Ophiostomatoid fungi associated with Ips subelongatus, including eight new species from northeastern China

Performance of two Ips bark beetles and their associated pathogenic fungi on hosts reflects a species-specific association in the beetle-fungus complex

When Ips bark beetles invade and colonize the host plants, their associated pathogenic fungal partners are carried into the phloem of the host trees. Host trees are lethally attacked by the beetle-fungus complex and the collective damage severely limits forestry production worldwide. It is of great importance to verify whether bark beetles and their associated fungi show concordant performance in terms of biology, physiology, and biochemistry on host trees. In this study, the two Ips bark beetles Ips typographus and Ips subelongatus (Coleoptera: Curculionidae, Scolytinae), their respective associated pathogenic fungi Endoconidiophora polonica and Endoconidiophora fujiensis, and their respective host plants Picea jezoensis and Larix olgensis were selected as test material. Cross-inoculation experiments were conducted indoors and outdoors to investigate the differences in reproduction and development of two beetles and infectivity of two fungi on two plants, as well as the differences in physiological responses of two plants to two fungal infections. The results showed that I. typographus and E. polonica had excellent host performance on P. jezoensis; however, neither successfully colonized and infected L. olgensis. In contrast, I. subelongatus and E. fujiensis showed strong host suitability on L. olgensis and some degree of suitability on P. jezoensis, although the host suitability of P. jezoensis for E. polonica was significantly higher than that for E. fujiensis. In addition, we found that the absolute amount of ergosterol accumulated on the lesion was positively correlated with lesion area. The ergosterol amount and lesion area were both strongly correlated with the release of host monoterpenes, but had no obvious correlation with the concentration of fungi-induced phenols on the lesion area and the side-chain oxidation of lignin in the xylem of the infected sites. Based on these results, we confirmed that "I. typographus-E. polonica" and "I. subelongatus-E. fujiensis" complexes both showed the most suitable consistent performances on their own traditional hosts, establishing a stable species-specific association relationship in these two beetle-fungus complexes, with the "I. subelongatus-E. fujiensis" complex showing broader host suitability. From the perspective of physiological responses of plants to fungal infections, monoterpenes are an important indicator of host suitability.

Identification of ophiostomatalean fungi associated with Tomicus pilifer infesting Pinus koraiensis in Northeastern China

Ophiostomatalean fungi usually facilitate bark beetles to infest tree hosts and seriously endanger the health of coniferous forests. Tomicus pilifer Spessivtsev is a common endemic bark beetle in Asia and primarily threatens Pinus koraiensis. Tomicus species have similar morphology; however, they can be differentiated by their genetic characteristics through phylogenetic analyses. To date, the 28S rDNA sequence of T. pilifer and the diversity of ophiostomatalean fungi associated with T. pilifer have not been reported. In this study, we aimed to clarify the taxonomic status of T. pilifer and identify ophiostomatalean fungi associated with T. pilifer infesting P. koraiensis in northeastern China. In total, 315 ophiostomatalean fungal strains were isolated from 62 adults of T. pilifer and 220 tissue samples from T. pilifer galleries in Jilin Province. Thirty-five representative strains were further identified by comparing their morphological and physiological characteristics and conducting the phylogenetic analysis of ITS, ITS2-LSU, TUB2, and TEF1-α. We identified nine species of ophiostomatalean fungi belonging to four genera, which included six novel species (Ceratocystiopsis changbaiensis sp. nov., Leptographium linjiangense sp. nov., Leptographium qieshaoense sp. nov., Ophiostoma piliferi sp. nov., Ophiostoma tonghuaense sp. nov., and Ophiostoma yaluense sp. nov.), two previously described species (Graphilbum interstitiale and Ophiostoma fuscum), and one undefined specie (Ceratocystiopsis sp. 1). To the best of our knowledge, this is the first report of G. interstitiale and O. fuscum in China and the fungal diversity of ophiostomatalean in T. pilifer. The dominant species were O. piliferi and L. qieshaoense, representing 39.37% and 35.87% of the isolates, respectively. The results of this study provide valuable information on the symbiotic relationship between bark beetles and ophiostomatalean fungi.

