Ips subelongatus is a major pest that infects larch plantations over large areas of northern and northeastern China. Ips species are closely associated with ophiostomatoid fungi that are morphologically well-adapted for dispersal by beetles. These associations result in important threat for coniferous forests worldwide. The aim of this study was to characterize the ophiostomatoid communities associated with I. subelongatus infesting Larix species and sympatric Pinus sylvestris var. mongolica in northeastern China forests. Morphological and multilocus phylogenetic approaches (based on six markers: ITS, LSU, 60S, β-tubulin, EF-1α, and CAL gene regions) allowed identifying 14 species of four genera (Ceratocystiopsis, Endoconidiophora, Leptographium and Ophiostoma). Eight species are showed to be new to science. Most strains resided in two Ophiostoma species complexes, viz. the O. clavatum and the O. ips complexes, all together accounting for 76.8% of all isolates. Ophiostoma hongxingense sp. nov., O. peniculi sp. nov., and O. subelongati sp. nov. (O. clavatum complex) and O. pseudobicolor sp. nov. (O. ips complex) were the four dominant species. The ophiostomatoid communities associated with larch bark beetles, I. cembrae and I. subelongatus, in Europe and Asia, China and Japan, also were compared. These comparisons showed distinct, specific assemblage patterns.

Ips nitidus is a well-known conifer pest that has contributed significantly to spruce forest disturbance in the Qinghai-Tibet Plateau and seriously threatens the ecological balance of these areas. We report a chromosome-level genome of I. nitidus determined by PacBio and Hi-C technology. Phylogenetic inference showed that it diverged from the common ancestor of I. typographus ∼2.27 mya. Gene family expansion in I. nitidus was characterized by DNA damage repair and energy metabolism, which may facilitate adaptation to high-altitude hypoxia. Interestingly, differential gene expression analysis revealed upregulated genes associated with high-altitude hypoxia adaptation and downregulated genes associated with detoxification after feeding and tunneling in fungal symbiont Ophiostoma bicolor-colonized substrates. Our findings provide evidence of the potential adaptability of I. nitidus to conifer host, high-altitude hypoxia and insight into how fungal symbiont assist in this process. This study enhances our understanding of insect adaptation, symbiosis, and pest management.

Ips is a genus of bark beetles found throughout the Northern Hemisphere, many of which are highly destructive to coniferous forests and plantations. Fungal symbionts, especially ophiostomatoid fungi, have contributed to the success of Ips bark beetles. Recently, climate change accelerated tree mortality caused by bark beetles and their fungal symbionts. However, the knowledge of ophiostomatoid fungi associated with Ips bark beetles is inadequate in China. Therefore, this study investigated the ophiostomatoid fungal communities associated with different Ips bark beetles from various coniferous forest areas of China. A total of 14,512 ophiostomatoid fungal strains were isolated from 1265 vigorous adult beetles and 826 fresh galleries belonging to 11 Ips species infesting 16 coniferous tree species, including pines, spruces, and larches, from 42 sampling sites in nine provinces or autonomous regions in northeast, northwest and southwest China. Based on a combination of morphological features and phylogenetic analysis, 71 taxa belonging to eight genera were identified (Ceratocystiopsis, Graphilbum, Grosmannia, Leptographium, Masuyamyces, and Ophiostoma in Ophiostomatales; Endoconidiophora and Graphium in Microascales), of which 38 species were described as new. Comparing patterns of fungal assemblages indicated that fungal symbionts genetically co-differentiated with their vectors. Host trees possibly reinforce the coarse species-specific association between ophiostomatoid fungi and Ips bark beetles. This study further demonstrates the high diversity of ophiostomatoid fungi associated with Ips bark beetles and provides insights into their symbiotic associations.

Climate change has exacerbated outbreaks of forest pests worldwide. In recent years, bark beetles have caused significant damage to coniferous forests of the Northern Hemisphere. Polygraphus bark beetles are widely distributed secondary pests. Recently, tree mortality caused by these beetles on the Qinghai-Tibet Plateau has been increasing; however, few studies have focused on their fungal associations. In the present study, we explored the diversity of ophiostomatalean fungi associated with these beetles on the north-eastern and southern Qinghai-Tibet Plateau. We isolated 442 ophiostomatalean strains from adult beetles and their fresh galleries, specifically targeting Polygraphus poligraphus and Polygraphus rudis infesting Picea crassifolia and/or Pinus griffithii. Based on phylogenetic and morphological features, we assigned the 442 strains to 16 species belonging to Grosmannia spp., Leptographium spp. and Ophiostoma spp. Amongst these, Ophiostoma maixiuense and Ophiostoma bicolor were the most frequently isolated species, accounting for 20.8% and 18.1% of the total number of ophiostomatalean assemblages, respectively. By comparing their fungal communities, we found that the different patterns of fungal assemblages of bark beetles from the north-eastern and southern Qinghai-Tibet Plateau may be influenced by biogeographic barriers and host tree species. The results of this study enhance our understanding of bark beetle fungal assemblages, especially Polygraphus, on the Qinghai–Tibet Plateau, with implications for forest management under changing climate.

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.

Pine wilt disease (PWD) is a devastating disease that occurs worldwide and affects conifers infested by the pine wood nematode (PWN, Bursaphelenchus xylophilus). PWD has caused serious economic and ecological losses in China. The mechanism of disease outbreak is complex, with the associated fungi, specifically ophiostomatoid fungi, thought to play an essential role. However, few ophiostomatoid fungal associates of PWD have been accurately identified. In the present study, we isolated fungi from bark beetles collected from Pinus thunbergii infested by the pine wood nematode on Laoshan Mountain, Shandong province. Three ophiostomatalean fungi were identified and assigned to Graphilbum and Ophiostoma based on phylogenetic analyses and comparison of morphological and cultural features, namely Gra. laoshanense sp. nov., Gra. translucens, and O. ips. This study increases the understanding of the diversity of ophiostomatoid fungi associated with PWD and provides resources for parsing this complex disease.