Geosmithia associated with bark beetles and woodborers in the western USA: taxonomic diversity and vector specificity

Fidelity or love the one you're with? Biotic complexity and tradeoffs can drive strategy and specificity in beetle-fungus by-product mutualisms

By-product mutualisms are ubiquitous yet seldom considered in models of mutualism. Most models represent conditional mutualisms that shift between mutualism and antagonism in response to shifts in costs and benefits resulting from changes in environmental quality. However, in by-product mutualisms, benefits arise as a part of normal life processes that may be costly to produce but incur little-to-no additional costs in response to the interaction. Without costs associated with the interaction, they do not have antagonistic alternate states. Here, we present a conceptual model that differs from traditional conditional models in three ways: (1) partners exchange by-product benefits, (2) interactions do not have alternate antagonistic states, and (3) tradeoffs are allowed among factors that influence environmental quality (rather than all factors that contribute to environmental quality being combined into a single gradient ranging from high to low). We applied this model to bark and ambrosia beetles (Curculionidae: Scolytinae), a diverse group that associates with fungi and that has repeatedly developed two distinct pathways to by-product mutualism. We used independent axes for each major factor influencing environmental quality in these systems, including those that exhibit tradeoffs (tree defense and nutritional quality). For these symbioses, tradeoffs in these two factors are key to which mutualism pathway is taken.

Profiling Walnut Fungal Pathobiome Associated with Walnut Dieback Using Community-Targeted DNA Metabarcoding

Walnut dieback can be caused by several fungal pathogenic species, which are associated with symptoms ranging from branch dieback to fruit necrosis and blight, challenging the one pathogen-one disease concept. Therefore, an accurate and extensive description of the walnut fungal pathobiome is crucial. To this end, DNA metabarcoding represents a powerful approach provided that bioinformatic pipelines are evaluated to avoid misinterpretation. In this context, this study aimed to determine (i) the performance of five primer pairs targeting the ITS region in amplifying genera of interest and estimating their relative abundance based on mock communities and (ii) the degree of taxonomic resolution using phylogenetic trees. Furthermore, our pipelines were also applied to DNA sequences from symptomatic walnut husks and twigs. Overall, our results showed that the ITS2 region was a better barcode than ITS1 and ITS, resulting in significantly higher sensitivity and/or similarity of composition values. The ITS3/ITS4_KYO1 primer set allowed to cover a wider range of fungal diversity, compared to the other primer sets also targeting the ITS2 region, namely, GTAA and GTAAm. Adding an extraction step to the ITS2 sequence influenced both positively and negatively the taxonomic resolution at the genus and species level, depending on the primer pair considered. Taken together, these results suggested that Kyo set without ITS2 extraction was the best pipeline to assess the broadest fungal diversity, with a more accurate taxonomic assignment, in walnut organs with dieback symptoms.

Thousand Cankers Disease in Walnut Trees in Europe: Current Status and Management

Thousand cankers disease (TCD) is a new deadly disease in walnut trees (Juglans spp.), which is plaguing commercial plantations, natural groves, and ornamental black walnut trees (Juglans nigra) in their native and invasion areas in the US and, more recently, in artificial plantations and amenity trees in the newly-invaded areas in Europe (Italy). This insect/fungus complex arises from the intense trophic activity of the bark beetle vector Pityophthorus juglandis in the phloem of Juglans spp. and the subsequent development of multiple Geosmithia morbida cankers around beetles' entry/exit holes. After an analysis of the main biological and ecological traits of both members of this insect/fungus complex, this review explores the options available for TCD prevention and management. Special focus is given to those diagnostic tools developed for disease detection, surveillance, and monitoring, as well as to existing phytosanitary regulations, protocols, and measures that comply with TCD eradication and containment. Only integrated disease management can effectively curtail the pervasive spread of TCD, thus limiting the damage to natural ecosystems, plantations, and ornamental walnuts.

