Bark Beetles Utilize Ophiostomatoid Fungi to Circumvent Host Tree Defenses

Bark beetles maintain symbiotic associations with a diversity of microbial organisms, including ophiostomatoid fungi. Studies have frequently reported the role of ophiostomatoid fungi in bark beetle biology, but how fungal symbionts interact with host chemical defenses over time is needed. We first investigated how inoculations by three fungal symbionts of mountain pine beetle affect the terpene chemistry of live lodgepole pine trees. We then conducted a complimentary laboratory experiment specifically measuring the host metabolite degradation by fungi and collected the fungal organic volatiles following inoculations with the same fungal species on lodgepole pine logs. In both experiments, we analyzed the infected tissues for their terpene chemistry. Additionally, we conducted an olfactometer assay to determine whether adult beetles respond to the volatile organic chemicals emitted from each of the three fungal species. We found that all fungi upregulated terpenes as early as two weeks after inoculations. Similarly, oxygenated monoterpene concentrations also increased by several folds (only in logs). A large majority of beetles tested showed a strong attraction to two fungal species, whereas the other fungus repelled the beetles. Together this study shows that fungal symbionts can alter host defense chemistry, assist beetles in overcoming metabolite toxicity, and provide possible chemical cues for bark beetle attraction.

Host Defense Metabolites Alter the Interactions between a Bark Beetle and its Symbiotic Fungi

Successful host plant colonization by tree-killing bark beetle-symbiotic fungal complexes depends on host suitability, which is largely determined by host defense metabolites such as monoterpenes. Studies have shown the ability of specific blends of host monoterpenes to influence bark beetles or their fungal symbionts, but how biologically relevant blends of host monoterpenes influence bark beetle-symbiotic fungal interaction is unknown. We tested how interactions between two host species (lodgepole pine or jack pine) and two fungal symbionts of mountain pine beetle (Grosmannia clavigera or Ophiostoma montium) affect the performance of adult female beetles in vitro. Beetles treated with the propagules of G. clavigera or O. montium or not treated (natural fungal load) were introduced into media amended with a blend of the entire monoterpene profile of either host species and beetle performance was compared. Overall, host blends altered beetle performance depending on the fungal species used in the beetle amendment. When beetles were amended with G. clavigera, their performance was superior over beetles amended with O. montium in either host blend. Furthermore, G. clavigera-amended beetles performed better in media amended with host blends than without a host blend; in contrast, O. montium-amended beetles performed better in media without a host blend than with a host blend. Overall, this study showed that host defense metabolites affect host suitability to bark beetles through influencing their fungal symbionts and that different species of fungal symbionts respond differentlly to host defense metabolites.

Aspen Leaves as a "Chemical Landscape" for Fungal Endophyte Diversity-Effects of Nitrogen Addition

Abiotic and biotic factors may shape the mycobiome communities in plants directly but also indirectly by modifying the quality of host plants as a substrate. We hypothesized that nitrogen fertilization (N) would determine the quality of aspen (Populus tremula) leaves as a substrate for the endophytic fungi, and that by subjecting the plants to N, we could manipulate the concentrations of positive (nutritious) and negative (antifungal) chemicals in leaves, thus changing the internal “chemical landscape” for the fungi. We expected that this would lead to changes in the fungal community composition, in line with the predictions of heterogeneity–diversity relationship and resource availability hypotheses. To test this, we conducted a greenhouse study where aspen plants were subjected to N treatment. The chemical status of the leaves was confirmed using GC/MS (114 metabolites, including amino acids and sugars), LC/MS (11 phenolics), and UV-spectrometry (antifungal condensed tannins, CTs), and the endophytic communities were characterized using culture-dependent sequencing. We found that N treatment reduced foliar concentrations of CT precursor catechin but not that of CTs. Nitrogen treatment also increased the concentrations of the amino acids and reduced the concentration of some sugars. We introduced beetle herbivores (H) as a second treatment but found no rapid changes in chemical traits nor strong effect on the diversity of endophytes induced by herbivores. A few rare fungi were associated with and potentially vectored by the beetle herbivores. Our findings indicate that in a controlled environment, the externally induced changes did not strongly alter endophyte diversity in aspen leaves.

