Newly Evolved Species of Tumbleweed Rolling Through California

Believe it or not, tumbleweeds—so strongly associated with wide-open expanses of the western United States—are actually weedy invaders from other continents. The most iconic and successful of these, the Russian thistle (Salsola tragus), came to America from central Asia in the 1870s, and is now found in 48 states.

Now, this species and another immigrant tumbler have given rise to a hybrid that has officially become a new species, named Salsola ryanii. Though only identified as a new species 14 years ago, it has already spread throughout the state, according to a study published in the American Journal of Botany.

"It's extremely rare to catch a new species in the act of appearing and expanding," says co-author Norm Ellstrand, a professor of genetics at University of California, Riverside, "and very exciting."

The plant was created when S. tragus and the other "parent," S. australis—from either Australia or South Africa (scientists aren't sure which)—created a hybrid, but no ordinary one. Usually, such plants are sterile. In this case, however, the plant underwent an unusual genetic event that led to a duplication of its entire genome. That allowed it to reproduce, and also make it incompatible with either of its parents, making it a new species.

Study first author, plant biologist Shana Welles, found the species at 15 sites throughout California. When it was first identified as a new species in 2002, it had only been spotted at three locations, all in the Central Valley—and the new locations come from several spots to the west, south and east of the valley.

The new species has very slight anatomical differences from its parents, though it cannot be told apart by an untrained eye. Like its tumbleweed parents, the new species is an annual plant that can quickly grow up to 5 or 6 feet in height and nearly as wide. After growing, it flowers and dries, and then its stem often breaks and it's carried through the wind, spreading its seeds as it bounces along.

Tumbleweeds are very good at growing in areas with loose soil that has been disturbed by human activity, such as roadsides, railroad tracks and ranchland. In these spots, it may outcompete other plants and grow rapidly. "In the Central Valley of California, they're typically the last things that are green in the fall," Welles says. But in more pristine areas, it can't establish a foothold.

Welles considers the new species to be foreign, as both parents are. But Ellstrand disagrees, in a tongue-in-cheek manner. Its parents "never had a chance to meet until they came here," rendezvoused in the Central Valley and "had the opportunity for romance for the first time in their history," he says, laughing. "I consider it native since it was born here."