Phylogeny of ambrosia beetle symbionts in the genus Raffaelea

How host species and body part determine the microbial communities of five ambrosia beetle species

The ambrosia beetles are farming insects that feed mainly on their cultivated fungi, which in some occasions are pathogens from forest and fruit trees. We used a culture-independent approach based on 16S and 18S rRNA gene metabarcoding analysis to investigate the diversity and composition of the bacterial and fungal communities associated with five ambrosia beetle species: four species native to America (Monarthrum dimidiatum, Dryocoetoides capucinus, Euwallacea discretus, Corthylus consimilis) and an introduced species (Xylosandrus morigerus). For the bacterial community, the beetle species hosted a broad diversity with 1,579 amplicon sequence variants (ASVs) and 66 genera, while for the fungal community they hosted 288 ASVs and 39 genera. Some microbial groups dominated the community within a host species or a body part (Wolbachia in the head-thorax of E. discretus; Ambrosiella in the head-thorax and abdomen of X. morigerus). The taxonomic composition and structure of the microbial communities appeared to differ between beetle species; this was supported by beta-diversity analysis, which indicated that bacterial and fungal communities were clustered mainly by host species. This study characterizes for the first time the microbial communities associated with unexplored ambrosia beetle species, as well as the factors that affect the composition and taxonomic diversity per se, contributing to the knowledge of the ambrosia beetle system.

Four New Species of Harringtonia: Unravelling the Laurel Wilt Fungal Genus

Symbiosis between beetles and fungi arose multiple times during the evolution of both organisms. Some of the most biologically diverse and economically important are mutualisms in which the beetles cultivate and feed on fungi. Among these are bark beetles and Harringtonia, a fungal genus that produces Raffaelea-like asexual morph and hosts the causal agent of laurel wilt, H. lauricola (formerly Raffaelea lauricola). In this study, we propose four new species of Harringtonia associated with beetles from Belize and Florida (USA). We hope to contribute towards a more robust and inclusive phylogenetic framework for future studies on these beetle-fungi relationships and their potential impact in crops and forests worldwide.

First Look Into the Ambrosia Beetle-Fungus Symbiosis Present in Commercial Avocado Orchards in Michoacán, Mexico

Most beetle-fungus symbioses do not represent a threat to agricultural and natural ecosystems; however, a few beetles are able to inoculate healthy hosts with disease-causing fungal symbionts. Here, we report the putative nutritional symbionts associated with five native species of ambrosia beetles colonizing commercial avocado trees in four locations in Michoacán. Knowing which beetles are present in the commercial orchards and the surrounding areas, as well as their fungal associates, is imperative for developing a realistic risk assessment and an effective monitoring system that allows for timely management actions. Phylogenetic analysis revealed five potentially new, previously undescribed species of Raffaelea, and three known species (R. arxi, R. brunnea, R. fusca). The genus Raffaelea was recovered from all the beetle species and across the different locations. Raffaelea lauricola (RL), which causes a deadly vascular fungal disease known as laurel wilt (LW) in Lauraceae species, including avocado, was not recovered. This study points to the imminent danger of native ambrosia beetles spreading RL if the pathogen is introduced to Mexico's avocado orchards or natural areas given that these beetles are associated with Raffaelea species and that lateral transfer of RL among ambrosia beetles in Florida suggests that the likelihood of this phenomenon increases when partners are phylogenetically close. Therefore, this study provides important information about the potential vectors of RL in Mexico and other avocado producing regions. Confirming beetle-fungal identities in these areas is especially important given the serious threat laurel wilt disease represents to the avocado industry in Mexico.

Metabarcoding of insect-associated fungal communities: a comparison of internal transcribed spacer (ITS) and large-subunit (LSU) rRNA markers

