Inducible pine rosin defense mediates interactions between an invasive insect-fungal complex and newly acquired sympatric fungal associates

Effects of Fusarium solani on the Growth and Development of Anoplophora glabripennis Larvae

Anoplophora glabripennis (Asian longhorned beetle) is a wood-boring pest that can inhabit a wide range of healthy deciduous host trees in native and non-native habitats. Lignocellulose degradation plays a major role in the acquisition of nutrients during the growth and development of A. glabripennis larvae. In this study, the lignocellulose degradation capacity of Fusarium solani, a fungal symbiont of A. glabripennis, was investigated in fermentation culture and in four host tree species. The impact of F. solani on larval growth and survival parameters was assessed. Fermentation culture demonstrated continuous and stable production of lignocellulolytic enzymes over the cultivation period. Furthermore, F. solani was able to degrade host tree lignocellulose, as shown by decreased soluble sugar and cellulose contents and an increase in protein content. No significant differences in larval survival were observed in larvae fed with or without F. solani. However, weight and head capsule width were higher in larvae fed on F. solani, and gut lignocellulose activities were elevated in fed larvae. Our results indicate a role for F. solani in the predigestion of lignocellulose during the colonization and parasitic stages of A. glabripennis larval development, and also the F. solani an important symbiotic partner to A. glabripennis, lowering barriers to colonization and development in a range of habitats.

Metabarcoding of mycetangia from the Dendroctonus frontalis species complex (Curculionidae: Scolytinae) reveals diverse and functionally redundant fungal assemblages

Dendroctonus-bark beetles are associated with microbes that can detoxify terpenes, degrade complex molecules, supplement and recycle nutrients, fix nitrogen, produce semiochemicals, and regulate ecological interactions between microbes. Females of some Dendroctonus species harbor microbes in specialized organs called mycetangia; yet little is known about the microbial diversity contained in these structures. Here, we use metabarcoding to characterize mycetangial fungi from beetle species in the Dendroctonus frontalis complex, and analyze variation in biodiversity of microbial assemblages between beetle species. Overall fungal diversity was represented by 4 phyla, 13 classes, 25 orders, 39 families, and 48 genera, including 33 filamentous fungi, and 15 yeasts. The most abundant genera were Entomocorticium, Candida, Ophiostoma-Sporothrix, Ogataea, Nakazawaea, Yamadazyma, Ceratocystiopsis, Grosmannia-Leptographium, Absidia, and Cyberlindnera. Analysis of α-diversity indicated that fungal assemblages of D. vitei showed the highest richness and diversity, whereas those associated with D. brevicomis and D. barberi had the lowest richness and diversity, respectively. Analysis of β-diversity showed clear differentiation in the assemblages associated with D. adjunctus, D. barberi, and D. brevicomis, but not between closely related species, including D. frontalis and D. mesoamericanus and D. mexicanus and D. vitei. A core mycobiome was not statistically identified; however, the genus Ceratocystiopsis was shared among seven beetle species. Interpretation of a tanglegram suggests evolutionary congruence between fungal assemblages and species of the D. frontalis complex. The presence of different amplicon sequence variants (ASVs) of the same genus in assemblages from species of the D. frontalis complex outlines the complexity of molecular networks, with the most complex assemblages identified from D. vitei, D. mesoamericanus, D. adjunctus, and D. frontalis. Analysis of functional variation of fungal assemblages indicated multiple trophic groupings, symbiotroph/saprotroph guilds represented with the highest frequency (∼31% of identified genera). These findings improve our knowledge about the diversity of mycetangial communities in species of the D. frontalis complex and suggest that minimal apparently specific assemblages are maintained and regulated within mycetangia.

Weathering of Antibacterial Melt-Spun Polyfilaments Modified by Pine Rosin

For many antibacterial polymer fibres, especially for those with natural functional additives, the antibacterial response might not last over time. Moreover, the mechanical performance of polymeric fibres degrades significantly during the intended operation, such as usage in textile and industrial filter applications. The degradation process and overall ageing can lead to emitted volatile organic compounds (VOCs). This work focused on the usage of pine rosin as natural antibacterial chemical and analysed the weathering of melt-spun polyethylene (PE) and poly lactic acid (PLA) polyfilaments. A selected copolymer surfactant, as an additional chemical, was studied to better integrate rosin with the molecular structure of the plastics. The results reveal that a high 20 w-% of rosin content can be obtained by surfactant addition in non-oriented PE and PLA melt-spun polyfilaments. According to the VOC analysis, interestingly, the total emissions from the melt-spun PE and PLA fibres were lower for rosin-modified (10 w-%) fibres and when analysed below 60 °C. The PE fibres of the polyfilaments were found to be clearly more durable in terms of the entire weathering study, i.e., five weeks of ultraviolet radiation, thermal ageing and standard washing. The antibacterial response against Gram-positive Staphylococcus aureus by the rosin-containing fibres was determined to be at the same level (decrease of 3–5 logs cfu/mL) as when using 1.0 w-% of commercial silver-containing antimicrobial. For the PE polyfilaments with rosin (10 w-%), full killing response (decrease of 3–5 logs cfu/mL) remained after four weeks of accelerated ageing at 60 °C.

