Genetic Diversity Of Cryphonectria Hypoviruses (CHV1)

The population diversity of Cryphonectria hypovirus (CHV1) and the transmission of hypovirulence were studied in this thesis.Efficiency of dsRNA transmission in Cryphonectria parasitica was tested by co-culture the dsRNA donor and recipient, and 6 conversed isolates were obtained. Virulence of dsRNA-contained isolates was determined by inoculation both in laboratory and field. The results showed that the conversion rate ranged from 0 to 100% within different pairs. Results of relative virulence tested by inoculation showed that the virulence of 6 transmitted isolates, 12 donors, 6 pairs of untransmitted isolates, and 6 pairs of transmitted isolate (donor) & wild type of donor (recipient) were extremely lower than that of recipient and wild type of donor. They were hypovirulence or mid-virulence. And no significant difference was observed among each other. Wild type of donor possesses comparative virulence to recipient, which exhibited high virulence. The results showed that the potentiality of the biological control of chestnut blight was higher than our expectation.A total of 43 hypovirus isolates (CHV1) of Cryphonectria parasitica from China, Japan, and Italy were analyzed by partial nucleotide measurement and restriction fragment length polymorphism (RFLP) for ORF A in virus genome to reveal the origin of CHV1 in Europe. By partial nucleotide measurement, 30 viruses belonged to 29 different haplotypes could be grouped into three subtypes. The viruses from China were subtypes I and III, but that from Japan were subtype II except Ja55 which was subtype III, and that from Italy were subtype III except IT 192 which was subtype I . Both subtypes I and III widely spreaded in China with different geographic character, whereas in Europe only one subtype spreaded widely. Subtype I viruses existed in southern China by the bound of Yangtze river except isolate 09314 which was from Hunan province, but subtype III viruses existed in northern China excepted isolates 228, 235 (both from Beijing), and 277 (from Liaoningprovince). The results suggested that gene flow appeared among the hypovirus populations. 4 genetically distinct RFLP types are obtained by RFLP analysis of the total 43 viral fragments, demonstrating that type I accounted for the most (31 viral isolates). The results showed higher homologous between the CHV1 populations from China and Italy than those from Japan and Italy, and more divergence in CHV1 population from China than that from Japan and Italy, indicating that CHV1 in Europe might be introduced from China.