The massive tumbleweed Salsola ryanii is a species that was once thought to be going extinct. Researchers have found that this tumbleweed is more likely to expand into new regions, however, and is here to stay. This tumbleweed carries two copies of its parents’ chromosomes. While that’s been thought of as a biological advantage for plants, this work shows that the offspring can grow faster and larger than its parents. The findings have been reported in the journal AoB Plants.
"Salsola ryanii is a nasty species replacing other nasty species of tumbleweed in the U.S.," noted study co-author Norman Ellstrand, a University of California Riverside (UCR) Distinguished Professor of Genetics. "It's healthier than earlier versions, and now we know why."
Almost all mammals, including humans, are diploid organisms, which means they carry two sets of chromosomes; one set comes from the female parent, and one from the male. In humans, if an egg that mistakenly carries two sets of chromosomes is fertilized, the resulting zygote is unlikely to survive.
In plants, however, it’s far more common for organisms to carry more than two sets of chromosomes - called polyploidy. Many common domesticated crops like apples, peanuts, and wheat are polyploid. Polyploidy can easily occur when two parents have similar chromosomes or when there is a genome duplication event. It’s often been assumed to confer some biological benefit in plants, otherwise, polyploidy would be rejected and it wouldn’t be seen so often.
"Typically, when something is new, and it's the only one of its kind, that's a disadvantage. There's nobody exactly like you to mate with," explained study co-author Shana Welles, a graduate student at the time of this research who is currently a postdoctoral fellow at Chapman University.
The evolutionary advantage of polyploidy had not been demonstrated, until this research. The scientists showed that a hybrid tumbleweed with more sets of chromosomes than its parents also grows more vigorously.
A tumbleweed that can grow higher faster is not something people living in the West are looking forward to. They can damage property and agriculture, and cause car accidents. Tumbleweeds buried the town of Victorville, California last year; they reached the second story of some buildings.
The range of Salsola ryanii is now expanding from its current small area. Climate change may encourage its growth, which tends to be in later winter. This work may help get it under control.
"It's one of the only things that's still green in late summer," Welles said. "They may be well-positioned to take advantage of summer rains if climate changes make those more prevalent. An ounce of prevention is a pound of cure."