The leafhopper vector Euscelidius variegatus responds differently to infection with two phytoplasmas

Phytoplasmas are plant pathogenic bacteria spread by hemipteran insects. They can establish either pathogenic or symbiotic relationships with their vectors. The leafhopper Euscelidius variegatus is an efficient natural vector of chrysanthemum yellows phytoplasma (CY) and a laboratory vector of Flavescence dorée phytoplasma (FD). These two genetically unrelated phytoplasmas have opposite effects on their common vector species: CY improves its fitness, while FD has a negative effect. To elucidate the molecular bases of these different responses, RNAseq analysis of E. variegatus infected with the two phytoplasmas was performed, providing the first de-novo sequence assembly for a phytoplasma insect vector. The insect genes differentially expressed between the two conditions (CY vs. FD) with a functional annotation were related mainly to immune response, movement and energy production. The differential expression of 10 of these genes was confirmed by RT-qPCR, and several biological experiments were carried out to further support the molecular evidences. Consistently with RNAseq results, measures of the phenoloxidase enzymatic activity, melanization and body pigmentation indicated a higher level of activation of the immune response in E. variegatus upon FD infection compared to CY infection, confirming the entomopathogenic effect of FD. In line with transcriptomic data, higher CO2 production and faster movements in CY-infected than in FD-infected insects, indicate that the long co-evolution between CY phytoplasma and E. variegatus could possibly have manipulated the vector behavior to increase its transmission.