Full taxonomic characterisation of fungal communities is necessary for establishing ecological associations and early detection of pathogens and invasive species. Complex communities of fungi are regularly characterised by metabarcoding using the Internal Transcribed Spacer (ITS) and the Large-Subunit (LSU) gene of the rRNA locus, but reliance on a single short sequence fragment limits the confidence of identification. Here we link metabarcoding from the ITS2 and LSU D1-D2 regions to characterise fungal communities associated with bark beetles (Scolytinae), the likely vectors of several tree pathogens. Both markers revealed similar patterns of overall species richness and response to key variables (beetle species, forest type), but identification against the respective reference databases using various taxonomic classifiers revealed poor resolution towards lower taxonomic levels, especially the species level. Thus, Operational Taxonomic Units (OTUs) could not be linked via taxonomic classifiers across ITS and LSU fragments. However, using phylogenetic trees (focused on the epidemiologically important Sordariomycetes) we placed OTUs obtained with either marker relative to reference sequences of the entire rRNA cistron that includes both loci and demonstrated the largely similar phylogenetic distribution of ITS and LSU-derived OTUs. Sensitivity analysis of congruence in both markers suggested the biologically most defensible threshold values for OTU delimitation in Sordariomycetes to be 98% for ITS2 and 99% for LSU D1-D2. Studies of fungal communities using the canonical ITS barcode require corroboration across additional loci. Phylogenetic analysis of OTU sequences aligned to the full rRNA cistron shows higher success rate and greater accuracy of species identification compared to probabilistic taxonomic classifiers.

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.

Fungal mutualisms and pathosystems: life and death in the ambrosia beetle mycangia

Ambrosia beetles and their microbial communities, housed in specialized structures termed mycangia, represent one of the oldest and most diverse systems of mutualism and parasitism described thus far. Comprised of core filamentous fungal members, but also including bacteria and yeasts, the mycangia represent a unique adaptation that allows beetles to store and transport their source of nutrition. Although perhaps the most ancient of "farmers," the nature of these interactions remains largely understudied, with the exception of a handful of emerging pathosystems, where the fungal partner acts as a potentially devastating tree pathogen. Such virulence is often seen during "invasions," where (invasive) beetles carrying the fungal symbiont/plant pathogen expand into new territories and presumably "naïve" trees. Here, we summarize recent findings on the phylogenetic relationships between beetles and their symbionts and advances in the developmental and genetic characterization of the mechanisms that underlie insect-fungal-plant interactions. Results on genomic, transcriptomic, and metabolomic aspects of these relationships are described. Although many members of the fungal Raffaelea-beetle symbiont genera are relatively harmless to host trees, specialized pathosystems including wilt diseases of laurel and oak, caused by specific subspecies (R. lauricola and R. quercus, in the USA and East Asia, respectively), have emerged as potent plant pathogens capable of killing healthy trees. With the development of genetic tools, coupled to biochemical and microscopic techniques, the ambrosia beetle-fungal symbiont is establishing itself as a unique model system to study the molecular determinants and mechanisms that underlie the convergences of symbioses, mutualism, parasitism, and virulence. KEY POINTS: • Fungal-beetle symbioses are diverse and ancient examples of microbial farming. • The mycangium is a specialized structure on insects that houses microbial symbionts. • Some beetle symbiotic fungi are potent plant pathogens vectored by the insect.

Ophiostomatalean fungi associated with wood boring beetles in South Africa including two new species

Ambrosia beetles are small wood inhabiting members of the Curculionidae that have evolved obligate symbioses with fungi. The fungal symbionts concentrate nutrients from within infested trees into a usable form for their beetle partners, which then utilize the fungi as their primary source of nutrition. Ambrosia beetle species associate with one or more primary symbiotic fungal species, but they also vector auxiliary symbionts, which may provide the beetle with developmental or ecological advantages. In this study we isolated and identified ophiostomatalean fungi associated with ambrosia beetles occurring in a native forest area in South Africa. Using a modified Bambara beetle trap, living ambrosia beetle specimens were collected and their fungal symbionts isolated. Four beetle species, three Scolytinae and one Bostrichidae, were collected. Five species of ophiostomatalean fungi were isolated from the beetles and were identified using both morphological characters and DNA sequence data. One of these species, Raffaelea sulphurea, was recorded from South Africa for the first time and two novel species were described as Ceratocystiopsis lunata sp. nov. and Raffaelea promiscua sp. nov.

Esteya floridanum sp. nov.: An Ophiostomatalean Nematophagous Fungus and Its Potential to Control the Pine Wood Nematode

The nematophagous fungal genus Esteya is reported as a natural enemy of the pine wood nematode Bursaphelenchus xylophilus, which causes pine wilt in Asia and Europe. During a survey of fungi associated with ambrosia beetles in Florida, an undescribed Esteya species was found. A phylogenetic analysis based on nuclear large subunit and β-tubulin DNA sequences supported this isolate as a new species, E. floridanum. Morphological and phylogenetic characteristics and a species description are provided here. The fungus was observed to kill the pine wood nematode in vitro. To evaluate the ability of E. floridanum to protect trees against the pine wood nematode in vivo, the effect of prophylactic inoculation was tested on Pinus koraiensis and Larix olgensis in Liaoning, China. The results suggest that the fungus is not a plant pathogen and that it delays wilt and postpones death of two conifer trees. This presents a potential new avenue for research on biocontrol of pine wilt disease and stresses the value of research on pest organisms in their native regions.