An invasive beetle-fungus complex is maintained by fungal nutritional-compensation mediated by bacterial volatiles

Mutualisms between symbiotic microbes and animals have been well documented, and nutritional relationships provide the foundation for maintaining beneficial associations. The well-studied mutualism between bark beetles and their fungi has become a classic model system in the study of symbioses. Despite the nutritional competition between bark beetles and beneficial fungi in the same niche due to poor nutritional feeding substrates, bark beetles still maintain mutualistic associations with beneficial fungi over time. The mechanism behind this phenomenon, however, remains largely unknown. Here, we demonstrated the bark beetle Dendroctonus valens LeConte relies on the symbiotic bacterial volatile ammonia, as a nitrogen source, to regulate carbohydrate metabolism of its mutualistic fungus Leptographium procerum to alleviate nutritional competition, thereby maintaining the stability of the bark beetle–fungus mutualism. Ammonia significantly reduces competition of L. procerum for carbon resources for D. valens larval growth and increases fungal growth. Using stable isotope analysis, we show the fungus breakdown of phloem starch into d-glucose by switching on amylase genes only in the presence of ammonia. Deletion of amylase genes interferes with the conversion of starch to glucose. The acceleration of carbohydrate consumption and the conversion of starch into glucose benefit this invasive beetle–fungus complex. The nutrient consumption–compensation strategy mediated by tripartite beetle–fungus–bacterium aids the maintenance of this invasive mutualism under limited nutritional conditions, exacerbating its invasiveness with this competitive nutritional edge.

High-Resolution Profiling of Gut Bacterial Communities in an Invasive Beetle using PacBio SMRT Sequencing System

Dendroctonus valens, an invasive bark beetle, has caused severe damage to Chinese forests. Previous studies have highlighted the importance of the gut microbiota and its fundamental role in host fitness. Culture-dependent and culture-independent methods have been applied in analyzing beetles’ gut microbiota. The former method cannot present a whole picture of the community, and the latter mostly generates short read lengths that cannot be assigned to species. Here, the PacBio sequencing system was utilized to capture full-length 16S rRNA sequences in D. valens gut throughout its ontogeny. A total of eight phyla, 55 families, 102 genera, and 253 species were identified. Bacterial communities in colonized beetles have the greatest richness but the lowest evenness in all life stages, which is different from those in young larvae. Pseudomonas sp., Serratia liquefaciens possess high abundance throughout its ontogeny and may serve as members of the core bacteriome. A phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) analysis predicted that gut microbiota in larvae are rich in genes involved in carbohydrate, energy metabolism. Gut microbiota in both larvae and colonized beetles are rich in xenobiotics and terpenoids biodegradation, which are decreased in dispersal beetles. Considering that the results are based mainly on the analysis of 16S rRNA sequencing and PICRUSt prediction, further confirmation is needed to improve the knowledge of the gut microbiota in D. valens and help to resolve taxonomic uncertainty at the species level.

Comparative analysis of the immune system of an invasive bark beetle, Dendroctonus valens, infected by an entomopathogenic fungus

Dendroctonus valens LeConte is one of the most economically important forest pest in China. Leptographium procerum, a mutualistic fungus can assist the host beetle in overcoming the pine's chemical defenses, and Beauveria bassiana, an entomopathogenic fungus has shown high beetle killing efficiency. Considering that the D. valens immune system remains unknown at the genomic level, a mutualistic and antagonistic fungus associated with the beetle provides an ideal model for studying immune interactions between the insect and associated fungi. Here, B. bassiana killed most tested larvae more effectively than L. procerum and Tween. The entomopathogenic fungus provoked stronger responses than the symbiotic fungus at the transcriptome level. We identified 185 immunity-related genes, including pattern recognition receptors, signal modulators, members of immune pathways (Toll, IMD, and JAK/STAT), and immune effectors. Quantitative real-time PCR analysis confirmed that several recognition receptors and effector genes were activated at 1 or 2 days post infection, while the effector genes were suppressed at 4 days post infection by B. bassiana, respectively. In contrast, effector genes were upregulated in response to L. procerum. Together, this study provides a comprehensive sequence resource and insight into the D. valens immune system and lays a basis for understanding the molecular aspects of the interaction between the host and associated fungi.

