Genome-level analyses resolve an ancient lineage of symbiotic ascomycetes

Symbionts out of sync: Decoupled physiological responses are widespread and ecologically important in lichen associations

A core vulnerability in symbioses is the need for coordination between the symbiotic partners, which are often assumed to be closely physiologically integrated. We critically re-examine this assumed integration between symbionts in lichen symbioses, recovering a long overlooked yet fundamental physiological asymmetry in carbon balance. We examine the physiological, ecological, and transcriptional basis of this asymmetry in the lichen Evernia mesomorpha. This carbon balance asymmetry depends on hydration source and aligns with climatic range limits. Differences in gene expression across the E. mesomorpha symbiosis suggest that the physiologies of the primary lichen symbionts are decoupled. Furthermore, we use gas exchange data to show that asymmetries in carbon balance are widespread and common across evolutionarily disparate lichen associations. Using carbon balance asymmetry as an example, we provide evidence for the wide-ranging importance of physiological asymmetries in symbioses.

Unravelling the Function of the Sesquiterpene Cyclase STC3 in the Lifecycle of Botrytis cinerea

The genome sequencing of Botrytis cinerea supplies a general overview of the map of genes involved in secondary metabolite synthesis. B. cinerea genomic data reveals that this phytopathogenic fungus has seven sesquiterpene cyclase (Bcstc) genes that encode proteins involved in the farnesyl diphosphate cyclization. Three sesquiterpene cyclases (BcStc1, BcStc5 and BcStc7) are characterized, related to the biosynthesis of botrydial, abscisic acid and (+)-4-epi-eremophilenol, respectively. However, the role of the other four sesquiterpene cyclases (BcStc2, BcStc3, BcStc4 and BcStc6) remains unknown. BcStc3 is a well-conserved protein with homologues in many fungal species, and here, we undertake its functional characterization in the lifecycle of the fungus. A null mutant ΔBcstc3 and an overexpressed-Bcstc3 transformant (OvBcstc3) are generated, and both strains show the deregulation of those other sesquiterpene cyclase-encoding genes (Bcstc1, Bcstc5 and Bcstc7). These results suggest a co-regulation of the expression of the sesquiterpene cyclase gene family in B. cinerea. The phenotypic characterization of both transformants reveals that BcStc3 is involved in oxidative stress tolerance, the production of reactive oxygen species and virulence. The metabolomic analysis allows the isolation of characteristic polyketides and eremophilenols from the secondary metabolism of B. cinerea, although no sesquiterpenes different from those already described are identified.

Metatranscriptomics reveals diversity of symbiotic interaction and mechanisms of carbon exchange in the marine cyanolichen Lichina pygmaea

Lichens are exemplar symbioses based upon carbon exchange between photobionts and their mycobiont hosts. Historically considered a two-way relationship, some lichen symbioses have been shown to contain multiple photobiont partners; however, the way in which these photobiont communities react to environmental change is poorly understood. Lichina pygmaea is a marine cyanolichen that inhabits rocky seashores where it is submerged in seawater during every tidal cycle. Recent work has indicated that L. pygmaea has a complex photobiont community including the cyanobionts Rivularia and Pleurocapsa. We performed rRNA-based metabarcoding and mRNA metatranscriptomics of the L. pygmaea holobiont at high and low tide to investigate community response to immersion in seawater. Carbon exchange in L. pygmaea is a dynamic process, influenced by both tidal cycle and the biology of the individual symbiotic components. The mycobiont and two cyanobiont partners exhibit distinct transcriptional responses to seawater hydration. Sugar-based compatible solutes produced by Rivularia and Pleurocapsa in response to seawater are a potential source of carbon to the mycobiont. We propose that extracellular processing of photobiont-derived polysaccharides is a fundamental step in carbon acquisition by L. pygmaea and is analogous to uptake of plant-derived carbon in ectomycorrhizal symbioses.