Ethanol-Enriched Substrate Facilitates Ambrosia Beetle Fungi, but Inhibits Their Pathogens and Fungal Symbionts of Bark Beetles

Bark beetles (sensu lato) colonize woody tissues like phloem or xylem and are associated with a broad range of micro-organisms. Specific fungi in the ascomycete orders Hypocreales, Microascales and Ophistomatales as well as the basidiomycete Russulales have been found to be of high importance for successful tree colonization and reproduction in many species. While fungal mutualisms are facultative for most phloem-colonizing bark beetles (sensu stricto), xylem-colonizing ambrosia beetles are long known to obligatorily depend on mutualistic fungi for nutrition of adults and larvae. Recently, a defensive role of fungal mutualists for their ambrosia beetle hosts was revealed: Few tested mutualists outcompeted other beetle-antagonistic fungi by their ability to produce, detoxify and metabolize ethanol, which is naturally occurring in stressed and/or dying trees that many ambrosia beetle species preferentially colonize. Here, we aim to test (i) how widespread beneficial effects of ethanol are among the independently evolved lineages of ambrosia beetle fungal mutualists and (ii) whether it is also present in common fungal symbionts of two bark beetle species (Ips typographus, Dendroctonus ponderosae) and some general fungal antagonists of bark and ambrosia beetle species. The majority of mutualistic ambrosia beetle fungi tested benefited (or at least were not harmed) by the presence of ethanol in terms of growth parameters (e.g., biomass), whereas fungal antagonists were inhibited. This confirms the competitive advantage of nutritional mutualists in the beetle’s preferred, ethanol-containing host material. Even though most bark beetle fungi are found in the same phylogenetic lineages and ancestral to the ambrosia beetle (sensu stricto) fungi, most of them were highly negatively affected by ethanol and only a nutritional mutualist of Dendroctonus ponderosae benefited, however. This suggests that ethanol tolerance is a derived trait in nutritional fungal mutualists, particularly in ambrosia beetles that show cooperative farming of their fungi.

Rapid Detection of Raffaelea lauricola Directly from Host Plant and Beetle Vector Tissues Using Loop-Mediated Isothermal Amplification

Since its introduction in 2002, laurel wilt disease has devastated indigenous lauraceous species in the southeastern United States. The causal agent is a fungal pathogen, Raffaelea lauricola, which, after being introduced into the xylem of trees by its vector beetle, Xyleborus glabratus, results in a fatal vascular wilt. Rapid detection and accurate diagnosis of infections is paramount to the successful implementation of disease management strategies. Current management operations to prevent the spread of laurel wilt disease are largely delayed by time-consuming laboratory procedures to confirm the diagnosis. In order to greatly speed up the operations, we developed a loop-mediated isothermal amplification (LAMP) species-specific assay that targets the β-tubulin gene region of R. lauricola, and allows for the rapid detection of the pathogen directly from host plant and beetle tissues. The assay is capable of amplifying as little as 0.5 pg of fungal DNA and as few as 50 conidia. The assay is also capable of detecting R. lauricola directly from wood tissue of artificially inoculated redbay saplings as early as 10 and 12 days postinoculation, when testing high-quality and crude DNA extracts, respectively. Finally, crude DNA extracts of individual adult female X. glabratus beetles were assayed and the pathogen was detected from all specimens. This assay greatly reduces the time required to confirm a laurel wilt diagnosis and, because LAMP technology is well suited to provide point-of-care testing, it has the potential to expedite and facilitate implementation of management operations in response to disease outbreaks.