Characterization and microsatellite marker development for a common bark and ambrosia beetle associate, Geosmithia obscura

Symbioses between Geosmithia fungi and wood-boring and bark beetles seldom result in disease induction within the plant host. Yet, exceptions exist such as Geosmithia morbida, the causal agent of Thousand Cankers Disease (TCD) of walnuts and wingnuts, and Geosmithia sp. 41, the causal agent of Foamy Bark Canker disease of oaks. Isolates of G. obscura were recovered from black walnut trees in eastern Tennessee and at least one isolate induced cankers following artificial inoculation. Due to the putative pathogenicity and lack of recovery of G. obscura from natural lesions, a molecular diagnostic screening tool was developed using microsatellite markers mined from the G. obscura genome. A total of 3256 candidate microsatellite markers were identified (2236, 789, 137 di-, tri-, and tetranucleotide motifs, respectively), with 2011, 703, 101 di-, tri-, and tetranucleotide motifs, respectively, containing markers with primers. From these, 75 microsatellite markers were randomly selected, screened, and optimized, resulting in 28 polymorphic markers that yielded single, consistently recovered bands, which were used in downstream analyses. Five of these microsatellite markers were found to be specific to G. obscura and did not cross-amplify into other, closely related species. Although the remaining tested markers could be useful, they cross-amplified within different Geosmithia species, making them not reliable for G. obscura detection. Five novel microsatellite markers (GOBS9, GOBS10, GOBS41, GOBS43, and GOBS50) were developed based on the G. obscura genome. These species-specific microsatellite markers are available as a tool for use in molecular diagnostics and can assist future surveillance studies.

Loop-Mediated Isothermal Amplification (LAMP) and SYBR Green qPCR for Fast and Reliable Detection of Geosmithia morbida (Kolařik) in Infected Walnut

Walnut species (Juglans spp.) are multipurpose trees, widely employed in plantation forestry for high-quality timber and nut production, as well as in urban greening as ornamental plants. These species are currently threatened by the thousand cankers disease (TCD) complex, an insect-fungus association which involves the ascomycete Geosmithia morbida (GM) and its vector, the bark beetle Pityophthorus juglandis. While TCD has been studied extensively where it originated in North America, little research has been carried out in Europe, where it was more recently introduced. A key step in research to cope with this new phytosanitary emergency is the development of effective molecular detection tools. In this work, we report two accurate molecular methods for the diagnosis of GM, based on LAMP (real-time and visual) and SYBR Green qPCR, which are complimentary to and integrated with similar recently developed assays. Our protocols detected GM DNA from pure mycelium and from infected woody tissue with high accuracy, sensitivity, and specificity, without cross-reactivity to a large panel of taxonomically related species. The precision and robustness of our tests guarantee high diagnostic standards and could be used to support field diagnostic end-users in TCD monitoring and surveillance campaigns.

Symbiosis and pathogenicity of Geosmithia and Talaromyces spp. associated with the cypress bark beetles Phloeosinus spp. and their parasitoids

Fungi associated with cypress bark beetles are practically unknown in the Eastern Mediterranean. Our study focused on the fungi associated with the body parts and galleries of two indigenous cypress bark beetles, Phloeosinus armatus and P. bicolor, sampled from Cupressus sempervirens trees in different regions in Israel. Arbitrarily primed PCR, performed on genomic DNA of 302 isolates, clustered the fungal population into five distinct groups. Multilocus phylogeny, split‐network analyses and morphological characterization identified the isolates as Geosmithia omnicola, Geosmithia langdonii, Geosmithia sp. 708b, Geosmithia cupressina sp. nov. CBS147103 and Talaromyces cupressi sp. nov. CBS147104. Of these fungal isolates, G. cupressina and T. cupressi are newly described, and their morphological features and phylogenetic designations are presented. Inoculation of intact cypress saplings in an outdoor net‐house revealed that only the representative isolate T. cupressi sp. nov. CBS147104 causes 100% disease incidence, whereas Geosmithia spp. isolates are not pathogenic. A number of these fungi were isolated from parasitoids that emerged from branch and stem sections colonized by P. armatus. This study suggests a long and stable association between Phloeosinus and Geosmithia species, and a possible role for additional associated fungal species as pathogens or endophytes of C. sempervirens trees in Israel.

