The relatively sudden appearance and destructiveness of the disease in Europe pointed to an exotic pathogen, but scientists didn't know where it came from. Tracing the origins of the pathogen back to California took some genetic sleuthing by Garbelotto, Della Rocca and their colleagues Catherine Eyre, a UC Berkeley post-doctoral researcher in ecosystem sciences, and Roberto Danti, a researcher at the Italian National Research Council.
The researchers used modern DNA fingerprinting techniques to analyze 96 S. cardinale isolates of diseased tree samples from seven Mediterranean countries, eight California counties, Chile and New Zealand.
S. cardinale is capable of reproducing asexually by creating genetically identical clones of itself, or sexually when a different variant is available for mating.
California emerged as a likely culprit because it hosts populations of the pathogen that are genetically diverse, a strong sign that the pathogen is endemic to the region, the researchers said. The study authors attribute the diversity to the likely sexual reproduction of two genetic variants of the pathogen found in California.
In contrast, the researchers found that one of the two variants of S. cardinale endemic to California is responsible for the epidemic of cypress canker in the Mediterranean, an indication that the fungi there all descended from a "founder" genotype that made its way to Europe.
The second variant found in California, incidentally, has been linked to the epidemic in countries in the Southern Hemisphere such as New Zealand and Chile.
Just how the pathogen moved from California outward is not yet clear, the researchers said. What they can say with certainty is that humans helped the pathogen along in its journey, since air and sea currents alone could not account for the discovery of identical genotypes thousands of miles apart. The paper reports that strai
|Contact: Sarah Yang|
University of California - Berkeley