Evidence for Succession and Putative Metabolic Roles of Fungi and Bacteria in the Farming Mutualism of the Ambrosia Beetle Xyleborus affinis

The bacterial and fungal community involved in ambrosia beetle fungiculture remains poorly studied compared to the famous fungus-farming ants and termites. Here we studied microbial community dynamics of laboratory nests, adults, and brood during the life cycle of the sugarcane shot hole borer, Xyleborus affinis We identified a total of 40 fungal and 428 bacterial operational taxonomic units (OTUs), from which only five fungi (a Raffaelea fungus and four ascomycete yeasts) and four bacterial genera (Stenotrophomonas, Enterobacter, Burkholderia, and Ochrobactrum) can be considered the core community playing the most relevant symbiotic role. Both the fungal and bacterial populations varied significantly during the beetle's life cycle. While the ascomycete yeasts were the main colonizers of the gallery early on, the Raffaelea and other filamentous fungi appeared after day 10, at the time when larval hatching happened. Regarding bacteria, Stenotrophomonas and Enterobacter dominated overall but decreased in foundresses and brood with age. Finally, inferred analyses of the putative metabolic capabilities of the bacterial microbiome revealed that they are involved in (i) degradation of fungal and plant polymers, (ii) fixation of atmospheric nitrogen, and (iii) essential amino acid, cofactor, and vitamin provisioning. Overall, our results suggest that yeasts and bacteria are more strongly involved in supporting the beetle-fungus farming symbiosis than previously thought.IMPORTANCE Ambrosia beetles farm their own food fungi within tunnel systems in wood and are among the three insect lineages performing agriculture (the others are fungus-farming ants and termites). In ambrosia beetles, primary ambrosia fungus cultivars have been regarded essential, whereas other microbes have been more or less ignored. Our KEGG analyses suggest so far unknown roles of yeasts and bacterial symbionts, by preparing the tunnel walls for the primary ambrosia fungi. This preparation includes enzymatic degradation of wood, essential amino acid production, and nitrogen fixation. The latter is especially exciting because if it turns out to be present in vivo in ambrosia beetles, all farming animals (including humans) are dependent on atmospheric nitrogen fertilization of their crops. As previous internal transcribed spacer (ITS) metabarcoding approaches failed on covering the primary ambrosia fungi, our 18S metabarcoding approach can also serve as a template for future studies on the ambrosia beetle-fungus symbiosis.

A new species of Raffaelea from beetle-infested Leucaena leucocephala

Species of Raffaelea (Ophiostomatales: Ascomycota) are obligate symbionts of ambrosia beetles, some of which pose a substantial threat to forest trees. Leucaena leucocephala is a small mimosoid tree species that is considered as an invasive weed in most of its introduced range globally. During a field expedition on the French island of Réunion, dying L. leucocephala trees were observed. Samples were taken from these trees and isolations made from symptomatic wood tissues that included beetle tunnels, but in the absence of the beetles themselves. Multiple isolates of a fungus resembling a Raffaelea species were obtained from the discoloured wood associated with the beetle tunnels. To determine their identity, microscopic examination was performed and DNA sequences for three gene regions (ITS, LSU, TUB) were obtained. Phylogenetic analyses based on these gene regions revealed that the isolates represent a new species of Raffaelea, described here as R. borbonica sp. nov. A pathogenicity test was conducted with the fungus, which was shown to cause lesions on the inoculated seedlings, but with a low level of aggressiveness.

High efficiency transformation and mutant screening of the laurel wilt pathogen, Raffaelea lauricola

The fungal pathogen, Raffaelea lauricola, is the causative agent of laurel wilt, a devastating disease affecting the Lauraceae family. The fungus is vectored by ambrosia beetles that carry the fungus in specialized structures (mycangia), with the fungus acting as a symbiont and food source for beetle larvae growing in tree galleries. In order to probe the molecular basis for plant pathogenicity and insect symbiosis of the laurel wilt fungus, molecular tools including establishment of efficient transformation protocols are required. Resistance marker profiling revealed susceptibility of R. lauricola to phosphinothricin, chlorimuron ethyl, hygromycin, and benomyl. Agrobacterium-mediated transformation using either the bar or sur marker resulted in 1-200 transformants/10⁵ spores. A second protocol using lithium acetate-polyethylene glycol (LiAc-PEG) treatment of fungal blastospores yielded 5-60 transformants/μg DNA/10⁸ cells. Transformants were mitotically stable (at least 5 generations), and > 95% of transformants showed a single integration event. R. lauricola strains expressing green and red fluorescent proteins (EGFP and RFP), as well as glucuronidase (GUS), were constructed. Using the Agrobacterium-mediated method, a random T-DNA insertion library was constructed, and genetic screens led to the isolation of developmental mutants as well as mutants displaying enhanced resistance to sodium dodecyl sulfate (SDS) or fluconazole, and those showing decreased susceptibility to biphenol. These results establish simple and reliable genetic tools for transformation of R. lauricola needed for genetic dissection of the symbiotic and virulent lifestyles exhibited by this fungus and establish a library of insertion mutants that can be used in various genetic screens to dissect molecular pathways. KEY POINTS: • Vectors and transformation protocols were developed for Raffaelea lauricola. • Method was used for construction of a random insertion mutant library. • Mutant library was validated by phenotypic screens for resistance and susceptibility to various agents.

