to see what they would weave when left on surfaces with different curvatures. They found that on particularly straight webs of lines, the silkworms would connect neighboring lines with silk, weaving directly onto the given shape. Using this knowledge they built asilk pavilion with 6,500 silkworms over a number of days.
Silkworms have been used in antibiotic discovery, as they have several advantageous traits compared to other invertebrate models.[20] Antibiotics such aslysocin E,[21] a non-ribosomal peptide synthesized byLysobacter sp. RH2180-5[22] and GPI0363[23] are among the notable antibiotics discovered using silkworms.In addition, antibiotics with appropriate pharmacokinetic parameters were selected that correlated with therapeutic activity in the silkworm infection model.[24]
Silkworms have also been used for the identification of novel virulence factors of pathogenic microorganisms. A first large-scale screening using transposon mutant library ofStaphylococcus aureus USA300 strain was performed which identified 8 new genes with roles in full virulence ofS. aureus.[25] Another study by the same team of researchers revealed, for the first time, the role of YjbH in virulence and oxidative stress tolerance in vivo.[26]
The domestic speciesB. mori, compared to the wild species (e.g.,B. mandarina), has increased cocoon size, body size, growth rate, and efficiency of its digestion. It has gained tolerance to human presence and handling, and also to living in crowded conditions. The domestic silk moths cannot fly, so the males need human assistance in finding a mate, and it lacks fear of potential predators. The native color pigments have also been lost, so the domestic silk moths areleucistic, since camouflage is not useful when they only live in captivity. These changes have madeB. mori entirely dependent upon humans for survival, and it does not exist in the wild.[27] The eggs are kept inincubators to aid in their hatching.
Silkworms and mulberry leaves placed on trays (Liang Kai'sSericulture c. 13th century)
Silkworms were first domesticated in China more than 5,000 years ago.[28][29]
PupaeSilkworm cocoons weighed and sorted (Liang Kai'sSericulture)
Silkworm breeding is aimed at the overall improvement of silkworms from a commercial point of view. The major objectives are improvingfecundity, the health of larvae, quantity of cocoon and silk production, and disease resistance. Healthy larvae lead to a healthy cocoon crop. Health is dependent on factors such as better pupation rate, fewer dead larvae in the mountage,[30] shorter larval duration (this lessens the chance of infection) and bluish-tinged fifth-instar larvae (which are healthier than the reddish-brown ones). Quantity of cocoon and silk produced are directly related to the pupation rate and larval weight. Healthier larvae have greater pupation rates and cocoon weights. Quality of cocoon and silk depends on a number of factors, including genetics.
In the U.S., teachers may sometimes introduce the insect life cycle to their students by raising domestic silk moths in the classroom as a science project. Students have a chance to observe complete life cycles of insects from eggs to larvae to pupae to moths.[31] Other than the U.S., this domestic silk moth is utilized in educational settings in several countries, including China, South Africa, Zimbabwe, Iran, and Taiwan.[32]
The fullgenome of the domestic silk moth was published in 2008 by the International Silkworm Genome Consortium.[13] Draft sequences were published in 2004.[33][34]
The genome of the domestic silk moth is mid-range with agenome size around 432 million base pairs. A notable feature is that 43.6% of the genome arerepetitive sequences, most of which are transposable elements. At least 3,000 silkworm genes are unique, and have no homologous equivalents in other genomes. The silkworm's ability to produce large amounts of silk correlates with the presence of specific tRNA clusters, as well as some clusteredsericin genes. Additionally, the silkworm's ability to consume toxic mulberry leaves is linked to specialized sucrase genes, which appear to have been acquired from bacterial genes.[13]
In 2018,Illumina's short reads for 137 strain genomes were published.[35] In 2022,Nanopore's long reads for 545 strain genomes were published.[36]
InAssam, India, they are boiled for extracting silk and the boiled pupae are eaten directly with salt or fried with chili pepper or herbs as a snack or dish. Live pupae may be eaten raw, boiled or fried.[37]
InKorea, they are boiled and seasoned to make a popular snack food known asbeondegi (번데기).[38]
InChina, street vendors sell roasted silk moth pupae.
InJapan, silkworms are usually served as atsukudani (佃煮), i.e., boiled in a sweet-sour sauce made withsoy sauce and sugar.
InVietnam, this is known asnhộng tằm, usually boiled, seasoned with fish sauce, then stir-fried and eaten as main dish with rice.
InThailand, roasted silkworm is often sold at open markets. They are also sold as packaged snacks.
Silkworms have also been proposed for cultivation by astronauts asspace food on long-term missions.[39]
In China, a legend indicates the discovery of the silkworm's silk was by an ancient empress namedLeizu, the wife of theYellow Emperor, also known as Xi Lingshi. She was drinking tea under a tree when a silk cocoon fell into her tea. As she picked it out and started to wrap the silk thread around her finger, she slowly felt a warm sensation. When the silk ran out, she saw a small larva. In an instant, she realized this caterpillar larva was the source of the silk. She taught this to the people and it became widespread. Many more legends about the silkworm are told.
