Left running head: Mase et al.

SUMMARY Sigma C protein sequences have been used to phylogenetically classify avian retrovirus (ARV) strains. However, the relationship between serotype and phylogenetic cluster classification of the five prototype serotype strains of ARV in Japan has not been established. Thus, used sigma C protein sequencing to characterize avian reoviruses (ARVs) isolated from chickens with tendonitis in Japan together with the five prototype serotype strains of ARV in Japan. Phylogenetic analysis of foreign ARVs based on the sigma C gene revealed that the five prototype serotype strains of ARV were each classified into different independent clusters. The two field isolates (JP/Tottori/2016 and JP/Nagasaki/2017) that were isolated from chickens with arthritis/tenosynovitis, were classified into different clusters. JP/Tottori/2016 was classified into cluster VI with strain CS-108 strain, and JP/Nagasaki/2017 was classified into cluster I with strain TS-142. Serologically, JP/Tottori/2016 was well-neutralized by antisera against the CS-108 strain, whereas JP/Nagasaki/2017 cross-reacted with antisera against both the CS-108 and TS-142 strains. Embryo pathogenicity tests revealed that the mortality of the two field isolates were 80% and 67%, respectively, whereas those of the five prototype strains were 0%–33%. Our findings will contribute to understanding the characteristics of ARV strains in Japan.