- Author: Travis M. Bean
What happens when two problematic invasive plant species hybridize? In the case of Salsola tragus and S. australis, both widespread and co-occuring, but genetically distinct species commonly referred to as Russian thistle, the result has been a new species, S. ryanii. So far S. ryanii is off to a running start, indicating that it's potentially just as problematic as its parent species, perhaps even more so, because while records exist showing the presence of S. tragus in San Bernardino County since the 1890s and S. australis for at least several decades, S. ryanii was first documented in 2002. For more on the significance of this point, refer to a previous UC Weed Science Blog post on the importance of lag times in invasion. In short, a single snapshot in time may not be the best predictor of a species' invasiveness, especially close to time of introduction.
In a paper published in the American Journal of Botany in March 2016, UC Riverside researchers Shana Welles and Norman Ellstrand documented the rapid expansion of this new species, an allopolyploid hybrid (meaning it has multiple complete sets of chromosomes derived from different species) of S. tragus and S. australis. Their paper has some very interesting information on the implications for alloploidy and hybridization for the evolution of invasiveness and why S. ryanii is such a valuable study organism, and I'd highly recommend reading it (and some of the references they draw upon) for more complete understanding of the evolutionary importance of their findings.
In terms of management implications, Welles and Ellstrand conducted a systematic collection of Salsola species from 53 sites in California, using genetic markers to identify the species of each individual, then compared the range of S. ryanii in 2002 (collections made by California Department of Food and Agriculture researchers). What they found was that in a single decade, this new species had increased from 2 populations to 15, and expanded its range into southern California. According to the authors, “to our knowledge, this is the fastest documented population number expansion of a newly formed allopolyploid species - perhaps the most dramatic known range expansion for any plant neospecies [newly formed species].”
Salsola species are notoriously good dispersers, with S. tragus exhibiting the “tumbling tumbleweed” action widely recognized as characteristic of the genus that facilitates long range dispersal, while S. australis lacks the tumbling dispersal behavior but is highly dispersed by wind. The morphological characteristics of S. ryanii are intermediate to its parent species, including the tumbling dispersal behavior, though other intermediate morphological characteristics related to fruits may enhance wind dispersal of seeds (meaning it's probably a good disperser as well).
In a continuation of their research, Welles and Ellstrand published another paper in Evolutionary Applications in July 2016, showing that S. ryanii is the result of at least three independent hybridization events between S. tragus and S. australis, and that there is gene flow between the descendants of these independent events. Translation: S. ryanii is likely good at propagule dispersal, it's entirely possible that new populations are forming where S. tragus and S. australis overlap (see CalFlora maps of their respective distributions in the first section), and those new populations are also breeding more S. ryanii. The bottom line is that this is a species on the move and one for managers to watch out for (although visual differentiation between the species is likely to be difficult to the untrained eye…).
Also, just for fun, check out the May 1895 UC Ag Experiment Station report “The Russian Thistle in California” by C. H. Shinn. My favorite quote is on page 13: “Since the plant is an annual it is easily killed during the growing season. For three months from the time it sprouts no good farmer need be afraid of it, but, according to Dakota experience, concerted action is necessary. The fundamental principle of the conflict must be: No Russian thistle should be allowed to produce seed.” If only it were that easy...
Luckily, we have added a few tools to our weed management toolbox since 1895, and on that note, check out these free online resources on managing Salsola brought to you by the University of California Agriculture and Natural Resources Statewide Integrated Pest Management Program./span>/span>