Genetic structure of rice black-streaked dwarf virus populations in China.

Rice black-streaked dwarf virus (RBSDV) is a double-stranded RNA (dsRNA) virus belonging to the genus Fijivirus in the family Reoviridae. The genome of RBSDV consists of ten dsRNA segments. Although RBSDV has caused significant economic losses to rice and maize production in the past few years in China, its molecular diversity and evolution remain largely unknown. To elucidate the factor(s) underlying the evolution of RBSDV, we determined segment 8 (S8; carrying ORF8 encoding the minor core capsid protein) sequences of 101 samples and segment 10 (S10; carrying ORF10 encoding the major capsid protein) sequences of 103 samples. The results show that both ORF8 and ORF10 are under negative selection. The S8 of three isolates and S10 of two isolates are recombinants. The RBSDV population in China can be classified into three groups according to S8 sequences or into two groups according to S10 sequences, irrespective of host or geographical origin. Of the RBSDV isolates with both S8 and S10 sequences available, 17 are between-group reassortants and 30 are between-subgroup reassortants. The RBSDV subpopulations from different geographical regions and hosts show frequent gene flow within or between subpopulations. The RBSDV population from maize is in a state of expansion. In this study, no new emergent population was detected. Taken together, the results indicate that, in addition to recombination and negative selection, reassortment and gene flow are important factors that drive evolution of RBSDV in China.