Comparative genomics of the diverse obligate symbionts of Adelgidae (Hemiptera: Aphidoidea)

Adelgids (Hemiptera: Sternorrhyncha: Adelgidae) are unusual among plant-sap-feeding hemipterans, in that all species appear to harbor two obligate bacterial endosymbionts. Moreover, multiple replacements of both endosymbionts have occurred over the history of the lineage. These changes appear to correlate with host-plant shifts, such that each adelgid lineage on a different secondary host has a different consortium of endosymbionts. Similar to other sternorrhynchans, we presume that adelgid symbionts perform a nutritional role for hosts, with genomes condensed to these functions. However, two observations hint at roles beyond supplementary nutrition: (i) some adelgids feed on more nutritionally complete cell contents, not phloem, and (ii) symbionts have been detected in salivary glands, where their products might interact directly with host plants. In this study, we take a comparative genomics approach to characterizing the metabolic capabilities of adelgid endosymbionts, as a first step towards discovering (a) the functional roles of symbionts in adelgids and (b) whether the combined capabilities of symbionts are similar across host lineages. We performed whole-genome sequencing for 3 adelgid species (Adelges tsugae, A. piceae, and A. cooleyi) and their obligate symbionts. Preliminary genome assemblies and annotation of symbiont genomes provide insight into the function of these symbiont consortiums and reveal characteristics of symbiont genomes that have been conserved across host lineages.