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.

Patterns of coevolution between ambrosia beetle mycangia and the Ceratocystidaceae, with five new fungal genera and seven new species

Ambrosia beetles farm specialised fungi in sapwood tunnels and use pocket-like organs called mycangia to carry propagules of the fungal cultivars. Ambrosia fungi selectively grow in mycangia, which is central to the symbiosis, but the history of coevolution between fungal cultivars and mycangia is poorly understood. The fungal family Ceratocystidaceae previously included three ambrosial genera (Ambrosiella, Meredithiella, and Phialophoropsis), each farmed by one of three distantly related tribes of ambrosia beetles with unique and relatively large mycangium types. Studies on the phylogenetic relationships and evolutionary histories of these three genera were expanded with the previously unstudied ambrosia fungi associated with a fourth mycangium type, that of the tribe Scolytoplatypodini. Using ITS rDNA barcoding and a concatenated dataset of six loci (28S rDNA, 18S rDNA, tef1-α, tub, mcm7, and rpl1), a comprehensive phylogeny of the family Ceratocystidaceae was developed, including Inodoromyces interjectus gen. & sp. nov., a non-ambrosial species that is closely related to the family. Three minor morphological variants of the pronotal disk mycangium of the Scolytoplatypodini were associated with ambrosia fungi in three respective clades of Ceratocystidaceae: Wolfgangiella gen. nov., Toshionella gen. nov., and Ambrosiella remansi sp. nov. Closely-related species that are not symbionts of ambrosia beetles are accommodated by Catunica adiposa gen. & comb. nov. and Solaloca norvegica gen. & comb. nov. The divergent morphology of the ambrosial genera and their phylogenetic placement among non-ambrosial genera suggest three domestication events in the Ceratocystidaceae. Estimated divergence dates for the ambrosia fungi and mycangia suggest that Scolytoplatypodini mycangia may have been the first to acquire Ceratocystidaceae symbionts and other ambrosial fungal genera emerged shortly after the evolution of new mycangium types. There is no evidence of reversion to a non-ambrosial lifestyle in the mycangial symbionts.

Genomic comparisons of the laurel wilt pathogen, Raffaelea lauricola, and related tree pathogens highlight an arsenal of pathogenicity related genes

Raffaelea lauricola is an invasive fungal pathogen and symbiont of the redbay ambrosia beetle (Xyleborus glabratus) that has caused widespread mortality to redbay (Persea borbonia) and other Lauraceae species in the southeastern USA. We compare two genomes of R. lauricola (C2646 and RL570) to seven other related Ophiostomatales species including R. aguacate (nonpathogenic close relative of R. lauricola), R. quercus-mongolicae (associated with mortality of oaks in Korea), R. quercivora (associated with mortality of oaks in Japan), Grosmannia clavigera (cause of blue stain in conifers), Ophiostoma novo-ulmi (extremely virulent causal agent of Dutch elm disease), O. ulmi (moderately virulent pathogen that cause of Dutch elm disease), and O. piceae (blue-stain saprophyte of conifer logs and lumber). Structural and functional annotations were performed to determine genes that are potentially associated with disease development. Raffaelea lauricola and R. aguacate had the largest genomes, along with the largest number of protein-coding genes, genes encoding secreted proteins, small-secreted proteins, ABC transporters, cytochrome P450 enzymes, CAZYmes, and proteases. Our results indicate that this large genome size was not related to pathogenicity but was likely lineage specific, as the other pathogens in Raffaelea (R. quercus-mongolicae and R. quercivora) had similar genome characteristics to the Ophiostoma species. A diverse repertoire of wood-decaying enzymes were identified in each of the genomes, likely used for toxin neutralization rather than wood degradation. Lastly, a larger number of species-specific, secondary metabolite, synthesis clusters were identified in R. lauricola suggesting that it is well equipped as a pathogen, which could explain its success as a pathogen of a wide range of lauraceous hosts.

