Comparison of the S RNA segments among Japanese isolates and Taiwanese isolates of Watermelon silver mottle virus.

The complete nucleotide sequences of the S RNA of two Japanese isolates of Watermelon silver mottle virus were determined. One was isolated from naturally infected watermelon and causes malformation on upper leaves of Tetragonia expansa. The other was isolated from melon and causes characteristic yellow necrotic lesions on upper leaves of T. expansa. The total nucleotide sequences of the S RNA of WS-O and WS-Y were 3553 nt and 3558 nt long, respectively. Both the nucleotide sequences and the deduced amino acid sequences of WS-O and WS-Y were quite similar even though the symptoms on T. expansa are quite different. They were also significantly similar to those of the Taiwanese isolates, Topso-W and Tospo-To. These results suggested that the Japanese isolates and the Taiwanese isolates of WSMoV were classified as one group not only serologically but also genetically. Within the S RNA sequences, the most variable region was the intergenic region between the N gene and the NSs gene. This was due to a 20 nt insertion between the Japanese isolates and the Taiwanese isolates.