DNA barcoding

The task of identifying Earth's estimated 10 million species has daunted biologists for centuries - fewer than two million have been named. Using a technique called DNA barcoding, researchers at Rockefeller University and two Canadian institutions have uncovered four new species of North American birds.

The result is an important step towards proving that the sequence of a short stretch of DNA - a so-called DNA barcode - can be used genetically to identify known species and to find new ones.

As the cost of DNA sequencing goes down, proponents of DNA barcoding envision developing a handheld device that amateur naturalists and others could take outdoors for species identification. Barcoding will enable rapid screening of large numbers of organisms and highlight those with novel barcodes that are likely to be new species.

Taxonomists traditionally have classified organisms on the basis of their physical characteristics. They use DNA too, but current techniques are labour intensive and difficult to compare.

Zoologist Paul Hebert, PhD, at the University of Guelph, proposes that a short DNA sequence from a gene found in all animals can be used to identify species because in each species the sequence varies slightly. He coined the term DNA barcode for this idea, analogous to the supermarket barcodes that allow quick identification of millions of items.

The technique depends on analysing a portion of a gene called cytochrome c oxidase I (COI) that is found in the mitochondria of cells of all animals. Mitochondrial DNA (mtDNA) is known to accumulate mutations three to five times faster than DNA in the nucleus. As a result, the mtDNA of closely related species differs more than the nuclear DNA of those species and can be used to tell them apart.

In earlier studies, Hebert and colleagues at the University of Guelph showed that the COI gene was easy to isolate from a variety of animals, and that species in a broad range of animal life, from flatworms to vertebrates, have distinct COI sequences.