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Genetic Structure of Populations of the Pinewood Nematode Bursaphelenchus xylophilus, the Pathogen of Pine Wilt Disease, Between and Within Pine Forests

First author: Laboratory of Molecular Microbiology, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Science, Chinese Academy of Science, Shanghai 200032, China; first and fourth authors: Symbiotic Function Research Unit, Asian Natural Environmental Science Center, the University of Tokyo, Midori-cho 1-1-8, Nishi-Tokyo, Tokyo 188-0002, Japan; second author: Experimental Station at Tanashi, University Forests, the University of Tokyo, Midori-cho 1-1-8, Nishi-Tokyo, Tokyo 188-0002, Japan; and third and fourth authors: Graduate School of Agricultural and Life Sciences, the University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan

We analyzed the genetic structure of Bursaphelenchus xylophilus populations within individual trees (subpopulations) in three distant pine forests (Tanashi, Tsukuba, and Chiba in Japan) based on the polymorphism of four microsatellite (SSR) markers. Most of the nematodes from subpopulations in Tanashi showed the same genotype over 2 years, indicating that nematodes of that genotype dominated there for years. In contrast, 16 and 15 genotypes were identified in nematode populations from Tsukuba and Chiba, respectively. Despite the high genetic diversity within the Tsukuba and Chiba populations, extremely low genetic diversity was observed within the subpopulations. The genetic difference between the Tsukuba and Chiba populations was significantly smaller than that between Tanashi and either Tsukuba or Chiba. Observed heterozygosity was significantly less than expected based on Hardy-Weinberg equilibrium. These findings are best explained by a founder effect, geographic isolation between populations, explosive nematode multiplication from a small number within individual trees, and the Wahlund effect.

Additional keywords: Japanese pine sawyer, Monochamus alternatus, Pinus densiflora, P. thunbergii.