Spider photo at right: Courtesy Mark Chappell.

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Now comes word that nature has done the same with spider silk -- "expanding the line" with silk that's strong and super stretchy, or strong and inelastic, or stretchy and sticky.

Take a little spider -- almost any spider -- and once you get over that silly "Eek a spider!" repulsion, you'll discover it makes seven kinds of silk -- one to wrap its prey, one to wrap its eggs, and five to construct the web itself.

Each type of silk comes from separate sets of spigots on the spinnerets, as biologists call the gizmos that spin proteins into silk. And each type of silk is composed of one or more specific proteins, made by one or more genes. "Look at the common garden spider, it has seven kinds of glands and seven kinds of silk," says Cheryl "Spiderwoman" Hayashi, assistant professor of biology at the University of California at Riverside, "and each has one or more gene expressed." Since some glands express two genes, each spider devotes at least seven genes to silk-making.

Each variety of silk is biologically engineered by evolution to perform its task to perfection. "Dragline" silk -- used to anchor webs and serve as its spokes -- has a higher tensile strength than the same weight of high-tensile steel. "Capture" silk -- which snares insects -- is stickier, and can stretch up to three times its length before breaking.

Cheryl's web
Silk may be elegant, but it's poorly understood. Although more than 34,000 species of spiders have been named, silk has been analyzed only in four closely-related orb-weaving spiders. (Orb webs are essentially round in shape.)

In a new study, Hayashi and colleagues analyzed silk in seven more genera, representing orb-web-weavers and other spiders (see "Extreme Diversity..." in the bibliography).

The researchers looked at the silk genes, and at the amino acids in silk -- and found common subunits, or "motifs," being replayed in different ways to produce silk with specific characteristics.

The analysis weaves a new view of the evolution of spider silks. Hayashi says orb-weaving spiders have been around at least 125 million years, based on fossil evidence and comparisons of their morphology and behavior. Some of their silk genes have changed little over that time, indicating that they were a successful solution to the problem of spider survival.

Better information on silk may shed light on similar proteins in other organisms. Oysters, for example, use a silk-like scaffold to organize the minerals for their shells. And humans use a protein called elastin in skin and blood vessels to make them -- you guessed it -- more elastic.

Got yer goat

Materials scientists have been itching to use silk in high-rent applications like spacecraft and medicine, but spiders can't be farmed like silkworms -- the little buggers are just too territorial. Silk genes have been stuck in bacteria, but the output of silk protein is ... microscopic.

Small tubes in the spinnerets connect to glands that make silk.
Courtesy Ed Nieuwenhuys; originally published in Leben am seidenen Faden, Die rätselvolle welt der Spinnen, 1975.

Nexia Biotechnologies, a Canadian firm, is trying to sidestep those difficulties by growing spider silk in genetically engineered goats. Nexia wants to market artificial silk for wound closures -- a thinner thread should promote healing -- and to replace Kevlar in bullet-proof vests.

The company has inserted a spider's gene into two goats, positioned so the milk will contain silk. They plan to breed the two critters to start a line of goats that make silk milk.

Then comes the tough task of spinning silk. "Whether you start with liquid proteins from spiders or with protein from recombinant bacteria," says Hayashi, "it's been very hard to replicate the spinning and come up with a thread as regular, as strong" as the threads that Charlotte famously wove for the benefit of her friend Wilbur the pig (see "Charlotte's Web" in the bibliography).

No word on how the silk proteins will be collected from the milk. Maybe Nexia can reverse nature. Instead of using spider's silk to catch flies, maybe they can use flies to catch spider's silk...

-- David Tenenbaum

Bibliography:
Extreme Diversity, Conservation, and Convergence of Spider Silk Fibroin Sequences, John Gatesy et al, Science, 30 March 2001, pp. 2603-5.

Charlotte's Web, E.B. White, New York, Harper, 1952.