Sap-sucking insects typically engage in obligate relationships with symbiotic bacteria that play nutritional roles in synthesizing nutrients unavailable or in scarce supply from the plant-sap diets of their hosts. Adelgids are sap-sucking insects with complex life cycles that involve alternation between conifer tree species. While all adelgid species feed on spruce during the sexual phase of their life cycle, each adelgid species belongs to a major lineage that feeds on a distinct genus of conifers as their alternate host. Previous work on adelgid symbionts had discovered pairs of symbionts within each host species, and unusual diversity across the insect family, but left several open questions regarding the status of bacterial associates. Here, we explored the consistency of symbionts within and across adelgid lineages, and sought evidence for facultative vs. obligate symbiont status. Representative species were surveyed for symbionts using 16S ribosomal DNA gene sequencing, confirming that different symbiont pairs were consistently present within each major adelgid lineage. Several approaches were used to establish whether symbionts exhibited characteristics of long-term, obligate mutualists. Patterns of symbiont presence across adelgid species and diversification with host insects suggested obligate relationships. Fluorescent in situ hybridization and electron microscopy localized symbionts to bacteriocyte cells within the bacteriome of each species (with one previously known exception), and detection of symbionts in eggs indicated their vertical transmission. Common characteristics of long-term obligate symbionts, such as nucleotide compositional bias and pleomorphic symbiont cell shape were also observed. Superimposing microbial symbionts on the adelgid phylogeny revealed a dynamic pattern of symbiont gains and losses over a relatively short period of time compared to other symbionts associated with sap-sucking insects, with each adelgid species possessing an older, “senior” symbiont and a younger “junior” symbiont. A hypothesis relating adelgid life cycles to relaxed constraints on symbionts is proposed, with the degradation of senior symbionts and repeated acquisition of more junior symbionts creating opportunities for repeated colonization of new alternate-conifer hosts by adelgids.

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.

Many plant-sap-feeding insects have maintained a single, obligate, nutritional symbiont over the long history of their lineage. This senior symbiont may be joined by one or more junior symbionts that compensate for gaps in function incurred through genome-degradative forces. Adelgids are sap-sucking insects that feed solely on conifer trees and follow complex life cycles in which the diet fluctuates in nutrient levels. Adelgids are unusual in that both senior and junior symbionts appear to have been replaced repeatedly over their evolutionary history. Genomes can provide clues to understanding symbiont replacements, but only the dual symbionts of hemlock adelgids have been examined thus far. Here, we sequence and compare genomes of four additional dual-symbiont pairs in adelgids. We show that these symbionts are nutritional partners originating from diverse bacterial lineages and exhibiting wide variation in general genome characteristics. Although dual symbionts cooperate to produce nutrients, the balance of contributions varies widely across pairs, and total genome contents reflect a range of ages and degrees of degradation. Most symbionts appear to be in transitional states of genome reduction. Our findings support a hypothesis of periodic symbiont turnover driven by fluctuating selection for nutritional provisioning related to gains and losses of complex life cycles in their hosts.

The planthopper genus Chionomus Fennah, 1971 (Hemiptera: Fulgoroidea: Delphacidae) currently includes three Neotropical species, removed from the polyphyletic genus Delphacodes Fieber, 1866. Morphological and molecular evidence further redefine Chionomus to include ten additional species (eight species removed from Delphacodes, two described as new, viz. Chionomus dolonus n. sp. and C. herkos n. sp.), with another four species synonymized. Phylogenetic analyses of morphological and molecular sequence data of the mitochondrial gene Cytochrome Oxidase I provide support for the monophyly of Chionomus. We use a mixed model Bayesian optimality criterion to define phylogenetic relationships among Chionomus and support paraphyly of the original definition of Chionomus (with respect to Delphacodes) and monophyly of the revised genus.