The underestimated fraction: diversity, challenges and novel insights into unicellular cyanobionts of lichens

Lichens are remarkable and classic examples of symbiotic organisms that have fascinated scientists for centuries. Yet, it has only been for a couple of decades that significant advances have focused on the diversity of their green algal and/or cyanobacterial photobionts. Cyanolichens, which contain cyanobacteria as their photosynthetic partner, include up to 10% of all known lichens and, as such, studies on their cyanobionts are much rarer compared to their green algal counterparts. For the unicellular cyanobionts, i.e. cyanobacteria that do not form filaments, these studies are even scarcer. Nonetheless, these currently include at least 10 different genera in the cosmopolitan lichen order Lichinales. An international consortium (International Network of CyanoBionts; INCb) will tackle this lack of knowledge. In this article, we discuss the status of current unicellular cyanobiont research, compare the taxonomic resolution of photobionts from cyanolichens with those of green algal lichens (chlorolichens), and give a roadmap of research on how to recondition the underestimated fraction of symbiotic unicellular cyanobacteria in lichens.

Genomic analysis of Coccomyxa viridis, a common low-abundance alga associated with lichen symbioses

Lichen symbiosis is centered around a relationship between a fungus and a photosynthetic microbe, usually a green alga. In addition to their main photosynthetic partner (the photobiont), lichen symbioses can contain additional algae present in low abundance. The biology of these algae and the way they interact with the rest of lichen symbionts remains largely unknown. Here we present the first genome sequence of a non-photobiont lichen-associated alga. Coccomyxa viridis was unexpectedly found in 12% of publicly available lichen metagenomes. With few exceptions, members of the Coccomyxa viridis clade occur in lichens as non-photobionts, potentially growing in thalli endophytically. The 45.7 Mbp genome of C. viridis was assembled into 18 near chromosome-level contigs, making it one of the most contiguous genomic assemblies for any lichen-associated algae. Comparing the C. viridis genome to its close relatives revealed the presence of traits associated with the lichen lifestyle. The genome of C. viridis provides a new resource for exploring the evolution of the lichen symbiosis, and how symbiotic lifestyles shaped evolution in green algae.

A cryptic root isolate belonging to Geoglossales from potted Rhododendron: its molecular phylogeny and ability to colonize an ericoid mycorrhizal host in vitro

Although the lifestyle of Geoglossales remains largely unknown, recent advancements have established a hypothesis regarding the ericoid mycorrhizal lifestyle of geoglossoid fungi. In this study, we focused on one isolate of Geoglossales sp. obtained from surface-sterilized roots of potted Rhododendron transiens. We aimed to reveal the phylogenetic position and in vitro colonizing ability of this species in the hair roots of ericoid mycorrhizal plants. Based on our multigene phylogenetic tree, this species is a sister of the genus Sarcoleotia which has not been reported from either other studies or field environment. Its ascocarps could not be obtained, and conspecific sequences were not found in the databases and repositories examined. The Geoglossales sp. colonized the vital rhizodermal cells of blueberries in vitro with hyphal coils. There were relatively large morphological variations of coils consistent with extraradical hyphae; however, overall, the colonization morphologically resembled those by Sarcoleotia globosa and representative ericoid mycorrhizal fungi. The taxonomy and ecological significance of the species remain to be resolved; nevertheless, our results suggest that the ericoid mycorrhizal lifestyle may be widespread within Geoglossales.

A phylogenetic assessment of a fungicolous lineage in Coniocybomycetes: Chaenotricha, a new genus of Trichaptum-inhabiting species

