Variable organization of symbiont-containing tissue across planthoppers hosting different heritable endosymbionts

Functional and Compositional Changes in Sirex noctilio Gut Microbiome in Different Habitats: Unraveling the Complexity of Invasive Adaptation

The mutualistic symbiosis relationship between the gut microbiome and their insect hosts has attracted much scientific attention. The native woodwasp, Sirex nitobei, and the invasive European woodwasp, Sirex noctilio, are two pests that infest pines in northeastern China. Following its encounter with the native species, however, there is a lack of research on whether the gut microbiome of S. noctilio changed, what causes contributed to these alterations, and whether these changes were more conducive to invasive colonization. We used high-throughput and metatranscriptomic sequencing to investigate S. noctilio larval gut and frass from four sites where only S. noctilio and both two Sirex species and investigated the effects of environmental factors, biological interactions, and ecological processes on S. noctilio gut microbial community assembly. Amplicon sequencing of two Sirex species revealed differential patterns of bacterial and fungal composition and functional prediction. S. noctilio larval gut bacterial and fungal diversity was essentially higher in coexistence sites than in separate existence sites, and most of the larval gut bacterial and fungal community functional predictions were significantly different as well. Moreover, temperature and precipitation positively correlate with most of the highly abundant bacterial and fungal genera. Source-tracking analysis showed that S. noctilio larvae at coexistence sites remain dependent on adult gut transmission (vertical transmission) or recruitment to frass (horizontal transmission). Meanwhile, stochastic processes of drift and dispersal limitation also have important impacts on the assembly of S. noctilio larval gut microbiome, especially at coexistence sites. In summary, our results reveal the potential role of changes in S. noctilio larval gut microbiome in the successful colonization and better adaptation of the environment.

The nutritional dimension of facultative bacterial symbiosis in aphids: Current status and methodological considerations for future research

Aphids are valuable models for studying the functional diversity of bacterial symbiosis in insects. In addition to their ancestral obligate nutritional symbiont Buchnera aphidicola, these insects can host a myriad of so-called facultative symbionts. The diversity of these heritable bacterial associates is now well known, and some of the ecologically important traits associated with them have been well documented. Some twenty years ago, it was suggested that facultative symbionts could play an important role in aphid nutrition, notably by improving feeding performance on specific host plants, thus influencing the adaptation of these insects to host plants. However, the underlying mechanisms have never been elucidated, and the nutritional role that facultative symbionts might perform in aphids remains enigmatic. In this opinion piece, I put forward a series of arguments in support of the hypothesis that facultative symbionts play a central role in aphid nutrition and emphasize methodological considerations for testing this hypothesis in future work. In particular, I hypothesize that the metabolic capacities of B. aphidicola alone may not always be able to counterbalance the nutritional deficiencies of phloem sap. The association with one or several facultative symbionts with extensive metabolic capabilities would then be necessary to buffer the insect from host plant-derived nutrient deficiencies, thus enabling it to gain access to certain host plants.

Nutrient supplementation by genome-eroded Burkholderia symbionts of scale insects

Hemipterans are known as hosts to bacterial or fungal symbionts that supplement their unbalanced diet with essential nutrients. Among them, scale insects (Coccomorpha) are characterized by a particularly large diversity of symbiotic systems. Here, using microscopic and genomic approaches, we functionally characterized the symbionts of two scale insects belonging to the Eriococcidae family, Acanthococcus aceris and Gossyparia spuria. These species host Burkholderia bacteria that are localized in the cytoplasm of the fat body cells. Metagenome sequencing revealed very similar and highly reduced genomes (<900KBp) with a low GC content (~38%), making them the smallest and most AT-biased Burkholderia genomes yet sequenced. In their eroded genomes, both symbionts retain biosynthetic pathways for the essential amino acids leucine, isoleucine, valine, threonine, lysine, arginine, histidine, phenylalanine, and precursors for the semi-essential amino acid tyrosine, as well as the cobalamin-dependent methionine synthase MetH. A tryptophan biosynthesis pathway is conserved in the symbiont of G. spuria, but appeared pseudogenized in A. aceris, suggesting differential availability of tryptophan in the two host species' diets. In addition to the pathways for essential amino acid biosynthesis, both symbionts maintain biosynthetic pathways for multiple cofactors, including riboflavin, cobalamin, thiamine, and folate. The localization of Burkholderia symbionts and their genome traits indicate that the symbiosis between Burkholderia and eriococcids is younger than other hemipteran symbioses, but is functionally convergent. Our results add to the emerging picture of dynamic symbiont replacements in sap-sucking Hemiptera and highlight Burkholderia as widespread and versatile intra- and extracellular symbionts of animals, plants, and fungi.

