Nucleotide sequences of P3 and pipo genes of Potato virus Y (PVY) from potato and tobacco were compared to investigate the effect of hosts on the population genetic structure. Meanwhile, mutation, natural selection and gene flow were evaluated to determine evolutionary forces responsible for the population genetic dynamics. The fixation indices of population differentiation (FST) of PVY from tobacco and potato were 0.116 and 0.120, respectively with significant difference, suggesting a moderate genetic differentiation between the two populations. Genetic variation analysis showed that nucleotide identities in P3 and pipo genes among the viral isolates from tobacco were respectively in the range of 85.2%-100% and 76.5%-100% while that from potato were respectively in the range of 95.7%-100% and 93.0%-100%, indicating higher genetic variation in PVY from tobacco than that from potato. Moreover, purifying selection was detected on the majority of polymorphic sites within P3 gene, suggesting that most of mutations in the gene were harmful and consequently being eliminated by natural selection. Conversely, positive selection was detected on two polymorphic sites, suggesting that these two mutations were beneficial to PVY. Neither purifying nor positive selection was detected in pipo gene, indicating neutral evolution of the gene. The values of gene flow (Nm) between PVY populations from tobacco and potato in P3 and pipo genes were 1.91 and 1.83, respectively, suggesting strong gene flow also contributes significantly to the population genetic dynamics of PVY population. In summary, this study indicates there was a significant genetic variation in PVY hosted by tobacco and potato, and mutation, natural selection and gene flow all contribute to the genetic diversity and population dynamic of the virus.
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