Geosmithia Species Associated With Bark Beetles From China, With the Description of Nine New Species

Fungi of the genus Geosmithia are frequently associated with bark beetles that feed on phloem on various woody hosts. Most studies on Geosmithia were carried out in North and South America and Europe, with only two species being reported from Taiwan, China. This study aimed to investigate the diversity of Geosmithia species in China. Field surveys in Fujian, Guangdong, Guangxi, Hunan, Jiangsu, Jiangxi, Shandong, Shanghai, and Yunnan yielded a total of 178 Geosmithia isolates from 12 beetle species. The isolates were grouped based on morphology. The internal transcribed spacer, β-tubulin, and elongation factor 1-α gene regions of the representatives of each group were sequenced. Phylogenetic trees were constructed based on those sequences. In total, 12 species were identified, with three previously described species (Geosmithia xerotolerans, G. putterillii, and G. pallida) and nine new species which are described in this paper as G. luteobrunnea, G. radiata, G. brevistipitata, G. bombycina, G. granulata (Geosmithia sp. 20), G. subfulva, G. pulverea (G. sp. 3 and Geosmithia sp. 23), G. fusca, and G. pumila sp. nov. The dominant species obtained in this study were G. luteobrunnea and G. pulverea. This study systematically studied the Geosmithia species in China and made an important contribution to filling in the gaps in our understanding of global Geosmithia species diversity.

Origin and evolution of fungus farming in wood-boring Coleoptera - a palaeontological perspective

Insect-fungus mutualism is one of the better-studied symbiotic interactions in nature. Ambrosia fungi are an ecological assemblage of unrelated fungi that are cultivated by ambrosia beetles in their galleries as obligate food for larvae. Despite recently increased research interest, it remains unclear which ecological factors facilitated the origin of fungus farming, and how it transformed into a symbiotic relationship with obligate dependency. It is clear from phylogenetic analyses that this symbiosis evolved independently many times in several beetle and fungus lineages. However, there is a mismatch between palaeontological and phylogenetic data. Herein we review, for the first time, the ambrosia system from a palaeontological perspective. Although largely ignored, families such as Lymexylidae and Bostrichidae should be included in the list of ambrosia beetles because some of their species cultivate ambrosia fungi. The estimated origin for some groups of ambrosia fungi during the Cretaceous concurs with a known high diversity of Lymexylidae and Bostrichidae at that time. Although potentially older, the greatest radiation of various ambrosia beetle lineages occurred in the weevil subfamilies Scolytinae and Platypodinae during the Eocene. In this review we explore the evolutionary relationship between ambrosia beetles, fungi and their host trees, which is likely to have persisted for longer than previously supposed.

Geosmithia associated with hardwood-infesting bark and ambrosia beetles, with the description of three new species from Poland

Geosmithia species (Hypocreales, Ascomycota) are associates of bark beetles and other arthropods. One species, Geosmithia morbida, is a virulent tree pathogen of Juglans nigra. To date, 10 Geosmithia spp. from conifer-infesting, and at least 23 species from hardwood associated bark beetles have been reported from Europe. The aim of this study was to survey Geosmithia spp. associated with 18 bark and ambrosia beetle species in hardwood ecosystems in Poland. In addition, we evaluated the pathogenicity of the six Geosmithia species by inoculating Acer, Fagus, Quercus, Tilia and Ulmus seedlings. Our surveys yielded a total of 1060 isolates from 2915 beetles and 1887 galleries. We identified isolates using morphology and ITS, β-tubulin and TEF1-α sequences. Altogether we identified 11 species including nine previously known and two new species described here as Geosmithia fagi sp. nov. and G. pazoutovae sp. nov. In addition, a sister species G. longistipitata sp. nov., associated with Picea trees, is described here. Bark beetles from hardwoods, with exeption of Dryocoetes alni, D. villosus, Scolytus ratzeburgi and ambrosia beetles, appear to be regular vectors of Geosmithia spp. Like in other parts of the world, most Geosmithia taxa exhibited a distinct level of vector/host specificity. None of Geosmithia isolates induced any disease symptoms under the conditions of our experiment. This study highlights the need for more intensive surveys across additional areas of Central and Northern Europe, insect vectors and host tree species in order to elucidate the Geosmithia species diversity in this region.

Phylogenetic Position of Geosmithia spp. (Hypocreales) Living in Juniperus spp. Forests (Cupressaceae) with Bark Beetles of Phloeosinus spp. (Scolytinae) from the Northeast of Mexico