Diversity of fungi associated with Monochamusalternatus larval habitats in Bursaphelenchusxylophilus-infected Pinusmassoniana and identification of two new ophiostomatalean species (Ascomycota, Ophiostomatales)

Bursaphelenchusxylophilus, a pathogenic pine wood nematode (PWN), is responsible for pine wilt disease (PWD), which has caused significant economic and ecological damage worldwide, particularly in East Asia. Multiple biological factors, such as the beetle vector Monochamus, symbiotic bacteria and associated fungi, are involved in the disease infection cycle. This study isolated and identified the fungal communities of Monochamusalternatus larval galleries and pupal chambers from different instars through field investigation, morphological observation and multi-locus DNA sequence analyses in Zhejiang Province, China. A total of 255 and 454 fungal strains were isolated from M.alternatus galleries and pupal chambers infected with PWN, from the 2nd-3rd and 4th-5th instar larvae, respectively. A total of 18 species of fungi were identified, 14 species were isolated from the 2nd-3rd instar larval galleries and six species from the galleries and pupal chambers of the 4th-5th instar larvae. Amongst them were six species belonging to four genera of ophiostomatalean fungi, including two novel species, Graphilbumxianjuensis sp. nov. and Ophiostomataizhouense sp. nov. and four known species, Ceratocystiopsisweihaiensis, Ophiostomaips, Sporothrixzhejiangensis and S.macroconidia. The findings revealed that the fungal diversity and abundance of the 2nd-3rd instar larvae differed markedly from those of the 4th-5th instar larvae. This difference could be the result of fungal succession. This study provides a thorough understanding of the fungi associated with PWD and lays the groundwork for future research.

Pathophysiology and transcriptomic analysis of Picea koraiensis inoculated by bark beetle-vectored fungus Ophiostoma bicolor

Ophiostomatoid fungi exhibit a complex relationship with bark beetles; exhausting of host tree defenses is traditionally regarded as one of the key benefits provided to beetle vectors. Ophiostoma bicolor is one of the dominant species of the mycobiota associated with Ips genus bark beetles which infect the spruce trees across the Eurasian continent. Host spruce trees resist fungal invasion through structural and inducible defenses, but the underlying mechanisms at the molecular level, particularly with respect to the interaction between bark beetle-associated fungi and host trees, remain unclear. The aim of this study was to observe the pathological physiology and molecular changes in Picea koraiensis seedlings after artificial inoculation with O. bicolor strains (TS, BH, QH, MX, and LWQ). This study showed that O. bicolor was a weakly virulent pathogen of spruce, and that the virulent of the five O. bicolor strains showed differentiation. All O. bicolor strains could induce monoterpenoid release. A positive correlation between fungal virulence and release of monoterpenoids was observed. Furthermore, the release rate of monoterpenoids peaked at 4 days post-inoculation (dpi) and then decreased from 4 to 90 dpi. Transcriptomic analysis at 4 dpi showed that many plant-pathogen interaction processes and mitogen-activated protein kinase (MAPK) metabolic processes were activated. The expression of monoterpenoid precursor synthesis genes and diterpenoid synthesis genes was upregulated, indicating that gene expression regulated the release rate of monoterpenoids at 4 dpi. The enriched pathways may reveal the immune response mechanism of spruce to ophiostomatoid fungi. The dominant O. bicolor possibly induces the host defense rather than defense depletion, which is likely the pattern conducted by the pioneers of beetle-associated mycobiota, such as Endoconidiophora spp.. Overall, these results facilitate a better understanding of the interaction mechanism between the dominant association of beetles and the host at the molecular level.

Ceratocystiopsis spp. associated with pine- and spruce-infesting bark beetles in Norway

The genus Ceratocystiopsis (Ophiostomatales, Ascomycota) includes 21 species, which can be found mainly in association with bark beetles in the Northern Hemisphere. A survey of Ceratocystiopsis species associated with bark beetles infesting Picea abies and Pinus sylvestris in Norway yielded 126 isolates, representing Ceratocystiopsis neglecta and Ceratocystiopsis rollhanseniana, and four species described herein as Ceratocystiopsis chalcographii, Ceratocystiopsis debeeria, Ceratocystiopsis norroenii and Ceratocystiopsis troendelagii. The new taxa were morphologically characterised and phylogenetically analysed on the basis of sequence data of multiple loci (ITS, LSU, beta-tubulin (TUB2), calmodulin (CAL) and translation elongation factor 1-alpha (TEF1) genes). Ceratocystiopsis norroenii and C. rollhanseniana were the most frequently isolated species, and the latter species had the wider vector range.

Ophiostomatoid fungi including a new species associated with Asian larch bark beetle Ips subelongatus, in Heilongjiang (Northeast China)

Ips subelongatus (Coleoptera, Scolytinae) is an important bark beetle species that infests Larix spp. in Asia. Individuals of this beetle are vectors of ophiostomatoid fungi, on their exoskeletons, that are transmitted to infested trees. In this study, the symbiotic assemblage of ophiostomatoid fungi associated with I. subelongatus in Northeast China was studied. Fungal isolates were identified based on their morphological characters and sequences of ITS, beta-tubulin, elongation factor 1-alpha and calmodulin gene regions. In total, 48 isolates were collected and identified, residing in six taxa. These included a novel species, described here as Ophiostoma gmelinii sp. nov.

