Identification, genome sequencing, and virulence variation analysis of MCRV-NH, an epidemic reovirus strain in cultured mud crab, Scylla paramamosain

Mud Crab Reovirus (MCRV) is a major pathogen of cultured mud crab Scylla paramamosain. Early studies showed that the mortality rate caused by MCRV was high. However, current studies show that MCRV has been widely present in cultured mud crab ponds, and the mortality rate has dramatically decreased, suggesting that it has become more infectious but less virulent. In this study, an epidemic MCRV strain named MCRV-NH was isolated from a representative farming areas of mud crabs to reveal the intrinsic molecular mechanism of the virulence variation of MCRV. Electron microscopy results showed that MCRV-NH was similar to the early reported MCRV or Scylla serrata reovirus (SsRV) in terms of morphology and structure. Viral infection experiments showed that no mud crab was dead during the infection period of 11 days even though MCRV-NH could rapidly proliferate in mud crabs at 21–23 °C, demonstrating that the mortality rate caused by MCRV-NH significantly decreased. Sequencing results of the MCRV-NH genome showed that eleven nucleotides were deleted in the sequencing region, and its nucleotide sequence identities with those of MCRV and SsRV were 99.43% and 99.33%, and the numbers of corresponding nucleotide mutations were 150 and 126, respectively. Evolutionary analysis showed that MCRV-NH belonged to the same species as MCRV and SsRV, and MCRV-NH should be a variant. In comparison with the open reading frames (ORFs) of MCRV and SsRV, the ORFs of MCRV-NH had no nucleotide deletions, but the numbers of nucleotide mutations were 100 and 123, and the numbers of amino acid mutations were 32 and 35, respectively. Eight mutated amino acids compared with the two reoviruses were located at the predicted active sites of the viral proteins. Among them, three mutations resulted in the generation of new phosphorylation sites, and two mutations led to the loss of the original phosphorylation sites. Meanwhile, a heptapeptide repeat sequence in VP9 disappeared, and a streptococcal histidine triad protein in VP6 and a Big-1 (bacterial Ig-like domain 1) domain in VP11 were generated due to key amino acid mutations in these viral proteins, which are involved in the infection and replication of MCRV. These changes in the active sites of the viral proteins may be responsible for the MCRV virulence variation. This study provided essential evidence to support the conclusion that MCRV-NH was a variant of MCRV with weaker pathogenicity and revealed the underlying possible molecular mechanism of the virulence variation of MCRV-NH.