Establishing a framework for a novel evolutionary role of endosymbionts in Adelgidae (Hemiptera: Sternorrhyncha)

The prospect of direct interactions between bacterial symbionts of insects and their host plants remains largely unexplored.  Adelgids (Hemiptera: Sternorrhyncha: Adelgidae), which feed on conifers and produce galls on spruce, offer an ideal study system for such interactions. Preliminary investigations on five Adelges species suggested that evolutionary changes in endosymbiont composition have mirrored patterns in host shifts by adelgids, and found symbionts in both the bacteriome and salivary glands of one species. This work indicates that obligate symbionts of adelgids might be released from a typical nutritional role, suggesting the hypothesis that symbionts are manipulating host-plant physiology to the benefit of the insect. If this were the case, we would expect to find a family-wide correlation between symbiont composition and adelgid host-plant shifts, given wider taxonomic sampling. Moreover, we expect that symbionts would be found in salivary glands of most adelgids. To test these ideas, we sampled a broader taxonomic range of species, including the other genus, Pineus. We reconstructed the phylogeny of adelgid symbionts using 16S rRNA sequences, and compared it with both the existing phylogeny of adelgids and host plant information. Our results support the hypothesis that, compared to other Sternorrhyncha, symbionts of adelgids have novel ecological roles that involve direct interactions with host plants, which have affected evolutionary changes in host plants. Future directions will include testing for presence of symbionts in salivary glands using in situ hybridizations with specific probes and genome sequencing of the symbionts to characterize their functions and roles in host-plant interactions.