Gut Bacterial Communities of Dendroctonus valens and Monoterpenes and Carbohydrates of Pinus tabuliformis at Different Attack Densities to Host Pines

Insects harbor a community of gut bacteria, ranging from pathogenic to obligate mutualistic organisms. Both biotic and abiotic factors can influence species composition and structure of the insect gut bacterial communities. Dendroctonus valens is a destructive forest pest in China. To overcome host pine defenses, beetles mass-attack the pine to a threshold density that can exhaust pine defenses. The intensity of pine chemical defenses and carbohydrate concentrations of pines can be influenced by beetle attack, both of which are known factors that modify beetle's gut microbiota. However, little is known to what extent variation exists in the beetle's gut communities, and host monoterpenes and carbohydrates at different attack densities. In this study, the gut bacterial microbiota of D. valens at low and high attack densities were analyzed, and monoterpenes and carbohydrates in host pine phloem were assayed in parallel. The results showed that no significant changes of gut bacterial communities of the beetles and concentrations of D-glucose, D-pinitol, and D-fructose in pine phloem were found between low and high attack densities. The concentrations of α-pinene, β-pinene, limonene at high attack densities were significantly higher than those at low attack densities. Our results suggested that different attack densities of D. valens influence monoterpenes concentration of host pines' phloem but have no significant impact on gut bacterial community structures of D. valens and carbohydrate concentration of host trees' phloem in early attack phase. Similar gut bacterial community structures of D. valens between low and high attack densities might be due to the quick adaptation of gut microbiota to high monoterpenes concentrations.

Effect of Oxygen on Verbenone Conversion From cis-Verbenol by Gut Facultative Anaerobes of Dendroctonus valens

Since its introduction from North America, Dendroctonus valens LeConte has become a destructive forest pest in China. Although gut aerobic bacteria have been investigated and some are implicated in beetle pheromone production, little is known about the abundance and significance of facultative anaerobic bacteria in beetle gut, especially with regards to effects of oxygen on their role in pheromone production. In this study, we isolated and identified gut bacteria of D. valens adults in an anaerobic environment, and further compared their ability to convert cis-verbenol into verbenone (a multi-functional pheromone of D. valens) under different O₂ concentrations. Pantoea conspicua, Enterobacter xiangfangensis, Staphylococcus warneri were the most frequently isolated species among the total of 10 species identified from beetle gut in anaerobic conditions. Among all isolated species, nine were capable of cis-verbenol to verbenone conversion, and the conversion efficiency increased with increased oxygen concentration. This O₂-mediated conversion of cis-verbenol to verbenone suggests that gut facultative anaerobes of D. valens might play an important role in the frass, where there is higher exposure to oxygen, hence the higher verbenone production. This claim is further supported by distinctly differential oxygen concentrations between gut and frass of D. valens females.

Bacterial volatile ammonia regulates the consumption sequence of d-pinitol and d-glucose in a fungus associated with an invasive bark beetle

Interactions among microbial symbionts have multiple roles in the maintenance of insect-microbe symbiosis. However, signals mediating microbial interactions have been scarcely studied. In the classical model system of bark beetles and fungal associates, fungi increase the fitness of insects. However, not all interactions are mutualistic, some of these fungal symbionts compete for sugars with beetle larvae. How this antagonistic effect is alleviated is unknown, and recent research suggests potential roles of bacterial symbionts. Red turpentine beetle (RTB), Dendroctonus valens LeConte, is an invasive pest in China, and it leads to wide spread, catastrophic mortality to Chinese pines. In the symbiotic system formed by RTB, fungi and bacteria, volatiles from predominant bacteria regulate the consumption sequence of carbon sources d-pinitol and d-glucose in the fungal symbiont Leptographium procerum, and appear to alleviate the antagonistic effect from the fungus against RTB larvae. However, active components of these volatiles are unknown. We detected 67 volatiles by Gas Chromatography-Mass Spectrometer (GC-MS). Seven of them were identified as candidate chemicals mediating bacteria-fungus interactions, among which ammonia made L. procerum consume its secondary carbon source D-pinitol instead of its preferred carbohydrate D-glucose. In conclusion, ammonia regulated the consumption sequence of these two carbon sources in the fungal symbiont.

