Genetic diversity and competition of Flavescence dorée phytoplasma populations between insect vectors and plants

Flavescence dorée phytoplasmas (FDp, 16SrV) are plant pathogenic non-cultivable bacteria associated with a severe and epidemic disease of grapevine and are transmitted by the Cicadellidae Deltocephalinae leafhoppers Scaphoideus titanus Ball (natural vector) and Euscelidius variegatus Kirschbaum (vector under laboratory conditions only). By living in two worlds, the plant and insect life style of phytoplasmas may drive differential gene expression, as known from literature, but also select different populations inside the plant and insect hosts. Genetic variation of phytoplasmas associated with FD (based on dnaK, vmpA and malG genes) in insects and plants revealed the presence of a wide range of genotypes infecting/co-infecting plants and vectors. FDp genetic variation is not homogeneous between insects and plants, as some genotypes are more represented in insects and others in plants. This skewed genotypic distribution indicates that insect vector drives phytoplasma populations that infect plants and, on the other way, host-plant drives the phytoplasma populations that can be acquired by and infect insects. The competition of the different genotypes inside plant and insect hosts and the presence of FD-infected plants inside and outside the vineyards, all of them representing source of inoculum for the vector, increase the complexity of this phytoplasma/plant/vector epidemiological system. Moreover, in the field, different plant species can harbour different FD phytoplasma genotypes and are visited by different vector/potential vector species. Some of these complex interrelationships have been studied at the field level as well as under controlled laboratory conditions. Although there is some convergence in the phytoplasma genotype composition infecting insects and plants at each site, a higher genetic variation of FD was consistently found in the insects compared to plants in all sites. Moreover, some genotypes were only found in vector or plant hosts. The ecological and evolutionary significance of this finding is discussed.