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Silk Genes Reveal Flexible Design

With seven kinds of silk, many spiders weave complex and resilient works of art. Now, the most extensive look yet at spider silk DNA reveals that the gene for a highly elastic silk has some marvelous architecture of its own: The DNA is a repetitive string of nucleotides grouped in a way that may be an engine for generating diversity. Although the finding may not have an immediate impact on the way artificial silk is designed, it has excited people who think about spider evolution.

Many arthropods make silk: spiders, silkworms, butterflies, and even some honeybees. Trying to mimic and improve this wonder fiber, outfits from the U.S. Army to DuPont have investigated its biomechanics and genetics. Since 1990, several teams have discovered 10 spider silk genes. But no one had looked at the genetics of one particular type of silk called flagelliform silk. So evolutionary biologist Cheryl Hayashi and molecular biologist Randolph Lewis of the University of Wyoming, Laramie, sequenced genes for the flagelliform silk of the tropical spiders Nephila clavipes and Nephila madagascariensis.

As Hayashi and Lewis sequenced the Flag gene, they found repeated stretches in the DNA that coded for three regular amino acid motifs. Curiously, these motifs kept turning up in the same order, the researchers report in the 25 February Science. But Hayashi and Lewis also came across 12 pieces of noncoding DNA, or introns, that are very similar--one pair is 99.9% identical. This suggests that the introns are more highly conserved than the exons, the coding parts of the genes. That's like taking better care of your CD cases than the compact discs themselves.

Hayashi and Lewis believe that all these peculiarities are tied to the monotony of the exons. When a sequence is highly repetitive, enzymes that copy the DNA will lose their place more often and make an error, causing mutations--thus increasing the chance that a new, improved type of silk will arise once in a while. On the other hand, the repetitive architecture may lead to "homogenization" of the gene, which occurs when one repeat overwrites another repeat. This tug-of-war between mutation and homogenization means that although the gene structure may stimulate innovation, the same architecture means that those improvements might be weeded out.

Spiders are a great animal for exploring the link between protein evolution and behavior, says Catherine Craig, an evolutionary biologist at Tufts University in Medford, Massachusetts. Because silk is directly extruded--rather than hidden inside the body like most proteins--it's much easier to study the physical effects of mutations on silk proteins and spider survival. Those kinds of experiments may be far off, but probing the genes behind the proteins is a key step. "Now we know the structure of the gene," Craig says, "and that's fantastic."