Partnering With a Pest: Genomes of Hemlock Woolly Adelgid Symbionts Reveal Atypical Nutritional Provisioning Patterns in Dual-Obligate Bacteria

Abstract

Nutritional bacterial symbionts enhance the diets of sap-feeding insects with amino acids and vitamins missing from their diets. In many lineages, an ancestral senior symbiont is joined by a younger junior symbiont. To date, an emergent pattern is that senior symbionts supply a majority of amino acids, and junior symbionts supply a minority. Similar to other hemipterans, adelgids harbor obligate symbionts, but have higher diversity of bacterial associates, suggesting a history of symbiont turnover. The metabolic roles of dual symbionts in adelgids and their contributions to the consortium are largely unexplored. Here, we investigate the symbionts of Adelges tsugae, the hemlock woolly adelgid (HWA), an invasive species introduced from Japan to the eastern United States, where it kills hemlock trees. The response of hemlocks to HWA feeding has aspects of a defensive reaction against pathogens, and some have speculated that symbionts may be involved. We sequenced the genomes of “Ca. Annandia adelgestsuga” and “Ca. Pseudomonas adelgestsugas” symbionts to detail their metabolic capabilities, infer ages of relationship, and search for effectors of plant defenses. We also tested the relationship of “Ca. Annandia” to symbionts of other insects. We find that both symbionts provide nutrients, but in more balanced proportions than dual symbionts of other hemipterans. The lesser contributions of the senior “Ca. Annandia” support our hypothesis for symbiont replacements in adelgids. Phylogenomic results were ambiguous regarding the position of “Ca. Annandia”. We found no obvious effectors of plant defenses related to insect virulence, but hypothetical proteins in symbionts are unknown players.

Comparative genomics of a quadripartite symbiosis in a planthopper host reveals the origins and rearranged nutritional responsibilities of anciently diverged bacterial lineages

“…Other bacterial symbionts (e.g., Baumannia ‐leafhoppers) are placed in a Sodalis clade (PP = 1). Although it is possible that these results also suffer from LBA, they are consistent with several previous symbiont phylogenomic efforts that used the same approaches, but with different datasets (Husnik et al ., ; Szabó et al ., ; Weglarz et al, ).…”

“…However, the approaches we implemented have also been used to infer the relationships of other Gammaproteobacteria ‐insect symbiont frameworks. These studies independently recovered similar results, but with different datasets (Husnik et al ., ; Szabó et al ., ; Weglarz et al, ).…”

“…Bayesian phylogenies with the recoded gene matrices were also inferred using Phylobayes_MPI v1.7 (settings: cat, gtr, dgam = 4, x = 10), which is robust to LBA reconstruction artefacts (Lartillot et al, 2013). The method was also implemented in insect symbiont frameworks and has been shown to better break LBA than other available methods (Husnik et al, 2011;Szabó et al, 2017;Weglarz et al, 2018). Two independent chains were run until parameters converged as determined with tracecomp and bpcomp with a 20% burn-in (e.g., maxdiff < 0.3 and minimum effective size > 50).…”

Evolutionary recent dual obligatory symbiosis among adelgids indicates a transition between fungus and insect associated lifestyles

“…It has been suggested that repeated replacement of symbionts among adelgids might be a consequence of periods with relaxed selection on symbiont functions due to different feeding behavior of adelgids on primary and secondary host trees -that is feeding on nutrient-rich parenchyma cells on spruce versus nutrient-poor phloem sap on alternate hosts -and multiple origins of hostalternating lifestyles(10). Annandia, the ancient symbiont of adelgids has lost many functions in essential amino acid biosynthesis, which could support this hypothesis(25), however the Vallotia -Profftia system does not follow this pattern.One of the most remarkable findings of our study is the evolutionary link between the betaproteobacterial insect symbiont, Vallotia, and endosymbionts of Rhizopus fungi supported by their close phylogenetic relationships and a high-level of genomic synteny. There are many possible scenarios that could explain the origin of these symbioses.…”

“…The only moderately reduced size and AT bias together with the low protein-coding density of the Vallotia and Profftia genomes was most similar to those of evolutionary young co-obligate partners of insects(63), for instance 'Ca. Pseudomonas adelgestsugas' in A. tsugae(25), Serratia symbiotica in Cinara cedri(64,65) and the Sodalis-like symbiont of Philaenus spumarius, the meadow spittlebug(66). However, compared to these systems involving a more ancient and a younger symbiont, similar genome characteristics of Vallotia and Profftia implied that both bacteria are evolutionarily recent symbionts in a phase of extensive gene inactivation typical for early stages of adaptation to an obligate host-restricted life-style(59,60).Although compared to their closest free-living relatives both Vallotia and Profftia have lost many genes in all functional categories, both retained a greater proportion of genes in translation-related functions and cell envelope biogenesis (Figures S2, S3).…”