When a Palearctic bacterium meets a Nearctic insect vector: genetic and ecological insights into the emergence of the grapevine flavescence dorée epidemics in Europe. Genetic diversity of 16SrV-C and 16SrV-D sub-groups phytoplasmas

Flavescence dorée (FD) epidemics spread in the South-European vineyards by the Deltocephalinae leafhopper vector Scaphoideus titanus of North American origin. However, the geographical and ecological origin of the etiological agents, i.e. phytoplasmas of taxonomic subgroups 16SrV-C and -D, remained unclear despite evidence for a plant host-range not restricted to grapevine. A survey of genetic diversity of FD phytoplasmas detected in grapevines, alders and clematis as well as their endemic leafhoppers was conducted in France, Hungary, Germany, Italy and Serbia. Hundred and twenty two genotypes were identified out of which only ten associated to FD outbreaks. Surprisingly, a higher number of 112 genotypes was detected in alder trees infected in more than 80% of the cases including both FD-free vineyards and vineyard-free areas. Transmission assays showed that the Macropsinae leafhopper Oncopsis alni transmitted genotypes that could not be transmitted by S. titanus, while the Deltocephalinae leafhoppers Allygus sp. and Orientus ishidae transmitted M38 and M50 genotypes that proved to be compatible with S. titanus. To search for phytoplasma genetic determinant responsible for the insect vector specificity, vmpA and vmpB genes were selected because they encode surface proteins with structural organization reminiscent of bacterial proteins involved in eukaryotic cell invasion. VmpA and vmpB phylogenetic analyses clearly discriminated 3 genetic clusters. Vmp-I cluster grouped all the alder phytoplasma genotypes only transmitted by the Macropsinae leafhopper O. alni, while clusters vmp-II and -III grouped the FD genotypes transmitted by the Deltocephalinae leafhoppers. Interestingly, in cluster vmp-I repeated domains evolved separately, whereas in clusters vmp-II and –III repeated domains seemed to evolv by duplications. Fluorescent beads coated with vmpA of cluster II exhibited enhanced adhesion to both Euscelidius variegatus epithelial cells and S. titanus midgut by comparison to vmpA of cluster I. Altogether these data indicate that FD phytoplasma is endemic to European alders and common in clematis. Its emergence as a grapevine epidemic pathogen is restricted to some genetic phytoplasma variants pre-existing in alders which compatibility to the introduced S. titanus vector certainly resulted from the pre-adaptation of Vmps to other Deltocephalinae leafhoppers already living on European alders.