Phytoplasmas modulate insect vector biology

Phytoplasmas are plant pathogenic bacteria transmitted by leafhoppers, planthoppers and psyllids. They can establish pathogenic, neutral or beneficial relationships with their vectors. The leafhopper Euscelidius variegatus (Hemiptera: Cicadellidae) is an efficient natural vector of chrysanthemum yellows phytoplasma (CYp) and a laboratory vector of Flavescence dorée phytoplasma (FDp). These two phylogenetically distant phytoplasmas have different effects on their common insect vector species: FDp negatively affects its fitness while CYp is not harmful. RNAseq analysis of E. variegatus infected with the two phytoplasmas was performed to underpin the molecular bases of these different insect responses. Among insect transcripts with a functional annotation, 97 were differentially modulated between the two conditions (CYp vs. FDp). The differentially expressed transcripts were mainly related to immune response, movement and energy production. RT-qPCR and biological experiments were carried out to support the molecular evidences. Measures of the phenoloxidase enzymatic activity, melanization and body pigmentation indicated a stronger level of activation of the immune response in E. variegatus infected with FDp rather than with CYp, confirming the detrimental effect of FDp on its vector. In the immunocompetence assay E. variegatus displayed a prompter response to stress upon infection with CYp compared to FDp. Higher CO2 production and faster movements were recorded in CYp-infected than in FDp-infected insects. Our data show that E. variegatus mostly perceives FDp as a pathogen, since it activates a long-lasting immune response, probably due to the lack of co-evolution between FDp and the laboratory vector E. variegatus. Conversely, the acquisition of CYp, transmitted by E. variegatus under natural conditions, does not affect fitness of the vector, primes its defenses and improves movement of the infected insect, enhancing the opportunity of pathogen transmission and providing an ecological advantage to both the vector and the phytoplasma.