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DNA Barcoding May Be Missing Some Stripes
1 December 2005 (All day)
A technique that categorizes living things based on small stretches of their genetic code might not be as powerful as some researchers have claimed. A new study of sea cowries–-marine gastropods famed for their exquisite shells–-indicates that DNA barcoding does not always allow taxonomists to tell the difference between closely related species. This raises questions about barcoding's ability to identify new lifeforms, one of its primary aims.
Much like supermarket barcodes that help computers distinguish tomato sauce from toilet paper, DNA barcoding culls a tiny sequence of DNA from one species and compares it to an equivalent sequence from another. The difference in the two sequences helps place a species on the right branch of its family tree. Some have hailed the method as the savior of taxonomy, claiming that in the near future handheld DNA barcoding devices will help identify new species on an unprecedented scale. But other scientists have concerns about the viability of the method, saying that it simply isn't possible to separate every species on the planet on the basis of a single DNA sequence.
To put DNA barcoding to its first truly comprehensive test, taxonomists Christopher Meyer and Gustav Paulay, of the Florida Museum of Natural History in Gainesville, spent more than ten years collecting tissue samples from sea cowries across the globe. This worldwide collaboration with other scientists, amateur naturalists, and collectors led to the creation of a dataset that broke the marine gastropods into 263 subsets from around the world, some of which were much more closely related than others.
When Meyer and Paulay compared the cowrie's DNA barcodes, they found a significant "barcoding gap." If much was already known about a cowrie's place in its family tree, DNA barcoding tended to confirm this placement. But when little was known about the species, the technique failed to clear up the picture. This is just when the tool is needed most, the team reports online this week in PLoS Biology, suggesting that DNA barcoding is not reliable enough to fuel new species discovery on its own. The findings have been welcomed by both advocates and critics of DNA barcoding. "The paper is an excellent step down the road toward discovering the best way to use DNA barcoding," says the Smithsonian Institution's David Schindel, the executive director of the Consortium for the Barcode of Life. Evolutionary biologist Felix Sperling, of the University of Alberta in Edmonton, Canada, says the results give a more realistic picture of the value of the technique. "Some of the early expectations that DNA barcoding could replace all of taxonomy are at last being reined in," he says.