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Sahara Mustard Fight Heats Up

 

Last updated 2/22/2017 at 1:59pm



“With the completion of Phase I of a three-phase project seeking a biocontrol agent for Sahara mustard (http://bit.ly/2lDcLke), the Tubb Canyon Desert Conservancy (TCDC), the University of California, Irvine (UCI), and the Steele/Burnand Desert Research Center have become the global leader in Sahara mustard genetic research,” said J. David Garmon, M.D., founding director and President of TCDC. Launched in Borrego Springs in January 2015, Phase I involved collecting hundreds of samples of Sahara mustard from 55 sites throughout the American southwestern—California, Arizona, Nevada, Utah, New Mexico, and Texas.

Working under the tutelage of Travis Huxman, Ph.D., Director of UCI’s Center for Environmental Biology and Director of the Steele/Burnand Research Center, primary investigator and Ph.D. candidate Daniel Winkler spent the first six months of 2015 on a 5000-mile journey collecting over 1000 individual samples of Sahara mustard. Once samples had been collected and catalogued, they were returned to the lab for processing.

While “processing” is a simple word, the actual processing of these samples was anything but simple. The initial steps involved using chemicals to break open the cells of each Sahara mustard sample so its genetic material, the deoxyribose nucleic acid (DNA), could be extracted and separated from other cellular structures and materials. Once the DNA from each sample was purified, a unique chemical tag was attached to it so it could be identified all along the process. The tagged strands of Sahara mustard DNA, each one approximately 500 million units long, was then chemically broken in to segments of a few 100 to a few 1000 units long.

“These tagged, 100-1000 unit long segments of DNA were then sequenced at a special laboratory at Oregon State University that uses cutting-edge technology,” said Mr. Winkler. “The amount of data we received from sequencing 1000 samples of Sahara mustard was equal to the number of letters contained in 24 complete sets of the Encyclopedia Britannica, or about 100 gigabytes. But now imagine those 24 sets of Encyclopedia Britannicas only used four letters—A, C, G, and T—over and over again in an order that is unique to each Sahara mustard plant.”

Mr. Winkler turned to the supercomputer at UCI to make sense of the billions of tagged DNA sequences. Over the course of weeks the computer matched, aligned, and overlapped billions of tiny segments of DNA, all the while looking for similarities and subtle differences in the emerging genetic code. “At this point,” Mr. Winkler says, “what we know for certain is that we have at least three genetically separate populations of Sahara mustard in the southwestern United States. About half the samples I collected appear to belong to population A and the other half to population B. And intriguingly, the samples from one site in Nipomo, CA did not belong to either population A or B, but were a genetically distinct population.”

Careful to not over interpret the data, Mr. Winkler acknowledges there could be additional populations of Sahara mustard in the American southwest that were simply not part of his extensive sample set. He also noted there is no geographic pattern to the locations of the populations of Sahara mustard he has studied. “But when you think about it, it makes sense there is not a geographic pattern with all of population A in one area and all of B in another. We know this weed is spread by human activity. Whether it is hiking, driving our cars, or hauling dirt, we are dispersing the seeds of Sahara mustard in a random pattern; and that is something this study demonstrates.” Mr. Winkler mused as to the possibility of Sahara mustards seeds that are currently growing next to the Borrego Springs airport hitching a ride on the tires or in the cowlings of private planes bound hundreds of miles away.

Mr. Winkler explained that Phase I raises important questions about Sahara mustard in the US, such as whether the three populations we now know about represent three separate introductions of Sahara mustard, or was there just one introduction and the genetic diversity we now see is a result of genetic evolution that has taken place over the century since the plant was first introduced into the US. He also wonders if that distinct population of Sahara mustard in Nipomo, CA might have been introduced from a completely different source such as Australia.

“Phase II will help us answer these questions,” Mr. Winkler said. “Once we sample, sequence, and build a family tree of Sahara mustard in its native range (Mediterranean basin and the Middle East), then we will be able to determine how many times Sahara mustard was introduced into the US; and perhaps even more importantly, we will be able to determine precisely where ‘our’ mustard came from.”

Dr. Garmon added, “Once we know where in its native range that first plant, or those first plants, came from, that’s when we begin Phase III, which is the identification of those biological agents—virus, fungus, bacteria, insects, etc.—that keep Sahara mustard in check on its home turf. Our colleagues at the USDA European Biological Control Laboratory in Montpellier, France have some early ideas, but the definitive answers will only come with the completion of Phases II and III.”

Garmon concluded, “We have come a long way in just two years in our understanding of the genetics of Sahara mustard in the United States. And we still have a long way to go to achieve our goal of countering Sahara mustard’s threat to our desert ecosystems. But we will get there with the brilliance of researchers like Mr. Winkler and the generosity of donors like Audrey Steele Burnand, Robert Johnson, and Susan Gilliland who have generously supported this effort from the beginning.”

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