Morphological and Phylogenetic Analyses Reveal a New Species of Ceratocystiopsis (Ophiostomataceae, Ophiostomatales) Associated with Ips subelongatus in Inner Mongolia (China) with Weak Host Pathogenicity

Ophiostomatoid fungi are known for their associations with bark beetles, and some species are important sources of tree diseases. Ceratocystiopsis is a genus of the ophiostomatoid fungi in order Ophiostomatales. The shortage of DNA barcodes for many species in this genus has resulted in the presence of many unnamed cryptic species. In this study, Ceratocystiopsis subelongati sp. nov. associated with Ips subelongatus infesting Pinus sylvestris var. mongolica in Inner Mongolia, China, was identified and described based on phylogenetic inference of multi-gene DNA sequences and morphological characteristics. The species is characterized by a hyalorhinocladiella- to sporothrix-like asexual state and an optimal growth temperature of 30 °C. Artificial inoculation tests in the field showed that it is mildly pathogenic to five-year-old larch trees, the main host of I. subelongatus. It is also the first described Ceratocystiopsis species associated with I. subelongatus in China. This discovery should provide new avenues for studying the symbiosis between bark beetles and ophiostomatoid fungi.

Abundance and Diversity of Ophiostomatoid Fungi Associated With the Great Spruce Bark Beetle (Dendroctonus micans) in the Northeastern Qinghai-Tibet Plateau

The role of several virulent tree pathogens in host death has been overlooked because of the aggressiveness of their associated bark beetles. The great spruce bark beetle (Dendroctonus micans) is a widely distributed beetle that infests coniferous plants in Eurasia; however, its associated fungi have been poorly studied. Therefore, in this study, we elucidated the diversity of ophiostomatoid fungi associated with D. micans in the northeastern Qinghai-Tibet Plateau through field investigation, laboratory isolation, and culture analyses. A total of 220 strains of ophiostomatoid fungi were isolated from adults and tunnel galleries of D. micans infesting Picea crassifolia. We identified that the isolated strains belonged to eight ophiostomatoid species, including five new species (Ophiostoma huangnanense sp. nov., Ophiostoma maixiuense sp. nov., Ophiostoma sanum sp. nov., Leptographium sanjiangyuanense sp. nov., and Leptographium zekuense sp. nov.), one undefined species (Ophiostoma sp. 1), and two known species (Ophiostoma bicolor and Endoconidiophora laricicola), using phylogenetic analysis of multigene DNA sequences and morphological characteristics. This is the first time that E. laricicola, a pioneer invader and virulent pathogen, has been reported in China. We found that E. laricicola was the dominant species, accounting for 40.91% of the total number of ophiostomatoid communities. This study enriched the knowledge of the fungal associates of D. micans and elucidated that it carried the virulent pathogen E. laricicola at a surprisingly high frequency. Our findings show increased species association between D. micans and ophiostomatoid fungi and provide a basis for understanding the occurrence of forest diseases and pests.

Ophiostomatoid species associated with pine trees (Pinus spp.) infested by Cryphaluspiceae from eastern China, including five new species

Cryphaluspiceae attacks various economically important conifers. Similar to other bark beetles, Cr.piceae plays a role as a vector for an assortment of fungi and nematodes. Previously, several ophiostomatoid fungi were isolated from Cr.piceae in Poland and Japan. In the present study, we explored the diversity of ophiostomatoid fungi associated with Cr.piceae infesting pines in the Shandong Province of China. We isolated ophiostomatoid fungi from both galleries and beetles collected from our study sites. These fungal isolates were identified using both molecular and morphological data. In this study, we recovered 175 isolates of ophiostomatoid fungi representing seven species. Ophiostomaips was the most frequently isolated species. Molecular and morphological data indicated that five ophiostomatoid fungal species recovered were previously undescribed. Thus, we proposed these five novel species as Ceratocystiopsisyantaiensis, C.weihaiensis, Graphilbumtranslucens, Gr.niveum, and Sporothrixvillosa. These new ophiostomatoid fungi add to the increasing number of fungi known from China, and this evidence suggests that numerous novel taxa are awaiting discovery in other forests of China.

