Molecular variability of Tobacco vein banding mosaic virus populations.

The incidence of Tobacco vein banding mosaic virus (TVBMV) on tobacco increases dramatically in China recently and it has caused great economic losses. To gain insights into the evolutionary mechanisms of TVBMV, a total of 40 TVBMV isolates were collected from different tobacco production regions in China and their genomic regions encoding helper component-proteinase (HC-Pro), the third protein (P3), the first 6K protein (6K1) and coat protein (CP) were sequenced. Phylogenetic analyses revealed that TVBMV isolates can be divided into two evolutionary divergent groups based on P3, the frame-shifting pipo and 6K1 genes, and three groups on HC-Pro and CP genes. The populations from most parts of mainland China (MC) showed frequent gene flow; those from Yunnan province in south western China always formed a separate group (YN) and also had frequent within-group gene flow. However, the gene flow between groups MC and YN was uncommon. Our results revealed that all the tested TVBMV genes were under negative selection and the HC-Pro gene was under the strongest constraints. Recombination events were identified in 13 of the 42 analyzed isolates. This study suggested that negative selection, gene flow and recombination were important evolutionary factors driving the genetic diversification of TVBMV.

Phylogenetic analyses of an isolate obtained from potato in 1985 revealed potato virus X was introduced to China via multiple events.

Potato virus X (PVX) is one of the most common plant viruses that cause great economic losses to solanaceous plants. We have previously reported the complete genomic sequence of the 2006 Chinese potato isolate FX21 and demonstrated that PVX isolates cluster into two groups: Eurasia and America. Here, we present the complete genomic sequence of one PVX isolate collected from potato in 1985 (PVX-1985). The genome of PVX-1985 is identical to that of FX21 in length and has the same genomic structure. PVX-1985, which like FX21 fell within the Eurasia group, clustered together with isolates from Europe, whereas FX21 clustered together with isolates of primarily Asian origin. Phylogenetic analyses of the complete genomic sequences and of CP gene sequences showed that Chinese PVX isolates have different origins and were introduced via multiple events. Though all the open reading frames of PVX are under negative/purifying selection, the central region of RNA-dependent RNA polymerase is under positive/diversifying selection.