Colonization Resistance of Symbionts in Their Insect Hosts

The symbiotic microbiome is critical in promoting insect resistance against colonization by exogenous microorganisms. The mechanisms by which symbionts contribute to the host's immune capacity is referred to as colonization resistance. Symbionts can protect insects from exogenous pathogens through a variety of mechanisms, including upregulating the expression of host immune-related genes, producing antimicrobial substances, and competitively excluding pathogens. Concordantly, insects have evolved fine-tuned regulatory mechanisms to avoid overactive immune responses against symbionts or specialized cells to harbor symbionts. Alternatively, some symbionts have evolved special adaptations, such as the formation of biofilms to increase their tolerance to host immune responses. Here, we provide a review of the mechanisms about colonization resistance of symbionts in their insect hosts. Adaptations of symbionts and their insect hosts that may maintain such symbiotic relationships, and the significance of such relationships in the coevolution of symbiotic systems are also discussed to provide insights into the in-depth study of the contribution of symbionts to host physiology and behavior.

Phylogeny and diversification of planthoppers (Hemiptera Fulgoromorpha) based on a comprehensive molecular dataset and large taxon sampling

Our understanding of the evolution of Fulgoromorpha (Insects, Hemiptera) has relied on molecular studies that have only considered either a limited number of taxa where all the families were not represented simultaneously, or a reduced number of genes.The absence of a global analysis comparing all the available data has thus led to significant biases in the analyzes, as evidenced by the incongruence of the results reported for planthopper phylogeny. Here we provide a phylogenetic and dating analysis of the Fulgoromorpha with a large sampling of 531 ingroup taxa, representing about 80% of the currently described suprageneric taxonomic diversity in this group. This study is based on most of the molecular sequences available to date and duly verified, for a set of nuclear and mitochondrial genes from a taxonomic sampling as complete as possible. The most significant results of our study are: (1) the unexpected paraphyly of Delphacidae whose Protodelphacida seem more related to Cixiidae than to other Delphacidae;(2) the group Meenoplidae-Kinnaridae recovered sister to the remaining Fulgoroidea families; (3) the early branching node of Tettigometridae sister of all the other families;(4) the Achilidae-Derbidae clade with Achilidae Plectoderini including Achilixiidae recovered as monophyletic as well as theFulgoridae-Dictyopharidae clade; and (5) the Tropiduchidae placed sister to the other so called 'higher' families (sec. Shcherbakov, 2006).Our divergence times analysis, calibrated with a set of duly verified fossils, suggests that the first diversification of planthoppers occurred in the Early Triassic around 240 Mya and those of the superfamilies Delphacoidea and Fulgoroidea in the Middle-Late Triassic around 210 Mya and 230 Mya, respectively. By the end of the Jurassic, all major planthopper lineages were originated, and all families, around 125 Mya, might havebeen driven in their distribution and evolution (in their first subfamilial divisions) by the geographical constraints of the Gondwanan break-up.Rapid evolutionary radiations occurred particularly in Fulgoridae around 125-130 Mya. Our results stress the importance of the good quality of the sequences used in the molecular analyzes and the primordial importance of a large sampling when analyzing the phylogeny of the group.