The Origins of Vibrant Colors in Domesticated Silkworm Cocoons

Silk, known as the queen of fibers, has a fascinating history that spans over 5,000 years. Humans domesticated silk from the wild moth known as Bombyx mandarina in China. The domesticated silk moth, Bombyx mori, is now reared all over the world, including in India, which is the second-largest producer of raw silk after China.

Both species of silkworms, also known as caterpillars, exclusively feed on the leaves of mulberry plants from the genus Morus. The domesticated moth is much larger than its wild ancestor, allowing it to produce longer silk fibers and build larger cocoons, reaching up to 900 meters in length. However, it relies entirely on human care for its survival and reproduction. Over time, the domesticated silk moth has lost the ability to fly and has also lost its pigmentation in both its caterpillar and adult stages, as it no longer needs camouflage.

Apart from the domesticated silk moth, there are other moths that produce different types of silk known as “wild” silks. These include muga, tasar, and eri silks, obtained from moth species such as Antheraea assama, Antheraea mylitta, and Samia cynthia ricini. These moths have a greater ability to survive independently of human care and their caterpillars feed on a wider variety of trees. Non-mulberry silks account for about 30% of the silk produced in India. However, these silks have shorter, coarser, and harder threads compared to the long, fine, and smooth threads of mulberry silks.

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While the ancestral mulberry moth creates dull brown-yellow cocoons, the domesticated silk moth produces cocoons in a stunning array of colors such as yellow-red, gold, flesh, pink, pale green, deep green, and white. Human handlers have historically selected the differently colored cocoons in the hope of breeding for colored silks. However, they soon discovered that the pigments responsible for the colors are water-soluble, causing them to gradually fade away. Instead, the vibrant colors seen in the market are achieved by using acid dyes.

Modern research has revealed that the pigments in the cocoon are derived from chemical compounds called carotenoids and flavonoids, which are produced by the mulberry leaves. Silkworms consume large quantities of these leaves, absorbing the chemicals in their midgut. The chemicals are then transported through the hemolymph, which is the equivalent of blood in arthropods, to the silk glands. There, the chemicals bind to the silk protein and are incorporated into the silk fiber. The mature caterpillars spin the silk fibers and pigments to create a single fiber, which they wrap around themselves to form the cocoon.

Mutant strains of silkworms, which have emerged through natural selection, have become valuable resources for scientists studying the molecular basis of silk moth domestication. Most research on silk domestication has been conducted in China and Japan, but scientists from India have also made important contributions to the field. Researchers from Southwest University in Chongqing, China, recently proposed a model to explain how different combinations of mutations give rise to the various colors of silk cocoons. They discovered that specific genes are responsible for the transport and absorption of carotenoids, which result in different colored cocoons.

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In a groundbreaking study, researchers from the University of Tokyo and Columbia University created hybrid silk moths by interbreeding domesticated and ancestral mulberry silk moths. They then selectively mutated a gene called apontic-like, derived from either the domesticated or ancestral moth. The results showed that the domesticated silkworm’s version of the gene had lost the ability to support the production of a pigment called melanin. This gene manipulation offers a unique opportunity to understand, gene by gene, the key steps that led to silk moth domestication.

Silk is a remarkable example of successful domestication, comparable to the domestication of basmati rice, alphonso mangoes, and golden retrievers. With the tools available today, scientists can create and compare genetically identical hybrid silk moths with variations in specific genes associated with domestication. This allows for a comprehensive understanding of the genetic changes that have occurred over thousands of years. Hopefully, similar techniques will become available for analyzing the domestication of other important crops and animals in the future.

D.P. Kasbekar, a retired scientist, notes that silk domestication research has been primarily focused in China and Japan, but Indian scientists have also made significant contributions, including those from the Centre for DNA Fingerprinting and Diagnostics in Hyderabad. The study of silk moth domestication continues to provide valuable insights into the remarkable genetic diversity that can evolve through artificial selection in a relatively short span of time.

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