Colours of Domesticated Silkworm Cocoons | 14 Nov 2023

Source: TH

Why in News?

Silk, often referred to as the “queen of fibres,” has been valued for its beauty and luxury for ages. Researchers have revealed the genetic factors behind the cocoon colors and adaptations of silk-producing insects, and how they have transformed the silk industry.

What Genetic Insights Does Silk Moth Domestication Uncover?

• Evolution of Silk Moth Domestication:
  • It is produced by the cocoons of the domesticated silk moth (Bombyx mori), which was derived from the wild silk moth (Bombyx mandarina) more than 5,000 years ago in China.
    • While the domesticated silk moth thrives worldwide, the ancestral moth still roams in regions like China, Korea, Japan, and far-eastern Russia.
• Types of Silk:
  • Wild Silk (Non-mulberry Silks):
    • Wild’ silks, which include the muga, tasar, and eri silks are obtained from other moth species: namely, Antheraea assama, Antheraea mylitta, and Samia cynthia ricini.
    • These moths survive relatively independently of human care, and their caterpillars forage on a wider variety of trees.
    • Non-mulberry silks comprise about 30% of all silk produced in India.
      • These silks have shorter, coarser, and harder threads compared to the long, fine, and smooth threads of the mulberry silks.
  • Mulberry Silk:
    • The most common and widely produced type of silk, accounting for about 90% of global silk production.
      • Derived from the cocoons of the domesticated mulberry silkworm (Bombyx mori), which feeds exclusively on mulberry leaves.
    • Has long, smooth, and lustrous fibers that can be woven or knitted into various fabrics with different textures and finishes.
    • Suitable for a wide range of applications, such as clothing, bedding, curtains, upholstery, and accessories.
• Cocoon Colours:
  • The ancestral mulberry moth makes (uniform) brown-yellow cocoons.
    • In contrast, domesticated silk moth cocoons come in an eye-catching palette of yellow-red, gold, flesh, pink, pale green, deep green or white.
  • The pigments that colour the silkworm cocoons are derived from chemical compounds called carotenoids and flavonoids, which are made by the mulberry leaves that the silkworms feed on.
    • The silkworms absorb the carotenoids and flavonoids and transport them to the silk glands, where they are taken up and bound to the silk protein.
    • The amount and type of pigments in the silk glands determine the colour and intensity of the silk threads, which are then extruded by the silkworms to form the cocoons.
  • The pigments that coloured the cocoons are water-soluble, so they gradually fade away.
    • The coloured silks we see in the market are instead produced by using acid dyes.
  • Mutations in genes responsible for carotenoids and flavonoids cause differently colored cocoons, providing insight into the molecular basis of silk diversity.