Partnerships Between Ambrosia Beetles and Fungi: Lineage-Specific Promiscuity Among Vectors of the Laurel Wilt Pathogen, Raffaelea lauricola

Nutritional mutualisms that ambrosia beetles have with fungi are poorly understood. Although these interactions were initially thought to be specific associations with a primary symbiont, there is increasing evidence that some of these fungi are associated with, and move among, multiple beetle partners. We examined culturable fungi recovered from mycangia of ambrosia beetles associated with trees of Persea humilis (silk bay, one site) and P. americana (avocado, six commercial orchards) that were affected by laurel wilt, an invasive disease caused by a symbiont, Raffaelea lauricola, of an Asian ambrosia beetle, Xyleborus glabratus. Fungi were isolated from 20 adult females of X. glabratus from silk bay and 70 each of Xyleborus affinis, Xyleborus bispinatus, Xyleborus volvulus, Xyleborinus saxesenii, and Xylosandrus crassiusculus from avocado. With partial sequences of ribosomal (LSU and SSU) and nuclear (β-tubulin) genes, one to several operational taxonomic units (OTUs) of fungi were identified in assayed individuals. Distinct populations of fungi were recovered from each of the examined beetle species. Raffaelea lauricola was present in all beetles except X. saxesenii and X. crassiusculus, and Raffaelea spp. predominated in Xyleborus spp. Raffaelea arxii, R. subalba, and R. subfusca were present in more than a single species of Xyleborus, and R. arxii was the most abundant symbiont in both X. affinis and X. volvulus. Raffaelea aguacate was detected for the first time in an ambrosia beetle (X. bispinatus). Yeasts (Ascomycota, Saccharomycotina) were found consistently in the mycangia of the examined beetles, and distinct, putatively co-adapted populations of these fungi were associated with each beetle species. Greater understandings are needed for how mycangia in ambrosia beetles interact with fungi, including yeasts which play currently underresearched roles in these insects.

Detecting Symbioses in Complex Communities: the Fungal Symbionts of Bark and Ambrosia Beetles Within Asian Pines

Separating symbioses from incidental associations is a major obstacle in symbiosis research. In this survey of fungi associated with Asian bark and ambrosia beetles, we used quantitative culture and DNA barcode identification to characterize fungal communities associated with co-infesting beetle species in pines (Pinus) of China and Vietnam. To quantitatively discern likely symbioses from coincidental associations, we used multivariate analysis and multilevel pattern analysis (a type of indicator species analysis). Nearly half of the variation in fungal community composition in beetle galleries and on beetle bodies was explained by beetle species. We inferred a spectrum of ecological strategies among beetle-associated fungi: from generalist multispecies associates to highly specialized single-host symbionts that were consistently dominant within the mycangia of their hosts. Statistically significant fungal associates of ambrosia beetles were typically only found with one beetle species. In contrast, bark beetle-associated fungi were often associated with multiple beetle species. Ambrosia beetles and their galleries were frequently colonized by low-prevalence ambrosia fungi, suggesting that facultative ambrosial associations are commonplace, and ecological mechanisms such as specialization and competition may be important in these dynamic associations. The approach used here could effectively delimit symbiotic interactions in any system where symbioses are obscured by frequent incidental associations. It has multiple advantages including (1) powerful statistical tests for non-random associations among potential symbionts, (2) simultaneous evaluation of multiple co-occurring host and symbiont associations, and (3) identifying symbionts that are significantly associated with multiple host species.

Fusarium oligoseptatum sp. nov., a mycosymbiont of the ambrosia beetle Euwallacea validus in the Eastern U.S. and typification of F. ambrosium

