Genomic characterization and evolution analysis of a mutant reovirus isolated from grass carp in Anhui

Grass carp fingerlings and yearlings are vulnerable to grass carp reovirus (GCRV), leading to a mortality rate of more than 80 %. A virulent reovirus, tentatively named GCRV-AH528, was isolated from a diseased grass carp in Anhui Province, China, in 2014. The isolated virus could be propagated in Ctenopharyngodon idellus kidney cells. However, the typical cytopathic effects of aquareovirus infection were not observed with this strain. Intraperitoneal injection with the isolated virus led to hemorrhage similar to the observed clinical symptoms. This confirmed that GCRV-AH528 was a pathogenic strain. Its genome contained 11 double-stranded RNA segments (S1-S11) ranging from 1,027 to 3,925 bp (accession numbers KR180368 to KR180378), and encoding 12 proteins instead of 11 proteins (Table 1). All segments had conserved terminal nucleotides, with GUAA^U/_CU at the 5′ end and U^U/_GCAUC at the 3′ end. Compared with registered GCRV strains [1–3], the GCRV-AH528 S1 segment had lost three continuous nucleotides (TAT) at nt 701-702, leading to the loss of a tyrosine, and the S4 segment exhibited two ORFs encoding two proteins, indicating that GCRV-AH528 was distinct from any known GCRV and was a mutant reovirus. Sequence comparisons showed that GCRV-AH528 possessed only seven proteins that were homologous to those of the first isolate, GCRV-873 from 1983 (18.1 %-47.0 % sequence identity), but eight that were homologous to members of the genus Orthoreovirus (13.9 %-45.2 % identity). A phylogenetic tree based on RdRp revealed that GCRV-AH528 clustered with members of the genus Aquareovirus and had genetic affinity with to members of the genus Orthoreovirus (Fig. 1). This study will help in recognizing and understanding the molecular epidemiology, genetic diversity, and evolutionary relationships of GCRV.

Table 1 Characteristics of GCRV-AH528 genome segments

Table 2 Sequences used for phylogenetic analysis

Fig. 1

Phylogenetic analysis based on amino acid sequences of RdRp from members of the family Reoviridae. The arrow indicates the position of GCRV-AH528. The phylogenetic tree was constructed by the neighbor-joining method using 1000 bootstrap replications. The abbreviations and RdRp GenBank accession numbers are presented in Table 2