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.

Fig. 1.

—Schematic of HWA immature showing locations of symbionts. Large red circles are bacteriocytes containing cells of the obligate “Ca. Annandia” symbiont, grouped into paired bacteriomes. Blue ovals represent cells of the obligate “Ca. Pseudomonas” symbiont free-living in the hemocoel. Yellow shapes are portions of central bacteriocytes containing the facultative “Ca. Serratia symbiotica” bacteria. “Ca. Pseudomonas” cells are not drawn to scale. (See von Dohlen et al. (2013) for more information.)

Fig. 2.

—Relationships between (A) genome size and GC content and (B) genome size and total gene number in HWA symbionts and other representative bacterial genomes.

Fig. 3.

—Conserved syntenic blocks and rearrangements between HWA symbiont genomes and relatives. Chromosomes are marked every 50 kb and inverted when appropriate for clarity. Ribbons between chromosomes indicate position and length of collinear syntenic blocks that are composed of a minimum of five genes. (A) Comparison of “Ca. Annandia adelgestsuga” to “Ca. Buchnera aphidicola” str. APS and “Ca. Ishikawaella capsulata”. (B) Comparison of “Ca. Pseudomonas adelgestsugas” and Pseudomonas aeruginosa PAO1.

Fig. 4.

—Reconstruction of essential amino acid pathways based on genes present in “Ca. Annandia adelgestsuga,” “Ca. Pseudomonas adelgestsugas,” and an unpublished transcriptome of HWA.

Fig. 5.

—Presence and absence of genes involved in central cellular processes related to protein synthesis and energy production for the symbionts of HWA, in comparison to representative obligate hemipteran symbionts. Colored boxes indicate gene presence. HWA symbionts are bolded and their gene boxes are outlined for clarity. Baumannia BGSS is “Ca. Baumannia cicadellinicola” str. BGSS, Ishikawaella is “Ca. Ishikawaella capsulata,” Buchnera APS and Cc are “Ca. Buchnera aphidicola” strains APS and Cc, respectively, Portiera is “Ca. Portiera aleyrodidarum,” Sulcia BGSS is “Ca. Sulcia muelleri” str. BGSS, and Carsonella is “Ca. Carsonella ruddii” from Heteropsylla texana. Genomes are arranged by size from largest (outer ring) to smallest (inner ring).

Fig. 6.

—Presence and absence of genes involved in central cellular processes related to bacterial replication, including DNA replication initiation, cell division, and synthesis of cell membrane components. For figure details, refer to legend for figure 5.