Silkworms: Textiles and Genetics

If it does become viable to mass produce spider-silk using genetically engineered silkworms the implications in the world of materials are staggering.  Spider-silk has a plethora of properties that have a wide range of applications.  Silkworms will, as a domesticated species, make all of it possible.

Spider-silk’s primary properties that make it attractive are it’s tensile strength, high breaking stress, light weight, electrical conductivity when mixed with carbon-nanotubes, and stretchiness.  In addition, some types of silk have anti-bacterial properties and can withstand the temperatures required to sterilize suture material.  This means spider-silk could be used to make stronger and more flexible sutures in surgery.

Spider-silk’s electrical properties have led to research into how the silk could be incorporated into electrical wiring to provide an organic alternative.  This wiring could be used in medical devices in place of non-degradable metal and plastic wiring.  The same silk-nanotube design for wiring is also being used as artificial muscle, which could be used in designing new prosthesis.  In terms of military and police applications, spider-silk could be used in place of Kevlar in bullet-proof vests and body armor to great effect.

Silkworms’ main product, silk, has had a tumultuous past, being the object of perhaps the longest running monopoly in history.  While silk production started in China around 3500 BCE, it wasn’t until 200 BCE in Korea that another culture was able to produce silk.  The secret of how to produce silk was closely guarded from western emissaries, with their belongings being thoroughly searched before leaving China.  Eventually the Chinese monopoly on the silk trade to Europe was broken around 550 AD by Byzantine missionaries, who smuggled larvae inside hollow walking sticks.

Before silk came to Europe, it was one of the primary goods traded along the silk road, an overland trade route across Eurasia.  

(http://www.east-site.com/silk-road)

    During the Han Dynasty, trade with the Roman Empire included silk along with other goods such as paper and “china” pottery.  These trade routes passed along not only goods, but also ideas and strategic inventions like gunpowder.  Without the silk trade, cross-cultural exchange between the East and West across Eurasia would have been much smaller.

    Silk had become an industrial good by the beginning of WWII, being used not only in clothing, but also for bicycle tires, artillery gunpowder bags, and parachutes.  The majority of America’s silk prior to WWII was imported from East Asia, and all Pacific trade suffered during the war.  The demand for silk-substitutes led to the development of artificial fibres like nylon, which soon moved from being expensive substitutes to the norm.  Silk also served a vital service during 20th century wars as thread for surgical sutures.

In modern times, silk is still a luxury cloth, but silkworms serve a new human need.  As one of the few domesticated lepidoptera, silkworms are the insect equivalent of white mice, being used in genetic experimentation.  This experimentation has led to the possibility of new goods being derived from silkworms.  Kraig Biocraft Laboratories has been researching the possibility of genetically engineering silkworms to produce a silkworm silk-spider silk hybrid fiber.  The project is beginning to reach the point of being economically viable, and has produced hybrid silk, essentially pure spider silk, and a silk designed to exceed the limitations of spider silk.

There are a few differences between wild and domesticated silkworms with most having to do with increasing dependence on humans and increasing silk production.  As well as modifications for increased production, domesticated silkworms have increased tolerance to living together in large groups as larvae.

    Compared to wild silkworms, domesticated silkworms exhibit greater growth rate, cocoon size, and efficiency of digestion.  All of these characteristics make these silkworms better producers of silk when compared to wild silkworms.  In terms of increasing dependence on humans, adult domesticated silkworms cannot fly and require incubation to gestate and hatch.    In addition to being bred to be more efficient, strains of silkworms are cultivated to have resistance to different diseases caused by bacteria and fungi.

    Within the last 50 years silkworms have become the equivalent of white mice in genetics research on insects and lepidoptera specifically.  This has led to research into genetically altering silkworms to provide new products, like spider-silk hybrid silk and gengineered silk designed to exceed the strength of regular spider-silk.

This BBC documentary outlines the physiology of the silkworm, and sets the stage for the next section, the history and historic impact of silkworms.

