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Incidence and Diversity of Double-Stranded RNAs Occurring in the Chestnut Blight Fungus, Cryphonectria parasitica, in China and Japan

First, second, and sixth authors: Department of Plant Pathology, Cornell University, Ithaca, NY, 14853; third author: Department of Plant Protection, Nanjing Agricultural University, Nanjing 210014, People's Republic of China; and fourth and fifth authors: Department of Plant Pathology, Rutgers University, New Brunswick, NJ 08903

Isolates of the chestnut blight fungus, Cryphonectria parasitica, were randomly sampled from 10 subpopulations in China and 8 subpopulations in Japan and screened for the presence of double-stranded (ds) RNA using an immunoblot procedure with a monoclonal antibody specific for dsRNA. The overall incidence of dsRNA in C. parasitica was 2 and 6% in China and Japan, respectively, much lower than the 28% found previously in North American populations. Genetic relatedness of dsRNAs within and among populations in China and Japan was examined using RNA-RNA hybridizations with labeled-dsRNA probes. The majority of Chinese and Japanese dsRNAs were members of a single hybridization group, related to Cryphonectria hypovirus 1 (CHV1) from Europe, and are referred to as CHV1-type dsRNAs. No evidence was obtained for genetic differentiation between CHV1-type dsRNAs sampled in China and Japan. Five Japanese isolates contained two genetically distinct dsRNAs. The larger segments (approximately 12 kilobases [kb]) were members of the CHV1 hybridization group, while the smaller segments (approximately 3 kb) did not hybridize with any known dsRNA from C. parasitica including the 2.7-kb dsRNA from isolate NB631 from New Jersey or dsRNA from isolate RC1 from Michigan. Two small dsRNA segments (approximately 1.8 and 2 kb) from one isolate sampled from Liaoning Province in northeastern China did not hybridize with any of the dsRNA probes tested including several described dsRNAs of similar size from C. parasitica in North America. Three dsRNAs from Anhui Province, China, hybridized to Cryphonectria hypovirus 2 (CHV2)-specific probes and are thus referred to as CHV2-type dsRNAs. Sequence analysis of 1,627 base pairs of these three CHV2-type dsRNAs from Anhui revealed that they were identical to each other in the region sequenced and very closely related to CHV2-NB58, isolated from New Jersey. We speculate that CHV2-NB58 may have been introduced into North America from this part of China. This is the first record of a North American C. parasitica dsRNA that is genetically related to a dsRNA from Asia.

Additional keywords: biological control , Castanea spp. , Endothia parasitica , hypovirulence , population structure .