Mutualistic Ophiostomatoid Fungi Equally Benefit from Both a Bark Beetle Pheromone and Host Tree Volatiles as Nutrient Sources

Interactions between mutualistic bark beetles and ophiostomatoid fungi have received considerable attention in recent years. Studies have shown how volatile organic compounds emitted from mutualist fungi affect the behaviors of several bark beetle species. However, we currently lack sufficient knowledge regarding whether bark beetle pheromones can influence mutualist fungi. Here, we measured growth and biomass of two mutualistic fungi of the mountain pine beetle in response to headspace of a beetle pheromone (trans-verbenol), a blend of host tree volatiles, the combination of both, or control (no volatile source) in vitro experiments consisting of a nitrogen-based medium. The surface area and ergosterol content of the mycelia were used as surrogates for fungal growth and biomass respectively. We found that both growth and biomass of Grosmannia clavigera and Ophiostoma montium were greater in medium exposed to any type of volatile sources than the control. While growth and ergosterol content of G. clavigera were highest in the combination treatment, there were no differences in growth or biomass among the types of volatiles introduced for O. montium. These results suggest that both mutualistic fungi can utilize both bark beetle pheromone and host tree volatiles as nutrient sources. Overall, these results support the on-going studies on the role of volatile organic compounds mediating mutualistic bark beetle-fungi interactions.

Individual Sick Fir Tree (Abies mariesii) Identification in Insect Infested Forests by Means of UAV Images and Deep Learning

Insect outbreaks are a recurrent natural phenomenon in forest ecosystems expected to increase due to climate change. Recent advances in Unmanned Aerial Vehicles (UAV) and Deep Learning (DL) Networks provide us with tools to monitor them. In this study we used nine orthomosaics and normalized Digital Surface Models (nDSM) to detect and classify healthy and sick Maries fir trees as well as deciduous trees. This study aims at automatically classifying treetops by means of a novel computer vision treetops detection algorithm and the adaptation of existing DL architectures. Considering detection alone, the accuracy results showed 85.70% success. In terms of detection and classification, we were able to detect/classify correctly 78.59% of all tree classes (39.64% for sick fir). However, with data augmentation, detection/classification percentage of the sick fir class rose to 73.01% at the cost of the result accuracy of all tree classes that dropped 63.57%. The implementation of UAV, computer vision and DL techniques contribute to the development of a new approach to evaluate the impact of insect outbreaks in forest.

Exposure to Fungal Volatiles Can Influence Volatile Emissions From Other Ophiostomatoid Fungi

Fungal volatile organic compounds (FVOCs) can act as intra- and inter-kingdom communication signals that influence the growth and behaviors of organisms involved in antagonistic or mutualistic relationships with fungi. There is growing evidence suggesting that FVOCs can mediate interactions between organisms within and across different ecological niches. Bark beetles have established mutualistic relationships with ophiostomatoid fungi which can serve as a food source and condition host plant tissues for developing beetle larvae. While the profiles (both composition and concentrations) of volatile emission from ophiostomatoid fungi can be influenced by abiotic factors, whether emissions from a given fungal species can be influenced by those from another is still unknown. Here, we analyzed FVOCs emitted from the two ophiostomatoid fungi, Grosmannia clavigera and Ophiostoma ips, associated with mountain pine beetle and pine engraver beetle, respectively, when each fungus was growing alone or in a shared headspace. We used two isolates of each fungus species. Overall, we detected a total of eight volatiles in both G. clavigera alone or in combination with O. ips including acetoin, ethyl acetate, cis-grandisol, isoamyl alcohol, isobutanol, 2-methyl-1-butanol, phenethyl acetate, and phenethyl alcohol. The profiles of volatiles emitted differed between the two fungal species but not between the two isolates of the same fungus. Six compounds were common between the species, whereas two compounds were detected only when G. clavigera was present. Moreover, the majority of volatiles were detected less frequently and at lower concentrations when the two fungi were grown together in a shared headspace. These results are likely due to reduced volatile emissions from O. ips in the presence of G. clavigera. However, changes in the profiles of fungal volatiles did not correspond with the observed changes in the growth of either species. Overall, these results suggest that the similarities in fungal volatiles among different species of fungi may reflect a common ecological niche and that the differences may correspond to species-specific adaptation to their respective host beetles or genetic factors.

Pathophysiological responses of pine defensive metabolites largely lack differences between pine species but vary with eliciting ophiostomatoid fungal species

Phytopathogenic ophiostomatoid fungi are common associates of bark beetles and contribute to beetle-associated mortality of trees. Mountain pine beetle outbreaks in Canada are facilitating novel associations between its vectored fungi (Grosmannia clavigera, Leptographium longiclavatum and Ophiostoma montium) and jack pine. How the induced defense-related metabolite responses of jack and lodgepole pines vary in response to the fungi is unknown. Understanding this variation is important to clarifying pine susceptibility to and the physiological impacts of infection. We used a comparative metabolite profiling approach to investigate the defense-related signaling, carbon utilization/mobilization, and synthesis responses of both pines to the fungi. Both pine species largely exhibited similar metabolite responses to the fungi. The magnitude of pine metabolite responses positively reflected pathogen virulence. Our findings indicate that pines can recognize and metabolomically respond to novel pathogens, likely due to signals common between the novel fungi and fungi coevolved with the pine. Thus, jack pine is likely as susceptible as lodgepole pine to infections by each of the MPB-vectored fungi. Furthermore, the magnitude of the metabolite responses of both pines varied by the eliciting fungal species, with the most virulent pathogen causing the greatest reduction in carbohydrates and the highest accumulation of defensive terpenes.

