Cryptic Invasion: Native reeds harbor aggressive alien

A mild-mannered reed native to the United States is getting blamed for mayhem created by an evil twin, says a Yale researcher.

Tall Phragmites australis plants, often just called Phragmites, have waved their tasseled tops over wetlands in North America for millennia, says Kristin Saltonstall. However in the 1970s, botanists became alarmed after seeing how extensively the species pushed into new territory and formed dense patches that crowded out other plants.

Perhaps an alien strain of the species was taking over by stealth. To investigate that possibility, Saltonstall analyzed genes from leaves of P. australis collected worldwide. She found that a strain now common in the United States has close relatives among Eurasian strains and could be an invader. This strain shows up only occasionally among Phragmites collected before 1910 but dominates modern herbarium collections, Saltonstall reports in the Feb. 19 Proceedings of the National Academy of Sciences.

Scientists have documented such invasions by foreign strains of native species in several marine creatures, including a crab. Saltonstall says she doesn’t know of other such cases on land. “There could be plenty of others,” she cautions, since these invaders hide in plain sight until someone does genetic testing.

The earliest traces of P. australis in North America come from 40,000-year-old sloth dung in the Southwest. More recent signs show up in cores of earth pulled from East Coast marshes. The cores indicate that at least 3,000 years ago, Phragmites grew in mixed communities of wetland species, not in the all-Phragmites blankets that have become common.

To sort out the strains, Saltonstall analyzed DNA in chloroplasts, the photosynthetic organelles in plant cells. Saltonstall compared two particularly variable regions in DNA harvested from 257 North American Phragmites patches and 88 populations from the rest of the world. The Smithsonian Institution even supplied her with a sample from Afghanistan.

Five mutations turned up in North American populations but were found nowhere else, says Saltonstall. She proposes that plants bearing these five markers represent long-standing North American strains.

In contrast, a strain now common in the eastern states didn’t bear the markers and fell instead into a cluster of Eurasian Phragmites. This strain accounted for 6 percent of the pre-1910 U.S. museum samples that Saltonstall checked but represented 61 percent of samples collected after 1960.

Now that Saltonstall can identify this exotic strain genetically, Cornell University Phragmites specialist Bernd Blossey is looking for ways to spot it on sight. “It may be easier than we expected,” he says.

He foresees efforts to find and protect native Phragmites, particularly along the East Coast.