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

Ophiostoma haidaense, sp. nov., a new member of the O. piceae species complex associated with yellow-cedar, Callitropsis nootkatensis

Ophiostoma haidanensis is described as a new species of the Ophiostoma piceae complex isolated from yellow-cedar (Callitropsis nootkatensis (D. Don) Oerst. ex D.P. Little) sapwood in the Haida Gwaii island archipelago and the North Coast of British Columbia, Canada. The fungus is characterized by the production of a typical sporothrix-like asexual morph but is distinguished morphologically from other members of the O. piceae species complex by its large, multiseptate primary conidia. Phylogenetic analysis of DNA sequences from the nuc rDNA internal transcribed spacer region ITS1-5.8S-ITS2 (ITS) and the β-tubulin (BTUB) and translation elongation factor 1-α (TEF1) genes supports the inclusion of O. haidensis as a distinct member within the O. piceae complex. To our knowledge, this is the first report of a blue stain fungus infecting yellow-cedar, an ecologically, culturally, and economically important conifer naturally distributed along the coastal forests of the Pacific Northwest in North America.

Molecular characterization of a novel endornavirus isolated from Ophiostoma bicolor associated with bark beetles

Ophiostoma bicolor is a pathogenic fungus associated with bark beetles that can cause serious damage to host plants. In this study, a novel fungal virus, "Ophiostoma bicolor endornavirus 1" (ObEV1), was obtained from O. bicolor, and its complete genome sequence was determined. ObEV1 has a single-stranded positive-sense (+ ss) RNA genome of 10,119 nucleotides. Sequence annotation and comparison showed that the viral genome has a single large open reading frame (ORF) encoding a polyprotein of 362.48 kDa. The polyprotein contains seven conserved domains: RNA-dependent RNA polymerase (RdRp), viral RNA helicase 1 (VHel1), viral methyltransferase (VMet), DEAD-like helicase (DEXDc), gliding-GltJ (G1), large tegument protein UL36 (PHA), and YlqF-related-GTPase (Y). Sequence comparisons and phylogenetic analysis showed that ObEV1 is a novel mycovirus belonging to the genus Betaendornavirus of the family Endornaviridae. This is the first report of a mycovirus in the ophiostomatoid fungus O. bicolor.

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.

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.