Fusarium oligoseptatum sp. nov. was isolated from the invasive Asian ambrosia beetle Euwallacea validis (Coleoptera, Scolytinae, Xyleborini) and from the galleries that females had constructed in dying Ailanthus altissima (tree-of-heaven) symptomatic for Verticillium wilt in south-central Pennsylvania, USA. This ambrosia fungus was cultivated by Euwallacea validis as the primary source of nutrition together with a second symbiont, Raffaelea subfusca . Female beetles transport their fungal symbionts within and from their natal galleries in paired pre-oral mycangia. Fusarium oligoseptatum was distinguished phenotypically from the 11 other known members of the Ambrosia Fusarium Clade (AFC) by uniquely producing mostly 1–2 septate clavate sporodochial conidia that were swollen apically. Phylogenetic analysis of multilocus DNA sequence data resolved F. oligoseptatum as a genealogically exclusive species-level lineage but evolutionary relationships with other members of the AFC were unresolved. Published studies have shown that F. oligoseptatum can be identified via phylogenetic analysis of multilocus DNA sequence data or a PCR multiplex assay employing species-specific oligonucleotide primers. In addition, to provide nomenclatural stability, an epitype was prepared from an authentic strain of F. ambrosium that was originally isolated from a gallery constructed in Chinese tea (Camellia sinensis ) by E. fornicatus in India, together with its lectotypification based on a published illustration.

Know your farmer: Ancient origins and multiple independent domestications of ambrosia beetle fungal cultivars

Bark and ambrosia beetles are highly specialized weevils (Curculionidae) that have established diverse symbioses with fungi, most often from the order Ophiostomatales (Ascomycota, Sordariomycetes). The two types of beetles are distinguished by their feeding habits and intimacy of interactions with their symbiotic fungi. The tree tissue diet of bark beetles is facilitated by fungi, while ambrosia beetles feed solely on fungi that they farm. The farming life history strategy requires domestication of a fungus, which the beetles consume as their sole food source. Ambrosia beetles in the subfamily Platypodinae originated in the mid-Cretaceous (119-88 Ma) and are the oldest known group of farming insects. However, attempts to resolve phylogenetic relationships and the timing of domestication events for fungal cultivars have been largely inconclusive. We sequenced the genomes of 12 ambrosia beetle fungal cultivars and bark beetle associates, including the devastating laurel wilt pathogen, Raffaelea lauricola, to estimate a robust phylogeny of the Ophiostomatales. We find evidence for contemporaneous diversification of the beetles and their associated fungi, followed by three independent domestication events of the ambrosia fungi genus Raffaelea. We estimate the first domestication of an Ophiostomatales fungus occurred ~86 Ma, 25 million years earlier than prior estimates and in close agreement with the estimated age of farming in the Platypodinae (96 Ma). Comparisons of the timing of fungal domestication events with the timing of beetle radiations support the hypothesis that the first large beetle radiations may have spread domesticated "ambrosia" fungi to other fungi-associated beetle groups, perhaps facilitating the evolution of new farming lineages.

Wood decay fungus Flavodon ambrosius (Basidiomycota: Polyporales) is widely farmed by two genera of ambrosia beetles

The ambrosia fungus Flavodon ambrosius is the primary nutritional mutualist of ambrosia beetles Ambrosiodmus and Ambrosiophilus in North America. F. ambrosius is the only known ambrosial basidiomycete, unique in its efficient lignocellulose degradation. F. ambrosius is associated with both native American beetle species and species introduced from Asia. It remains unknown whether F. ambrosius is strictly a North American fungus, or whether it is also associated with these ambrosia beetle genera on other continents. We isolated fungi from the mycangia and galleries of ambrosia beetles Ambrosiodmus rubricollis, Ambrosiodmus minor, Ambrosiophilus atratus, and Ambrosiophilus subnepotulus in China, South Korea, and Vietnam. Phylogenetic analyses suggest that all Asian and North American isolates represent a single haplotype. These results confirm Flavodon ambrosius as the exclusive mutualistic fungus of multiple Ambrosiodmus and Ambrosiophilus beetle species around the world, making it the most widespread known ambrosia fungus species, both geographically and in terms of the number of beetle species. The Flavodon-beetle symbiosis appears to employ an unusually strict mechanism for maintaining fidelity, compared to the symbioses of the related Xyleborini beetles, which mostly vector more dynamic fungal communities.

The Ambrosia Symbiosis: From Evolutionary Ecology to Practical Management

The ambrosia beetle-fungus farming symbiosis is more heterogeneous than previously thought. There is not one but many ambrosia symbioses. Beetle-fungus specificity is clade dependent and ranges from strict to promiscuous. Each new origin has evolved a new mycangium. The most common relationship with host trees is colonization of freshly dead tissues, but there are also parasites of living trees, vectors of pathogenic fungi, and beetles living in rotten trees with a wood-decay symbiont. Most of these strategies are driven by fungal metabolism whereas beetle ecology is evolutionarily more flexible. The ambrosia lifestyle facilitated a radiation of social strategies, from fungus thieves to eusocial species to communities assembled by attraction to fungal scent. Although over 95% of the symbiotic pairs are economically harmless, there are also three types of pest damage: tree pathogen inoculation, mass accumulation on susceptible hosts, and structural damage. Beetles able to colonize live tree tissues are most likely to become invasive pests.

