Whether or not we like these eight-legged creatures, we have to admire one thing about spiders — their ability to churn out certain fibers that, although they might look fragile, the individual threads are extremely strong — pound for pound they are stronger than steel.
Garb and her colleague, Cheryl Hayashi, set out to untangle the intricacies of spider silk. Now, as reported in Discover magazine, they've cracked the genetic code for egg-sack silk after studying material from spider abdominal glands where it's produced. The findings may one day enable scientists to produce extremely strong, lightweight materials.
"Spider silks would be very useful for human application such as in making medical sutures or high performance ropes or even used in ballistic applications such as filling bullet proof vests," Hayashi says. This could explain why, for more than a decade, many scientists have been trying to characterize spider silk.
Hayashi and Garb are hoping to pass on to science something that's a natural for spiders — the ability to choose from a tool bag of silks to match the task at hand. "A typical garden spider can spin seven different types of silks," explains Hayashi, adding that spiders use silk to cover their eggs, make burrows and build homes for themselves, and of course, for capturing food in their silky webs. "Where spiders are picking and choosing to use a particular fiber type for a particular application now material scientists may be able to match a silk for an application," she says.
Egg-case silk holds particular interest for Garb and Hayashi because it has properties that make it strong, water repellant and a natural temperature regulator, that protects fragile spider eggs. "Wrapping eggs is an essential function for successful reproduction," says Hayashi. "Without covering their eggs, their eggs would be very vulnerable to predators, to parasites, to mold or just desiccation from the environment or even just getting too wet."
Of the four spider species the researchers studied — out of more than 37,000 identified spider species — all use versions of the same gene to produce the egg-case silk. The gene codes for a protein produced in the spider abdominal gland. "These glands are called silk glands and these silk glands secrete proteins," Hayashi says. "We sequenced these genes and then we could figure out what the predicted protein of these genes, what the gene product would be."
Spider egg sack image: Jessica Garb
When spinning silk to cover eggs, the spiders Garb and Hayashi studied make many silk proteins copies that come together and align themselves inside the gland as a liquid. Then, as the proteins exit the body through a very narrow nozzle, called a spigot, the proteins form a solid fiber. "So that fiber is basically just lots and lots of this silk proteins all aligned together," Garb explains. "Probably many millions of this protein molecule all bundled up together. And so it emerges from that spigot as a fiber. And the spider has many, many of these glands. So it has lots of these tiny spigots."
The gene has been around long enough to lead Garb to believe that it is has probably specialized for that function over time. "We know that these silks evolved from sort of ancestral silk, and this happened approximately the same time when spiders first came into existence. And we know from the fossil record that this happened some 350 to 400 million years ago," Garb says. "This specific egg case silk has been around at least 125 million years and… it must have diverged from some sort of ancestral type silk even farther back in time. And that's really remarkable in the sense that spiders have diversified their use of silk so remarkably well." Although the genes are part of the same gene family, geneticists are still trying to work out how they relate to each other.
Next, researchers will need to figure out how to produce huge amounts of spider silk protein if they hope to harness its properties for mass-production. But don't expect spider farms anytime soon. Spiders are too territorial to share the same space. Garb suggests instead that scientists might splice the silk gene into crop plants where it can be extracted after harvest. Another enterprising group has already experimented with putting other spider silk genes into goats to harvest the protein from the goats' milk.
Until scientists work out what comes naturally to spiders, we'll have to wait for futuristic super-strong fabrics.