Geosmithia members are mitosporic filamentous fungi commonly recorded and isolated from bark beetles of the Scolytinae subfamily and their respective host’s species. This genus includes 18 species formally described and 38 phylogenetic species recorded in several localities from Africa, Asia, Australia, Europe, and North and South America, where they exhibit frequent associations with phloeophagous and wood-boring bark beetles. Among phloephagous bark beetle species, specifically, in members of the genus Phloeosinus Chapuis, almost 10% of Geosmithia strains have been isolated. By its physiographic elements and high bark beetle and conifer species richness, Mexico is a potential region to host a high diversity of Geosmithia species and potential new species. In the present study, we systematically sampled and isolated, cultured, and molecularly identified members of the Geosmithia species associated with Phloeosinus spp. and their Juniperus spp. host trees at the north of Sierra Madre Oriental, at Nuevo Leon State, Mexico. Phylogenetic analyses based on 378 internal transcribed spacer region (ITS) sequences supported the presence of strains from Geosmithia langdonii-Geosmithia sp. 32 clade associated with Phloeosinus serratus vector and with Juniperus coahuilensis (JC) host, and the presence of strains from Geosmithia sp. 21-Geosmithia xerotolerans clade with Phloeosinusdeleoni and Juniperus flaccida (JF) in this geographical region. The genetic and morphological differences found in our strains with respect to those previously described in the species from both clades (Geosmithia langdonii-Geosmithia sp. 32 and Geosmithia sp. 21-G. xerotolerans) suggest that both Geosmithia lineages from Nuevo Leon correspond to two potential new species in the genus.

Comparative eco-physiology revealed extensive enzymatic curtailment, lipases production and strong conidial resilience of the bat pathogenic fungus Pseudogymnoascus destructans

The genus Pseudogymnoascus encompasses soil psychrophilic fungi living also in caves. Some are opportunistic pathogens; nevertheless, they do not cause outbreaks. Pseudogymnoascus destructans is the causative agent of the white-nose syndrome, which is decimating cave-hibernating bats. We used comparative eco-physiology to contrast the enzymatic potential and conidial resilience of P. destructans with that of phylogenetically diverse cave fungi, including Pseudogymnoascus spp., dermatophytes and outdoor saprotrophs. Enzymatic potential was assessed by Biolog MicroArray and by growth on labelled substrates and conidial viability was detected by flow cytometry. Pseudogymnoascus destructans was specific by extensive losses of metabolic variability and by ability of lipid degradation. We suppose that lipases are important enzymes allowing fungal hyphae to digest and invade the skin. Pseudogymnoascus destructans prefers nitrogenous substrates occurring in bat skin and lipids. Additionally, P. destructans alkalizes growth medium, which points to another possible virulence mechanism. Temperature above 30 °C substantially decreases conidial viability of cave fungi including P. destructans. Nevertheless, survival of P. destructans conidia prolongs by the temperature regime simulating beginning of the flight season, what suggests that conidia could persist on the body surface of bats and contribute to disease spreading during bats active season.

Four mycangium types and four genera of ambrosia fungi suggest a complex history of fungus farming in the ambrosia beetle tribe Xyloterini

Ambrosia beetles farm fungal cultivars (ambrosia fungi) and carry propagules of the fungal mutualists in storage organs called mycangia, which occur in various body parts and vary greatly in size and complexity. The evolution of ambrosia fungi is closely tied to the evolution and development of the mycangia that carry them. The understudied ambrosia beetle tribe Xyloterini included lineages with uncharacterized ambrosia fungi and mycangia, which presented an opportunity to test whether developments of different mycangium types in a single ambrosia beetle lineage correspond with concomitant diversity in their fungal mutualists. We collected representatives of all three Xyloterini genera (Trypodendron, Indocryphalus, and Xyloterinus politus) and characterized their ambrosia fungi in pure culture and by DNA sequencing. The prothoracic mycangia of seven Trypodendron species all yielded Phialophoropsis (Microascales) ambrosia fungi, including three new species, although these relationships were not all species specific. Indocryphalus mycangia are characterized for the first time in the Asian I. pubipennis. They comprise triangular prothoracic cavities substantially smaller than those of Trypodendron and unexpectedly carry an undescribed species of Toshionella (Microascales), which are otherwise ambrosia fungi of Asian Scolytoplatypus (Scolytoplatypodini). Xyloterinus politus has two different mycangia, each with a different ambrosia fungus: Raffaelea cf. canadensis RNC5 (Ophiostomatales) in oral mycangia of both sexes and Kaarikia abrahamsonii (Sordariomycetes, genus incertae sedis with affinity for Distoseptisporaceae), a new genus and species unrelated to other known ambrosia fungi, in shallow prothoracic mycangia of females. In addition to their highly adapted mycangial mutualists, Trypodendron and X. politus harbor a surprising diversity of facultative symbionts in their galleries, including Raffaelea. A diversity of ambrosia fungi and mycangia suggest multiple ancestral cultivar captures or switches in the history of tribe Xyloterini, each associated with unique adaptations in mycangium anatomy. This further supports the theory that developments of novel mycangium types are critical events in the evolution of ambrosia beetles and their coadapted fungal mutualists.