New Raffaelea species (Ophiostomatales) from the USA and Taiwan associated with ambrosia beetles and plant hosts

Raffaelea (Ophiostomatales) is a genus of more than 20 ophiostomatoid fungi commonly occurring in symbioses with wood-boring ambrosia beetles. We examined ambrosia beetles and plant hosts in the USA and Taiwan for the presence of these mycosymbionts and found 22 isolates representing known and undescribed lineages in Raffaelea. From 28S rDNA and β-tubulin sequences, we generated a molecular phylogeny of Ophiostomatales and observed morphological features of seven cultures representing undescribed lineages in Raffaelea s. lat. From these analyses, we describe five new species in Raffaelea s. lat.: R. aguacate, R. campbellii, R. crossotarsa, R. cyclorhipidia, and R. xyleborina spp. nov. Our analyses also identified two plant-pathogenic species of Raffaelea associated with previously undocumented beetle hosts: (1) R. quercivora, the causative agent of Japanese oak wilt, from Cyclorhipidion ohnoi and Crossotarsus emancipatus in Taiwan, and (2) R. lauricola, the pathogen responsible for laurel wilt, from Ambrosiodmus lecontei in Florida. The results of this study show that Raffaelea and associated ophiostomatoid fungi have been poorly sampled and that future investigations on ambrosia beetle mycosymbionts should reveal a substantially increased diversity.

Geographic variation in mycangial communities of Xyleborus glabratus

Factors that influence fungal communities in ambrosia beetle mycangia are poorly understood. The beetle that is responsible for spreading laurel wilt in SE USA, Xyleborus glabratrus, was examined at three sites along a 500 km N-S transect in Florida, each populated by host trees in the Lauraceae. Fungal phenotypes were quantified in mycangia of individual females that were collected from a site in Miami-Dade County (MDC), 25.8N, with swamp bay (Persea palustris), one in Highlands County (HC), 27.9N, with silkbay (P. humulis) and swamp bay and another in Alachua County (AC), 29.8N, with redbay (P. borbonia). Based on combined LSU, SSU and beta-tubulin datasets the most prominent phenotypes were Raffaelea lauricola (cause of laurel wilt), R. subalba, R. subfusca, R. fusca, R. arxii and an undescribed Raffaelea sp. Mean numbers of colony forming units (CFUs) of R. lauricola varied by location (P < 0.003), and a multivariate analysis, which accounted for the presence and relative abundance of fungal species, indicated that there were significant variations in mycangial communities among the sites; thus climate and vegetation might have affected fungal diversity and the relative abundance of these fungi in the mycangia of X. glabratus Statistically it was unlikely that any of the species influenced the presence and prevalence of another species.

Isolation of Fungi and Bacteria Associated with the Guts of Tropical Wood-Feeding Coleoptera and Determination of Their Lignocellulolytic Activities

The guts of beetle larvae constitute a complex system where relationships among fungi, bacteria, and the insect host occur. In this study, we collected larvae of five families of wood-feeding Coleoptera in tropical forests of Costa Rica, isolated fungi and bacteria from their intestinal tracts, and determined the presence of five different pathways for lignocellulolytic activity. The fungal isolates were assigned to three phyla, 16 orders, 24 families, and 40 genera; Trichoderma was the most abundant genus, detected in all insect families and at all sites. The bacterial isolates were assigned to five phyla, 13 orders, 22 families, and 35 genera; Bacillus, Serratia, and Pseudomonas were the dominant genera, present in all the Coleopteran families. Positive results for activities related to degradation of wood components were determined in 65% and 48% of the fungal and bacterial genera, respectively. Our results showed that both the fungal and bacterial populations were highly diverse in terms of number of species and their phylogenetic composition, although the structure of the microbial communities varied with insect host family and the surrounding environment. The recurrent identification of some lignocellulolytic-positive inhabitants suggests that particular microbial groups play important roles in providing nutritional needs for the Coleopteran host.