The lodgepole × jack pine hybrid zone in Alberta, Canada: a stepping stone for the mountain pine beetle on its journey East across the boreal forest?

Historical data show that outbreaks of the tree killing mountain pine beetle are often preceded by periods of drought. Global climate change impacts drought frequency and severity and is implicated in the range expansion of the mountain pine beetle into formerly unsuitable habitats. Its expanded range has recently reached the lodgepole × jack pine hybrid zone in central Alberta, Canada, which could act as a transition from its historical lodgepole pine host to a jack pine host present in the boreal forest. This field study tested the effects of water limitation on chemical defenses of mature trees against mountain pine beetle-associated microorganisms and on beetle brood success in lodgepole × jack pine hybrid trees. Tree chemical defenses as measured by monoterpene emission from tree boles and monoterpene concentration in needles were greater in trees that experienced water deficit compared to well-watered trees. Myrcene was identified as specific defensive compound, since it significantly increased upon inoculation with dead mountain pine beetles. Beetles reared in bolts from trees that experienced water deficit emerged with a higher fat content, demonstrating for the first time experimentally that drought conditions benefit mountain pine beetles. Further, our study demonstrated that volatile chemical emission from tree boles and phloem chemistry place the hybrid tree chemotype in-between lodgepole pine and jack pine, which might facilitate the host shift from lodgepole pine to jack pine.

Effect of water stress and fungal inoculation on monoterpene emission from an historical and a new pine host of the mountain pine beetle

The mountain pine beetle (Dendroctonus ponderosae, MPB) has killed millions of lodgepole pine (Pinus contorta) trees in Western Canada, and recent range expansion has resulted in attack of jack pine (Pinus banksiana) in Alberta. Establishment of MPB in the Boreal forest will require use of jack pine under a suite of environmental conditions different from those it typically encounters in its native range. Lodgepole and jack pine seedlings were grown under controlled environment conditions and subjected to either water deficit or well watered conditions and inoculated with Grosmannia clavigera, a MPB fungal associate. Soil water content, photosynthesis, stomatal conductance, and emission of volatile organic compounds (VOCs) were monitored over the duration of the six-week study. Monoterpene content of bark and needle tissue was measured at the end of the experiment. β-Phellandrene, the major monoterpene in lodgepole pine, was almost completely lacking in the volatile emission profile of jack pine. The major compound in jack pine was α-pinene. The emission of both compounds was positively correlated with stomatal conductance. 3-Carene was emitted at a high concentration from jack pine seedlings, which is in contrast to monoterpene profiles of jack pine from more southern and eastern parts of its range. Fungal inoculation caused a significant increase in total monoterpene emission in water deficit lodgepole pine seedlings right after its application. By 4 weeks into the experiment, water deficit seedlings of both species released significantly lower levels of total monoterpenes than well watered seedlings. Needle tissue contained lower total monoterpene content than bark. Generally, monoterpene tissue content increased over time independent from any treatment. The results suggest that monoterpenes that play a role in pine-MPB interactions differ between lodgepole and jack pine, and also that they are affected by water availability.

Mountain pine beetle host-range expansion threatens the boreal forest

The current epidemic of the mountain pine beetle (MPB), an indigenous pest of western North American pine, has resulted in significant losses of lodgepole pine. The leading edge has reached Alberta where forest composition shifts from lodgepole to jack pine through a hybrid zone. The susceptibility of jack pine to MPB is a major concern, but there has been no evidence of host-range expansion, in part due to the difficulty in distinguishing the parentals and their hybrids. We tested the utility of a panel of microsatellite loci optimized for both species to classify lodgepole pine, jack pine and their hybrids using simulated data. We were able to accurately classify simulated individuals, and hence applied these markers to identify the ancestry of attacked trees. Here we show for the first time successful MPB attack in natural jack pine stands at the leading edge of the epidemic. This once unsuitable habitat is now a novel environment for MPB to exploit, a potential risk which could be exacerbated by further climate change. The consequences of host-range expansion for the vast boreal ecosystem could be significant.