Spiders spin a web filled with lessons

Next time you eat a chili pepper, think tarantula. The creepy spider and the fiery vegetable use a similar chemical tactic to discourage attackers. This latest discovery of disparate organisms evolving similar solutions to a common problem illustrates the treasure trove of information on the development of earthly life that spiders represent. Scientists are sifting through that treasure molecule by molecule.

Organisms don't necessarily evolve a similar response to a common need. For example, we use our forelimbs to lift objects. Elephants use their noses.

But sometimes the response is good enough for it to develop in organisms as unrelated as plants and animals. Jan Siemans at the University of California in San Francisco and colleagues saw this in action when they took a good look at how venom from a West Indian tarantula works. In last week's issue of Nature, they reported that they found three peptide molecules "that target the capsaicin receptor" in an animal's nervous system.

Wait a minute. Capsaicin is the stuff that gives chili peppers their bite. It stimulates that receptor to evoke a sensation of pain. The entirely separate evolution of chilies and tarantulas produced a way to use the capsaicin receptor to scare off predators, or, in the case of chilies, enhance the world's cuisines.

Apparent cases of such parallel evolution are not always what they seem to be. For example, scientists had believed the iconic orb web design of two different spider groups had evolved independently. Then Jessica Garb at the University of California in Riverside and colleagues traced the evolutionary history recorded in the spiders' DNA. Their research, reported in the June issue of Science, shows that instead of having evolved separately in each spider group, the orb web originated in a common ancestor that lived at least 136 million years ago. "A lot of people had said over the years that the orb web was a pinnacle of adaptive design. Our work confirms that not only is this web type very old, it was also lost in certain lineages of spiders," Dr. Garb explains. Evolution can discard an adaptation as well as create it.

Perhaps the gem of spider evolution is the super-strong insoluble silk on which spiders swing and with which they weave their webs. Industrial and academic engineers are going after spider silk, hoping to learn how to make artificial materials as good as, or better than, the spiders' product. One of the latest insights comes from Gareth McKinley's laboratory at the Massachusetts Institute of Technology. In the current issue of the Journal of Experimental Biology, the group describes physical processes it has discovered that turn a watery protein solution into the tough fiber produced by the golden-silk orb-weaving spider.

"The amazing thing nature has found is how to spin a material out of an aqueous solution and produce a fiber that doesn't redissolve," Professor McKinley commented in a press release.

Research engineers like McKinley look forward to using spider know-how to design tougher plastics, better body armor, stronger parachutes, and other novel materials. Research on tarantula venom points the way to uncovering subtle effects of other plant or animal toxins on nervous systems. Either way, we can learn a lot from spiders.

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