Ophiostomatoid fungi associated with pine bark beetles and infested pines in south-eastern Australia, including Graphilbum ipis-grandicollis sp. nov

The ophiostomatoid fungi are an assemblage of ascomycetes which are arguably best-known for their associations with bark and ambrosia beetles (Curculonidae) and blue stain (sap stain) of many economically important tree species. These fungi are considered a significant threat to coniferous forests, which has resulted in numerous studies characterising the diversity of bark beetles and their ophiostomatoid associates globally. The diversity of ophiostomatoid fungi present in Australian pine plantations, however, remains largely undetermined. The aims of this study were therefore to reconsider the diversity of ophiostomatoid fungi associated with Pinus in Australia, and to establish the baseline of expected taxa found within these plantation ecosystems. To achieve this, we reviewed Australian plant pathogen reference collections, and analysed samples collected during forest health surveillance programs from the major pine growing regions in south-eastern Australia. In total, 135 ophiostomatoid isolates (15 from reference collections and 120 collected during the current study) were assessed using morphological identification and ITS screening which putatively distinguished 15 taxonomic groups. Whole genome sequencing (WGS) of representative isolates from each taxon was performed to obtain high-quality sequence data for multi-locus phylogenetic analysis. Our results revealed a greater than expected diversity, expanding the status of ophiostomatoid fungi associated with Pinus in Australia to include 14 species from six genera in the Ophiostomatales and a single species residing in the Microascales. While most of these were already known to science, our study includes seven first records for Australia and the description of one new species, Graphilbum ipis-grandicollis sp. nov.. This study also provides an early example of whole genome sequencing (WGS) approaches replacing traditional PCR-based methods for taxonomic surveys. This not only allowed for robust multi-locus sequence extraction during taxonomic assessment, but also permitted the rapid establishment of a curated genomic database for ophiostomatoid fungi which will continue to aid in the development of improved diagnostic resources and capabilities for Australian biosecurity.

Chemical signal interactions of the bark beetle with fungal symbionts, and host/non-host trees

The symbiosis between the bark beetle (Ips subelongatus) and its fungal symbiont (Endoconidiophora fujiensis) poses a serious threat to larch forests. However, the signaling pathways between these symbiotic partners and their host/non-host trees are not fully understood. Inoculation of the host larch (Larix principis-rupprechtii) with two strains of E. fujiensis induced a rapid and long-term release of monoterpenes. Although the fungi had a level of tolerance to these compounds, many monoterpenes inhibited fungal growth in culture. Moreover, monoterpenes with stronger inhibitory effects on fungal growth exhibited weaker synergistic effects on the attraction of I. subelongatus to aggregation pheromone. Surprisingly, individual isomers of aggregation pheromone components promoted fungal symbiont growth in a culture medium. Non-host volatiles (NHVs) were tested and shown to completely inhibit the growth of fungal symbionts in culture but had no effects on beetle responses to aggregation pheromone, with the exception of (Z)-3-hexen-1-ol. These results reveal convergence and mutualism patterns in the evolution of I. subelongatus and E. fujiensis with respect to host tree volatiles but not in response to NHVs. Ultimately, we put forward a hypothesis that host plants are ecological and evolutionary determinants of bark beetle-fungus symbioses in terms of their complex signaling interactions.

Ophiostomatoid fungi associated with mites phoretic on bark beetles in Qinghai, China

Bark beetle galleries are complex ecosystems where many microbes and other arthropods co-exist with the beetles. Fungi isolated from these galleries are often referred to as ‘beetle associates’, but the nature of these associations are poorly understood. The possibility that many of these fungi might in fact be mite associates is often overlooked. Several recent studies explored the diversity of fungi from conifer-infesting bark beetles and their galleries in China, but only one study considered phoretic mites and their fungi from conifer-infesting bark beetles in Yunnan, southwestern China. We studied the mites and fungi from galleries of four spruce-infesting bark beetle species in the high altitude forests of Qinghai province, western China. Mites were identified based on morphological characteristics, and fungi based on DNA sequences of four gene regions. In total, 173 mite individuals were collected belonging to 18 species in 11 genera. A total of 135 fungal isolates were obtained from the mites, representing 14 taxa from the Ophiostomatales. The most frequently isolated fungus was Ophiostoma nitidum, which represented 23.5% of the total isolates. More fungal species were found from fewer mites and bark beetle species than from the study in Yunnan. Although we could not elucidate the exact nature of interactions between mites and their fungi, our results re-enforce that these organisms should not be ignored in pest risk assessments of bark beetles, that often focus only on the beetles and their fungi. Three new species are described: Grosmannia zekuensis, O. manchongi, and O. kunlunense spp. nov., and our data revealed that O. typographi, recently described from China, is a synonym of O. ainoae.

Correction to: Ophiostomatoid fungi associated with Ips subelongatus, including eight new species from northeastern China

Following publication of the original article [1], the authors reported that Table 1 contained some errors in the GenBank number of BT gene. The correct Table 1 is presented below.