April 18, 2010 | 6
NEW YORK—Will exotic orchids soon be subjected to the same genetic scrutiny as some luxury caviars? That is just one of the coding conundrums that scientists convened at the New York Botanical Garden in the Bronx to discuss on a cloudy mid-April afternoon.
If plant DNA barcoding proves to be as useful as it has been in animals, it could help prevent illegal trade of endangered species, certify timber types, identify invasive species and even reveal what’s really in herbal supplements—and other plant-based products.
Biologists developed the technique of DNA barcoding in the past decade, a process that involves sequencing a short genetic marker in organisms in an attempt to quickly identify them by species. The technique has helped in efforts to track inaccurately labeled animal products, such as caviar and tuna as well as in preventing trade in endangered animal species.
DNA barcoding has been slower to take off in plants because their mitochondrial genome—the best one for sequencing—is not as distinct from species to species as it is in animals. And plants, especially orchids, are particularly prone to hybridization, creating individuals that are extremely close genetically to other species and occasionally even other genera. Botanists, however, eventually settled upon two gene locations (rbcL and matK) to begin standardizing genetic barcodes for plants. (The technique was published in 2009 in Proceedings of the National Academy of Sciences and is still in the midst of an 18-month-long trial phase during which the efficacy and practicality of using these two gene regions is being evaluated across the field.)
Even for these two gene locations, testing is still only about 70 percent accurate, so additional markers might need to be used in many cases, explained Peter Hollingsworth, of the Royal Botanic Garden in Edinburgh, at the April 16 symposium.
In many cases, however, that 70 percent is more than enough information to tell if a plant or plant-based product is—or isn’t—what it is claiming to be. One practical application is testing dietary herbal supplements, which are not monitored by the U.S. Food and Drug Administration (FDA) to the extent that chemically manufactured drugs or even whole and processed foods are. David Baker, a professor of obstetrics, gynecology and reproductive medicine at Stony Brook University, saw many postmenopausal patients who were taking herbal supplements in lieu of much-maligned hormone-replacement therapy. In particular, many women were taking black cohosh (Actaea racemosa), which is thought to reduce symptoms by binding to estrogen receptors. Baker, however, wondered just what was really in the "numerous little white plastic bottles" that his patients were bringing in to show him. And in fact, after testing more than two dozen varieties of commercially available black cohosh supplements, he found that several contained a different species entirely. The erroneous Actaea concerned him because some varieties in the genus contain toxins, and some trials of the herb (often conducted with commercially prepared pills) have resulted in harmful side effects, while other trials have shown no benefit, suggesting that not all of the trials were testing the same substance.
Although the prospects for plant identification via DNA barcoding appear numerous, not everyone is convinced that the technique will prove to be the final word in plant sciences. Rob DeSalle, curator of invertebrate zoology at the American Museum of Natural History in New York, who calls himself "somewhat of a skeptic of barcoding," noted that for describing new species, the technique is far from perfect. "In taxonomy—unlike horseshoes or grenades—close enough doesn’t count," he said. He advocates for a dual-track approach, one that uses simple genetic markers to determine distinctions among species in the field but uses more detailed systematic biology approaches to describe new finds back at the lab. "DNA barcoding is just another tool in the taxonomist’s toolbox," DeSalle said.
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