The globally distributed genus Trichaptum is one of the most species-rich among polypores in terms of hosting other fungi. Among Trichaptum-associates, there is a group of mazaediate lichenized fungi (Coniocybomycetes, Ascomycota) that previously had an uncertain phylogenetic position. DNA sequences - mitochondrial small subunit (mtSSU), nuclear large subunit rDNA (nuLSU), and internal transcribed spacer (ITS) - were obtained from 29 specimens collected from Europe and North America. Maximum likelihood and Bayesian inference analyses of these three gene loci were used to infer phylogenetic position and relationships among lineages. Statistical tests were used to find which phenotypical characteristics distinguish species. The molecular sequence data provide evidence that the fungicolous specimens form a distinct lineage within Coniocybomycetes sister to the combined clade of Chaenotheca s. lat. and Sclerophora. Considering its phylogenetic placement and strict specialization, we describe a new genus - Chaenotricha. This fungicolous lineage contains three species based on molecular characteristics. Morphological characters mostly overlap except for spore size and stalk length of apothecia. We provide a new combination, Chaenotricha obscura, for the only previously described species for which we designate an epitype, and introduce a new species - Chaenotricha cilians. The third lineage remains undescribed because of a small sample size, which did not allow us to clearly delineate species boundaries. Citation: Suija A, McMullin RT, Lõhmus P (2023). A phylogenetic assessment of a fungicolous lineage in Coniocybomycetes: Chaenotricha, a new genus of Trichaptum-inhabiting species. Fungal Systematics and Evolution 12: 255-269. doi: 10.3114/fuse.2023.12.13.

Next-generation fungal identification using target enrichment and Nanopore sequencing

BACKGROUND

Rapid and accurate pathogen identification is required for disease management. Compared to sequencing entire genomes, targeted sequencing may be used to direct sequencing resources to genes of interest for microbe identification and mitigate the low resolution that single-locus molecular identification provides. This work describes a broad-spectrum fungal identification tool developed to focus high-throughput Nanopore sequencing on genes commonly employed for disease diagnostics and phylogenetic inference.

RESULTS

Orthologs of targeted genes were extracted from 386 reference genomes of fungal species spanning six phyla to identify homologous regions that were used to design the baits used for enrichment. To reduce the cost of producing probes without diminishing the phylogenetic power, DNA sequences were first clustered, and then consensus sequences within each cluster were identified to produce 26,000 probes that targeted 114 genes. To test the efficacy of our probes, we applied the technique to three species representing Ascomycota and Basidiomycota fungi. The efficiency of enrichment, quantified as mean target coverage over the mean genome-wide coverage, ranged from 200 to 300. Furthermore, enrichment of long reads increased the depth of coverage across the targeted genes and into non-coding flanking sequence. The assemblies generated from enriched samples provided well-resolved phylogenetic trees for taxonomic assignment and molecular identification.

CONCLUSIONS

Our work provides data to support the utility of targeted Nanopore sequencing for fungal identification and provides a platform that may be extended for use with other phytopathogens.

OMICS and Other Advanced Technologies in Mycological Applications

Fungi play many roles in different ecosystems. The precise identification of fungi is important in different aspects. Historically, they were identified based on morphological characteristics, but technological advancements such as polymerase chain reaction (PCR) and DNA sequencing now enable more accurate identification and taxonomy, and higher-level classifications. However, some species, referred to as "dark taxa", lack distinct physical features that makes their identification challenging. High-throughput sequencing and metagenomics of environmental samples provide a solution to identifying new lineages of fungi. This paper discusses different approaches to taxonomy, including PCR amplification and sequencing of rDNA, multi-loci phylogenetic analyses, and the importance of various omics (large-scale molecular) techniques for understanding fungal applications. The use of proteomics, transcriptomics, metatranscriptomics, metabolomics, and interactomics provides a comprehensive understanding of fungi. These advanced technologies are critical for expanding the knowledge of the Kingdom of Fungi, including its impact on food safety and security, edible mushrooms foodomics, fungal secondary metabolites, mycotoxin-producing fungi, and biomedical and therapeutic applications, including antifungal drugs and drug resistance, and fungal omics data for novel drug development. The paper also highlights the importance of exploring fungi from extreme environments and understudied areas to identify novel lineages in the fungal dark taxa.

Mycology: Metagenomes illuminate evolutionary relationships and reframe symbiotic interactions

An intriguing new study leverages newly generated metagenomes to remap the evolution of the most species-rich clade of fungi, highlighting how some of the most intriguing and visible manifestations of symbioses - lichens - may arise.