The Good, the Bad and the Ugly: Which processes shape phytoplasma-insect-plant relationship?

The association between insect herbivores and vascular plants represents one of the greatest success stories in terrestrial evolution. Microbes play important roles in mediating plant- insect trophic interactions, for example by supplementing nutritionally unbalanced diets, endosymbionts help facilitate adaptation of herbivores to their host plants. Parasitic bacteria (such as phytoplasmas) that manipulate host behavior and phenotype may have even more dramatic effects on evolutionary trajectories in insect-plant associations. Phytoplasmas, a diverse group of bacterial plant pathogens, alter the phenotypes of infected plants and hemipteran insect vectors, thereby affecting functional traits (such as diet breadth) and mediating host shifts (in ecological time) and diversification (in evolutionary time). Previous research focusing on bipartite patterns of hemipteran insect-phytoplasma associations has shown that strict co-speciation appears to be quite rare. In contrast, our preliminary cophylogenetic analyses reveal that host shifts among distantly related phytoplasmas occurred frequently in many different hemipteran lineages. We provide a first insight toward understanding coevolutionary and cospeciation processes of phytoplasmas and their hemipteran hosts. We reveal that subnetworks of phytoplasma linageages (e.g., 16SrX group) are confined to particular host groups (i.e., Psyllidae) including taxa that are strongly restricted to specific biogeographic region.Future research may benefit from exploring the patterns of host-phytoplasma phylogenetic congruence to predict possible host switches of phytoplasmas among host plants or vectors.