| Two major factors affecting the chestnut blight fungus, Cryphonectria parasitica, have been studied: viruses and transposable elements. In the first part a new species of viruses in the family Hypoviridae, named CHV4, has been characterized. The complete nucleotide sequence and the genome structure were determined from the double stranded (ds) RNA present in the isolate SR2. CHV4-SR2 is closely related to CHV3-GH2 in terms of genome organization, sequence similarity and presence of a glucosyltransferase homologue. CHV4-SR2 does not affect the phenotype of the fungal host, as other hypoviruses do. Since CHV4 isolates are widespread in the U.S., a small-scale study on population variability was undertaken through sequence analysis of three regions of the viral genome. Although the limited sampling, the CHV4 dsRNAs may be subdivided into two groups, the main and the NJ group, suggesting two independent founder effects.; In the second part two Class II transposons, Crypt1 and Crypt2, have been identified and characterized. Crypt1 is related to the hAT family and Crypt2 to the Fot1 family. Activity of Crypt1 was assessed by the identification of a recent transposition event in the flat mutant 747ss3, which originated from a virus-infected C. parasitica isolate. Crypt1 is present in a population of C. parasitica in Southern China as single copy, interrupted by the insertion of Crypt2. This inactive copy was found to be ubiquitous in C. parasitica. The distribution of the transposons was examined in three Cryphonectria species: parasitica, radicales and sp., a new unidentified species from Asia sympatric to C. parasitica. C. parasitica harbors both transposons, C. radicalis neither and C. sp. only Crypt2, but in lower copy number than C. parasitica. According to the transposon profile, a potential hybrid between C.p. and C. sp. has been identified in the Japanese isolate YM2. The possibility of horizontal transfer of transposons in this fungal genus is discussed. |