Fungal metabolic profile dataset was not influenced by long-term in vitro preservation of strains

Comparative ecophysiology is highly valuable approach to reveal adaptive traits linked with specific ecological niches. Although long-term in vitro preserved fungal isolates are often used for analyses, only sparse data is available about the effect of such handling on fungal physiology. The purpose of our data is to show the effect of long-term in vitro preservation of fungal strains on their metabolic profiles. This data is related to research paper “Adaptive traits of bark and ambrosia beetle-associated fungi” (Veselská et al., 2019). Biolog MicroPlates™ for Filamentous fungi were used to compare metabolic profiles between freshly isolated and long-term in vitro preserved strains of two Geosmithia species. Additionally, carbon utilization profiles of 35 Geosmithia species were assessed, including plant pathogen G. morbida and three ambrosia species. Data also shows differences in carbon utilization profiles among diverse ecology types presented in the genus Geosmithia.

Assessment of Alternative Candidate Subcortical Insect Vectors From Walnut Crowns in Habitats Quarantined for Thousand Cankers Disease

Thousand cankers disease (TCD) results from the combined activity of the fungal pathogen, Geosmithia morbida Kolařík, Freeland, Utley, and Tisserat and its principle vector, Pityophthorus juglandis (Blackman) (Coleoptera: Curculionidae: Scolytinae) in Juglans L. spp. and Pterocarya Kunth spp. host plants. TCD has been reported from the eastern and western United States. To evaluate potential for other beetle species to vector the fungus in east Tennessee, specimens were collected using ethanol-baited traps that were suspended beneath crowns of TCD-symptomatic trees. Associations of G. morbida with insect species collected in traps were assessed in an unsuccessful, preliminary culture-based fungal assay, and then with a molecular-based detection method. For culture-based assays, rinsate from washed, individual insects was plated on nutrient media and growing colonies were subcultured to obtain axenic G. morbida cultures for identification. For the molecular-based method, G. morbida presence was detected by amplifying the previously developed, species-specific microsatellite locus GS004. Capillary electrophoresis was used to detect the amplified amplicons and representative reactions were validated using Sanger sequencing. Eleven beetle species were found to carry G. morbida, including Cnestus mutilatus (Blandford), Dryoxylon onoharaensum (Murayama), Hylocurus rudis (LeConte), Monarthrum fasciatum (Say), Monarthrum mali (Fitch), Xyleborinus saxesenii (Ratzeburg), Xylosandrus crassiusculus (Motschulsky), Xylosandrus germanus (Blandford) (all Coleoptera: Curculionidae: Scolytinae), Stenomimus pallidus (Boheman) (Coleoptera: Curculionidae: Cossoninae), Oxoplatypus quadridentatus (Olivier) (Coleoptera: Curculionidae: Platypodinae), and Xylops basilaris (Say) (Coleoptera: Bostrichidae). These findings raise concerns that alternative subcortical insect species that already occur within quarantined habitats can sustain incidence of introduced G. morbida and contribute to spread within the native range of black walnut, Juglans nigra L., in the eastern United States.

Two new Geosmithia species in G. pallida species complex from bark beetles in eastern USA

Species of Geosmithia are cosmopolitan but understudied fungi, and most are associated with phloem-feeding bark beetles on various woody hosts. We surveyed 207 bark and ambrosia beetles from 37 species in the eastern USA for associated fungi. The community is dominated by species in the G. pallida species complex (GPSC) and included several Geosmithia isolates that appear to be new to science. The new Geosmithia isolates exhibited the characteristic brownish-colored colonies typical for the G. pallida species complex and were phylogenetically resolved as two genealogically exclusive lineages based on a concatenated multilocus data set based on the internal transcribed spacers (ITS) of the nuc rDNA (ITS1-5.8S-ITS2 = ITS), and the translation elongation factor 1-α (TEF1-α), β-tubulin (TUB2), and RNA polymerase II second largest subunit (RPB2) genes. Two new Geosmithia species, G. brunnea and G. proliferans, are proposed, and their morphological traits and phylogenetic placements are presented.