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Spotted.
Gentile Francesco Ficetola collects water to test for the DNA of bullfrogs (inset).

Credit: Claude Miaud; Mathieu Berroneau (inset)

Invaders Betrayed by DNA

By Erik Stokstad
ScienceNOW Daily News
9 April 2008

Scientists have hit upon a way to spy on invasive wetland species without ever having to see them: They simply detect their DNA in the water. The technique worked for bullfrogs, and such DNA scans could eventually be used in rapid surveys of biodiversity.

The North American bullfrog (Rana catesbeiana) has successfully invaded countries around the world, including at least five in Europe. Tracking the frog in southwest France, Gentile Francesco Ficetola has tromped through more than 2500 wetlands. Could there be an easier way to keep tabs on invasive species? Ficetola, a postdoc at the University of Milan in Italy, teamed up with Pierre Taberlet of Joseph Fourier University in Grenoble, France, who specializes in the detection of ancient DNA. "Our goal was to see if we could detect animals without having them in hand," Taberlet says.

First, the team created DNA primers to amplify short mitochondrial sequences specific to bullfrogs. As a test in the lab, the researchers grew tadpoles in aquariums. The primers detected traces of bullfrog DNA in water from these tanks. The tests were negative for a control tank filled with water collected from above the elevation where bullfrogs live. Then the researchers went to nine ponds, ranging in size up to 10,000 square meters. In three ponds, where they had seen just one to two adult bullfrogs that weren't reproducing, the DNA test came up positive. They also collected water from three more ponds with dozens of adults and teeming with tadpoles, and they found much more frog DNA in these samples. That means the technique could be used to gauge the abundance of a species, Taberlet says. Another three ponds, apparently devoid of frogs, came up negative, the team reports this week in Biology Letters. Taberlet says his group never got a false negative during the study. The frog DNA could be coming from urine, feces, mucus, or perhaps dead frogs.

The study represents "an extremely important move forward in the quest to find practical means of assessing biodiversity," says Karen Armstrong, who studies molecular diagnostics and biosecurity at Lincoln University in Canterbury, New Zealand. Ultimately, as computing power and bioinformatics improve, researchers might be able to detect rare or invasive species, or keep tabs on biodiversity for thousands of species.