Shedding light on the genetic diversity and evolutionary dynamics of geographic populations of WVMV: a case study for the spread of emerging potyviruses in Europe?

Wisteria vein mosaic virus (WVMV) is a member of the genus Potyvirus associated with Wisteria mosaic disease (WMD), the most serious disease affecting Wisteria spp. In 2022, severe symptoms of WMD were observed on the leaves of a Chinese wisteria (W. sinensis) tree growing in an urban area in Apulia (Italy). The presence of WVMV was ascertained by RT-PCR analysis. Although the occurrence of WVMV in Italy had been posited in the late 1960s, no molecular information had been reported for any Italian isolate prior to this study. Subsequent phylogenetic analyses based on NIb and CP genes placed the WVMV Italian isolate within a large clade identified in the genus Potyvirus as the BCMV supergroup. Based on the increasing number of reports of the virus worldwide, we attempted an exploratory analysis of its genetic diversity and possible mechanisms that may have shaped its geographic population structure. Relying on the N-terminus of the CP, available for twenty WVMV isolates from Europe, Asia, and Oceania, sixteen different haplotypes were identified. A high haplotype diversity was found, particularly relevant in the European population. The measured dN/dS ratio led to the assumption that the target region is under purifying selection. Tests evaluating the neutrality of nucleotide variability showed different results for the European and Asian groups. The estimation of inter-population genetic differentiation showed a high level of gene flow between the two populations. Overall, our results provide a possible approach to understanding the mechanisms of WVMV emergence in Europe and draw attention to its further spread and the increasing threat of this and other neglected potyvirus species to the ornamental nursery sector.