Sexual variation of bacterial microbiota of Dendroctonus valens guts and frass in relation to verbenone production

Gut microbiota are widely involved in insect biology, and many factors can influence the microbiota in guts and frass. Dendroctonus valens is a very destructive forest pest in China, and the mass-attacking behavior is regulated by several semiochemicals, including verbenone, a multifunctional pheromone. The beetle harbors a variety of bacteria in its guts and frass and some of them are capable of verbenone production. D. valens is characterized by monogamy and female-initiated attacking behavior. Whether the bacterial communities fluctuate according to sex, and whether the variation influences the verbenone production, remains to be determined. In this study, the bacterial microbiota in D. valens guts and frass were analyzed, and verbenone production by their crude bacterial suspensions was compared in vitro. Bacterial diversity in female frass is more abundant compared to male frass, and the percentages and total amounts of main genera like Lactococcus and Pseudomonas in female frass are significantly higher than those in male frass. The verbenone produced by the female frass suspension is significantly higher than male frass. This study presents a comprehensive comparison of bacterial communities in guts and frass between both sexes of D. valens, highlighting the potential significance of female frass microbiota in verbenone production.

Pine Defensive Monoterpene α-Pinene Influences the Feeding Behavior of Dendroctonus valens and Its Gut Bacterial Community Structure

The exposure to plant defense chemicals has negative effects on insect feeding activity and modifies insect gut microbial community composition. Dendroctonus valens is a very destructive forest pest in China, and harbors a large diversity and abundance of gut microorganisms. Host pine defensive chemicals can protect the pines from attack by the holobiont. In this study, boring length of D. valens feeding on 0 mg/g α-pinene and 9 mg/g α-pinene concentration in phloem media for 6 and 48 h were recorded, and their gut bacterial communities were analyzed in parallel. Nine milligram per gram α-pinene concentration significantly inhibited boring length of D. valens and altered its gut microbial community structure after 6 h. The inhibition of boring length from 9 mg/g α-pinene in diets ceased after 48 h. No significant differences of the bacterial communities were observed between the beetles in 0 and 9 mg/g α-pinene concentration in phloem media after 48 h. Our results showed that the inhibition of the feeding behavior of D. valens and the disturbance to its gut bacterial communities in 9 mg/g α-pinene concentration in phloem media after 6 h were eliminated after 48 h. The resilience of gut bacterial community of D. valens may help the beetle catabolize pine defense chemical.

Does cryptic microbiota mitigate pine resistance to an invasive beetle-fungus complex? Implications for invasion potential

Microbial symbionts are known to assist exotic pests in their colonization of new host plants. However, there has been little evidence linking symbiotic invasion success to mechanisms for mitigation of native plant resistance. The red turpentine beetle (RTB) was introduced with a fungus, Leptographium procerum, to China from the United States and became a destructively invasive symbiotic complex in natural Pinus tabuliformis forests. Here, we report that three Chinese-resident fungi, newly acquired by RTB in China, induce high levels of a phenolic defensive chemical, naringenin, in pines. This invasive beetle-fungus complex is suppressed by elevated levels of naringenin. However, cryptic microbiotas in RTB galleries strongly degrade naringenin, and pinitol, the main soluble carbohydrate of P. tabuliformis, is retained in L. procerum-infected phloem and facilitate naringenin biodegradation by the microbiotas. These results demonstrate that cryptic microbiota mitigates native host plant phenolic resistance to an invasive symbiotic complex, suggesting a putative mechanism for reduced biotic resistance to symbiotic invasion.

Inducible pine rosin defense mediates interactions between an invasive insect-fungal complex and newly acquired sympatric fungal associates

Mutualism between insects and fungi drives insect evolutionary diversification and niche expansion; for invasive insects, however, mechanisms by which they maintain mutualistic relationships with beneficial fungi have not been clearly explored. Here, we report that an invasive herbivorous insect, the red turpentine beetle (RTB), with its co-invasive mutualistic fungus, Leptographium procerum, has newly acquired a set of sympatric fungi during invasion, which could potentially outcompete the RTB mutualistic fungus. Host pine Pinus tabuliformis exhibited more rosin-based responses to the sympatric fungi than to RTB mutualistic fungus and, in return, the rapidly induced rosin suppressed the sympatric fungi more significantly than L. procerum. In addition, from direct fungal pairing competitions, we found that the antagonistic effects of sympatric fungi on L. procerum were drastically reduced under induced rosin defense. Our results together with previous findings imply that pine oleoresin defense (turpentine and rosin) might have been exploited by the invasive mutualistic fungus L. procerum, which helps to explain its invasion success and, by extension, its mutualistic partner RTB in China.