Humans first started domesticating silkworms around 3500 BCE in China, before the first dynasty government.  How silkworm sericulture started is likely that ancient Chinese took notice of the cocoons of wild silkworms on mulberry trees, leading to the deliberate raising of the larvae in controlled groves of mulberry trees.  In the Chinese legend of the Silk Goddess, an empress named Leizu discovered that the mulberry trees in her garden where being stripped of their leaves by something.  One day while drinking her tea in the garden, a white nut fell into her cup.  Looking closely, she found that the trees were full of these nuts.  The nut that had fallen in her hot tea had softened to the point where threads began to come loose from it.  Pulling on these threads she found that the nut was a cocoon containing a moth larva.  Recognizing the value of the thread Leizu had more trees planted and the threads spun into the first silk.  A different legend says that a girl made a deal with a horse, that if he found and returned her father from a war she would marry it.  After the horse succeeds and the father returns he learns of the deal and kills the horse, hanging its hide.  One day the hide blows onto and around the girl, transforming her into a silkworm.  The relationship between horses and silkworms is drawn based on the shape of the silkworm’s head.

    Silk as a commodity was at first limited to nobility due to its quality and the effort involved in getting enough thread to produce it.  As sericulture became more advanced and societies had more resources to put into silk production, silk became one of the main export goods of China, especially across Eurasia to the Middle Eastern and European empires.

    As insects, the relationship between silkworms and humans is severely one sided.  Silkworms receive food and shelter from natural predators and in return are boiled alive in their cocoons.  Lepidoptera in general do not have as complex social structure as other insects, let alone mammals.  Silkworms themselves have almost no social structure as the adults do not live very long, and are entirely dependant on humans to survive.  There has been little impact on humans by the domestication of silkworms itself.  By contrast the silkworm’s products, silk, food, and genetic experimentation, have had great impact on people and history.

The evolution of the modern silkworm can be traced through its full taxonomy classification: Animilia Arthropoda Insecta Macrolepidoptera Bombicidae Mori.  As the larvae of moths, silkworms fall under the Order Lepidoptera, the same as butterflies as well as moths.  The fossil record for Lepidoptera is lacking due to their soft-bodies and lack of hard exo-skeleton.  There are a few fossils though, and these allow us to track the history of the Lepidoptera as far back as the Triassic when they diverged, along with Trichoptera from a common ancestor.  The superfamily Apoditrysia contains all of the large modern moths and butterflies, including the silkworm, and split off during the early hylogenetic_chart_of_Lepidoptera.svg">Cretaceous.  

    In more recent times, the silkworm was bred from its wild state, Bombyx Mandarina, to its domestic one, Bombyx Mori.  Over the past 5000 years the domestication process split Mori off of the Chinese Silkworm, rather than the Japanese Silkworm, which diverged approximately 23,600 years ago.

There are a variety of modern silkworm variants.  By far the most common silkworm is the Bombyx Mori, which is raised in China and India and provides the majority of silk in the form of mulberry silk.  The silkworm Antheraea mylitta produces Tasar silk, which is not as smooth as mulberry silk, and is used as a less fine cloth for interiors and furnishing.  Oak Tasar is a subtype of this silk produced by Antheraea pernyi, and is a finer type of Tasar.  By far the most major difference between wild and domesticated silkworms is that the adult Bombyx have lost the ability to fly.

There is some evidence of horizontal gene transfer (HGT) playing a role in silkworm evolution.  Horizontal gene transfer is any asexual process which results in the transfer of genes from one organism to another.  In silkworms the plausible mechanisms for HGT are transformation and transposable elements.  Transformation is the absorption of gene fragments by a cell from its environment, while transposable elements are pieces of genetic material that can be passed between organisms, sometime transferring antibiotic resistance.  HGT is one of the ways in which organisms evolve over time, and in silkworms it may have resulted in disease resistance.  These gene transfers are a legacy from the ancestors of modern domesticated silkworms, and are primarily from bacteria.  The research into what these gene fragments do in silkworms is ongoing and mainly focuses on finding new pesticides and how insects adapt to them.