Where do domesticated silkworm cocoons get their wild colours from?

Where do domesticated silkworm cocoons get their wild colours from?

Subject: Science and Tech

Section: Msc

About:

Silk, the queen of fibres, is drawn or reeled from cocoons of the silk moth (Bombyx mori). Humans domesticated it more than 5,000 years ago in China, from the wild moth (Bombyx mandarina). India is the world’s second largest producer of raw silk after China.

Silkworms

• Caterpillars, also known as silkworms, of both these species feed exclusively on leaves of mulberry plants (genus Morus).
• The domesticated moth-is much larger than its wild progenitor, and thus extrudes a longer silk fibre to build its larger cocoon, up to 900 meters long.But it depends wholly on human care for its survival and reproduction.
• Since having been domesticated, it has lost the ability to fly, and since its need for camouflage no longer exists, it has also lost its caterpillar and adult-stage pigmentation.

Carotenoids and flavonoids

• ‘Wild’ silks – which include the muga, tasar, and eri silks – are obtained from other moth species: namely, Antheraea assama, Antheraea mylitta, and Samia cynthiaricini.
• 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.
• The ancestral mulberry moth makes 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.
• Human handlers selected the differently coloured cocoons whenever they emerged, possibly in the hope of breeding for coloured silks.
• We know today that the cocoon’s pigments are derived from chemical compounds called carotenoids and flavonoids, which are made by the mulberry leaves.
• Silkworms feed voraciously on the leaves, absorb the chemicals in their midgut, transport them via the hemolymph – arthropods’ analogue of blood – to the silk glands, where they are taken up and bound to the silk protein.
• Mature caterpillars then spin out the silk proteins and associated pigment into a single fibre. The caterpillar wraps the fibre around itself to build the cocoon.

Mutant strains

• The adult moth hatches from the cocoon. In this process, the fibre is broken in many places.
• Superior quality silk however comes from an unbroken fibre, so unhatched cocoons are used for reeling.
• The differently coloured cocoons arise from mutations in genes responsible for the uptake, transport, and modification of carotenoids and flavonoids.
• The mutant strains have become a valuable resource for scientists to study the molecular basis of how, in a relatively short span of 5,000 years, artificial selection generated such spectacular diversity.

The gene called apontic-like

• Domesticated and ancestral mulberry silk moths can be interbred to produce hybrid offspring.
• The hybrid caterpillars, like their wild parent, made the pigment called melanin.
• But when the B. mandarina-derived copy of apontic-like was mutated, the hybrid failed to make melanin.
• Both versions of the apontic-like gene make the same protein. Therefore, the difference between them was attributable to differences in sequences that regulate when and where the gene was turned ‘on’ or ‘off’.

Conclusion

• Silk is an acme of domestication, comparable in its success to basmati rice, alphonso mangoes, and the golden retriever. Today, the tools are at hand for scientists to make and compare genetically identical hybrid silk moths that differ only in which of a gene’s two parental versions is inactivated: domesticated or ancestral.
• This paves the way for scientists to work out – gene by gene – all the key steps that led to silk moth domestication. Hopefully, someday soon, similar techniques will become available for us to analyse domestication in rice, mangoes, and dogs.