Molecular Population Analyses of Phytophthora ramorum
Kelly Ivors1, Katy Hayden2, Matteo Garbelotto2, and David Rizzo3
Phytophthora ramorum, the causal agent of Sudden Oak Death, is presumed to be an introduced aggressive pathogen in the United States and is responsible for the mortality of at least 3 species of native California oaks (Quercus spp.), and tanoak (Lithocarpus densiflora). Amplified Fragment Length Polymorphism (AFLP) analyses were conducted to determine the genetic variability and sub-structuring of this pathogen isolated from different hosts and locations in North America and Europe. Among the North American isolates tested, we found 12 different AFLP genotypes. A single genotype accounted for 82% of all isolates; no other genotype had more than 2 isolates represented. In contrast, each of the isolates tested in a recent AFLP study of P. ramorum in Europe represented a different genotype, as also observed with the seven European isolates used in our study.
The main genotype of P. ramorum was recovered throughout the known geographic range of the pathogen including the northernmost (Oregon) and southernmost locations (Big Sur), a distance of approximately 650 km. These results indicate a largely clonal population in North America with no subdivisions based on geography or host. Such data strongly suggest that the pathogen may move from one host to another with no evidence of host specificity between different isolates. The apparent limited gene pool of P. ramorum in North America is suggestive of an introduced or emergent organism, but the actual origin and global genetic structure of the pathogen remains unknown. Although sample size from the European population was limited, AFLP results place the European and North American populations in two separate clades. The results and implications of these population analyses will be further discussed.
1Department of ESPM-ES, University of California, Berkeley, CA 94720; (510) 643-4282; kivors@nature.berkeley.edu
2Department of ESPM-ES, University of California, Berkeley, CA 94720
3Department of Plant Pathology, University of California, Davis, CA 95616
|
