Evidence that Cymbidium mosaic virus (CymMV) isolates divide into two subgroups based on nucleotide diversity of coat protein and replicase genes

Abstract

The genetic diversity of Cymbidium mosaic virus (CymMV, family Flexiviridae) was assessed by analysing the nucleotide sequences of coat protein (CP) and partial RNA-dependent RNA polymerase (RdRp) genes. Thirty CymMV sequences from vanilla isolates, obtained in this work by direct sequencing of RT-PCR products, were compared to the sequences from ornamental orchid isolates available in GenBank. The CymMV population exhibited overall low genetic diversity (pi=0.054 and pi=0.053 for CP and RdRp genes, respectively). Phylogenetic analyses of the 85 CP and 37 RdRp sequences revealed the segregation of the isolates into two congruent monophyletic clusters; however these two subgroups did not cluster in amino sequence analysis because most of the nt mutations were synonymous. Nevertheless, the subgrouping was confirmed by highly significant Kst tests for the CP and RdRp genes. Analysis of population genetic parameters and distribution of synonymous and nonsynonymous mutations revealed that both genes were under negative selection with no recombination events. These results suggested that the CymMV isolates found in cultivated orchids worldwide have a dual origin and are expanding as if following bottlenecks.