The Chinese guarded their knowledge of silk, but, according to one story, a Chinese princess given in marriage to aKhotan prince brought to the oasis the secret of silk manufacture, "hiding silkworms in her hair as part of her dowry", probably in the first half of the first century AD.[40] About AD 550, Christian monks are said to havesmuggled silkworms hidden in a hollow stick out of China, selling the secret to the eastern Romans.
According to a Vietnamese folk tale, silkworms were originally a beautiful housemaid running away from her gruesome masters and living in the mountain, where she was protected by the mountain god. One day, a lecherous god from the heaven came down to Earth to seduce women. When he saw her, he tried to rape her but she was able to escape and was hidden by the mountain god. The lecherous god then tried to find and capture her by setting a net trap around the mountain. With the blessing ofGuanyin, the girl was able to safely swallow that net into her stomach. Finally, the evil god summons his fellow thunder and rain gods to attack and burn away her clothes, forcing her to hide in a cave. Naked and cold, she spit out the net and used it as a blanket to sleep. The girl died in her sleep, and as she wished to continue to help other people, her soul turned into silkworms.[citation needed]
Bombyx mori is essentially monophagous, exclusively eating mulberry leaves (Morus spp.). By developing techniques for using artificial diets, the amino acids needed for development are known.[41] The various amino acids can be classified into five categories:
Those which, when removed, cause larval development to stop entirely: lysine, leucine, isoleucine, histidine, arginine, valine, tryptophan, threonine, phenylalanine, methionine
Those which, when removed, impede later stages of larval development: glutamate and aspartate
Semi-essential amino acids, with negative effects that can be eliminated by supplementing with other amino acids: proline (ornithine can be substituted)
Non-essential amino acids that can by replaced through biosynthesis by the larvae: alanine, glycine, serine
Non-essential amino acids that can be removed with no effect at all: tyrosine
Beauveria bassiana, a fungus, destroys the entire silkworm body. This fungus usually appears when silkworms are raised under cold conditions with high humidity. This disease is not passed on to the eggs from moths, as the infected silkworms cannot survive to the moth stage. This fungus, however, can spread to other insects.
Grasserie, also known as nuclear polyhedrosis, milky disease, or hanging disease, is caused by infection with theBombyx mori nucleopolyhedrovirus (akaBombyx mori nuclear polyhedrosis virus, genusAlphabaculovirus). If grasserie is observed in the chawkie stage, then the chawkie larvae must have been infected while hatching or during chawkie rearing. Infected eggs can be disinfected by cleaning their surfaces prior to hatching. Infections can occur as a result of improper hygiene in the chawkie rearing house. This disease develops faster in early instar rearing.
Pébrine is a disease caused by a parasitic microsporidian,Nosema bombycis. Diseased larvae show slow growth, undersized, pale and flaccid bodies, and poor appetite. Tiny black spots appear on larval integument. Additionally, dead larvae remain rubbery and do not undergo putrefaction after death.N. bombycis kills 100% of silkworms hatched from infected eggs. This disease can be carried over from worms to moths, then to eggs and worms again. This microsporidium comes from the food that the silkworms eat. Female moths pass the disease to the eggs, and 100% of silkworms hatching from the diseased eggs die in their worm stage. To prevent this disease, eggs from infected moths are ruled out by checking the moth's body fluid under a microscope.
Flacherie infected silkworms look weak and are colored dark brown before they die. The disease destroys the larva's gut and is caused by viruses or poisonous food.
Several diseases caused by a variety of funguses are collectively namedMuscardine.
^Hideaki Maekawa; Naoko Takada; Kenichi Mikitani; et al. (1988). "Nucleolus organizers in the wild silkwormBombyx mandarina and the domesticated silkwormB. mori".Chromosoma.96 (4):263–269.doi:10.1007/BF00286912.S2CID12870165.
^Rasmussen SW (April 1977). "The transformation of the Synaptonemal Complex into the 'elimination chromatin' in Bombyx mori oocytes".Chromosoma.60 (3):205–21.doi:10.1007/BF00329771.PMID870294.
^Hamamoto, H.; Urai, M.; Ishii, K.; et al. (2015). "Lysocin E is a new antibiotic that targets menaquinone in the bacterial membrane. Nat".Chem. Biol.11 (2):127–133.doi:10.1038/nchembio.1710.PMID25485686.
^Panthee, S.; Hamamoto, H.; Suzuki, Y.; Sekimizu, K. (2017). "In silico identification of lysocin biosynthetic gene cluster from Lysobacter sp. RH2180-5".J. Antibiot.70 (2):204–207.doi:10.1038/ja.2016.102.PMID27553855.S2CID40912719.
^Hong-Song Yu1; Yi-Hong Shen; Gang-Xiang Yuan; et al. (2011). "Evidence of selection at melanin synthesis pathway loci during silkworm domestication".Molecular Biology and Evolution.28 (6):1785–99.doi:10.1093/molbev/msr002.PMID21212153.{{cite journal}}: CS1 maint: numeric names: authors list (link)
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