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

Genomics: A window into the molecular mystery box of lichen symbiosis

How is a symbiosis built? Lichen metagenomic and metatranscriptomic surveys comparing growth stages, experimental treatments and environmental settings identify a catalogue of candidate genes - and microbial partners - in a developing model system.

Phylogeny, evolution and a re-classification of the Lichinomycetes

The Lichinomycetes is an independent lichenized lineage within the Ascomycota comprising ca. 390 species and 50 genera. Very few studies have dealt with family and genus classification using molecular data and many groups are in need of thorough revision. Thus, we constructed a multilocus phylogeny (mtSSU, RPB2 and mcm7 gene regions) including 190 specimens of Lichinomycetes belonging to 126 species. Ancestral state reconstruction analyses were carried out to trace the evolution of selected characters. The current classification scheme of the Lichinomycetes based on morphological and anatomical characters is in great conflict with the phylogenetic relationships resulting from the present study. The results suggest substantial non-monophyly at the family and genus levels. A revised classification is proposed here and an overview of genera accepted in the Lichinomycetes is given. Ancestral Lichinomycetes are reconstructed as crustose with pycnoascocarps and octosporous asci. We used a combination of characters to delineate groups including the ascoma development and the type of asci. The revised classification includes 11 new genera, five resurrected genera, and 54 new combinations distributed in four families (three emended and one new). Three new species are also described. Taxonomic novelties: New family: Lichinellaceae M. Schultz & M. Prieto. New genera: Allopyrenis M. Schultz & M. Prieto, Gonotichia M. Schultz & M. Prieto, Lapismalleus M. Schultz & M. Prieto, Lingolemma M. Schultz & M. Prieto, Paludolemma M. Schultz & M. Prieto, Paracyphus M. Schultz & M. Prieto, Peltolemma M. Schultz & M. Prieto, Pseudocarpon M. Schultz & M. Prieto, Pseudotichia M. Schultz & M. Prieto, Pycnolemma M. Schultz & M. Prieto, Tichocyphus M. Schultz & M. Prieto. New species: Paracyphus gotlandicus M. Schultz & M. Prieto, Pseudocarpon persimile M. Schultz & M. Prieto, Tichocyphus gotlandicus M. Schultz & M. Prieto. New combinations: Allopyrenis grumulifera (Nyl.) M. Schultz & M. Prieto, Allopyrenis haemaleella (Nyl.) M. Schultz & M. Prieto, Allopyrenis impolita (Th. Fr.) M. Schultz & M. Prieto, Allopyrenis phaeococca (Tuck.) M. Schultz & M. Prieto, Allopyrenis reducta (Th. Fr.) M. Schultz & M. Prieto, Allopyrenis sanguinea (Anzi) M. Schultz & M. Prieto, Allopyrenis tenuis (Henssen) M. Schultz & M. Prieto, Cladopsis densisidiata (Aptroot et al.) M. Schultz & M. Prieto, Cladopsis foederata (Nyl.) M. Schultz & M. Prieto, Cladopsis guyanensis (M. Schultz et al.) M. Schultz & M. Prieto, Cladopsis palmana (J. Steiner) M. Schultz & M. Prieto, Cladopsis polycocca (Nyl.) M. Schultz & M. Prieto, Forssellia canariensis (Henssen) M. Schultz & M. Prieto, Forssellia concordatula (Nyl.) M. Schultz & M. Prieto, Gonotichia octosporella (Lettau) M. Schultz & M. Prieto, Lapismalleus lugubris (A. Massal.) M. Schultz & M. Prieto, Lemmopsis lutophila (Arnold) M. Schultz & M. Prieto, Lempholemma segregatum (Nyl.) M. Schultz & M. Prieto, Lichinella baicalensis (Makryi) M. Schultz, Lichinella etoshica (Brusse) M. Schultz, Lichinella lusitanica (Henssen) M. Schultz, Lichinella pulvinata (E. Dahl) M. Schultz, Lichinella schleicheri (Hepp) M. Schultz, Lichinella terrestris (Makryi) M. Schultz, Lingolemma lingulatum (Tuck.) M. Schultz & M. Prieto, Paludolemma syreniarum (C.J. Lewis & M. Schultz) M. Schultz & M. Prieto, Peltolemma socotranum (M. Schultz) M. Schultz & M. Prieto, Phylliscum aotearoa (Henssen & B. Bartlett) M. Schultz & M. Prieto, Phylliscum cylindrophorum (Vain.) M. Schultz, Phylliscum laatokkaense (Vain.) M. Schultz & M. Prieto, Phylliscum neglectum (Henssen) M. Schultz & M. Prieto, Phylliscum permiscens (Nyl.) M. Schultz & M. Prieto, Phylliscum rhodostictum (Taylor) M. Schultz & M. Prieto, Porocyphus antarcticus (Cromb.) M. Schultz & M. Prieto, Porocyphus macrosporus (Henssen et al.) M. Schultz & M. Prieto, Porocyphus minutissimus (Henssen) M. Schultz, Porocyphus rosulans (A. Henssen) M. Schultz, Porocyphus tasmanicus (A. Henssen) M. Schultz, Porocyphus willeyi (Tuck.) M. Schultz & M. Prieto, Pseudotichia vermiculata (Nyl.) Schultz & M. Prieto, Pycnolemma polycarpum (M. Schultz) M. Schultz & M. Prieto, Synalissina botryosa (A. Massal.) M. Schultz & M. Prieto, Synalissina cladodes (Tuck.) M. Schultz & M. Prieto, Synalissina condensata (Arnold) M. Schultz & M. Prieto, Synalissina degeliana (P.M. Jørg.) M. Schultz & M. Prieto, Synalissina dispansa (H. Magn.) M. Schultz & M. Prieto, Synalissina intricatissima (J. Steiner) M. Schultz & M. Prieto, Synalissina isidiodes (Nyl. ex Arnold) M. Schultz & M. Prieto, Synalissina vesiculifera (Henssen) M. Schultz & M. Prieto, Thelignya arnoldii (Frauenf.) M. Schultz & M. Prieto, Thelignya lacustris (P.M. Jørg. & R. Sant.) M. Schultz & M. Prieto, Thelignya neglecta (Erichsen) M. Schultz & M. Prieto, Thelignya obtenebrans (Nyl.) M. Schultz, Thyrea osorioi (Henssen) M. Schultz. New status and combination: Gonotichia depauperata (Servít) M. Schultz & M. Prieto. Emended description: Lempholemma Körb., Lichina C. Agardh, Thelignya A. Massal., Lichinaceae Nyl., Phylliscaceae Th. Fr., Porocyphaceae Körb. Resurrection: Cladopsis Nyl., Collemopsis Nyl. ex Crombie, Forssellia Zahlbr., Pleopyrenis Clem., Synalissina Nyl. Citation: Prieto M, Wedin M, Schultz M (2024). Phylogeny, evolution and a re-classification of the Lichinomycetes. Studies in Mycology 109: 595-655. doi: 10.3114/sim.2024.109.09.

Genome-wide differentiation corresponds to climatic niches in two species of lichen-forming fungi

Lichens can withstand fluctuating environmental conditions such as hydration-desiccation cycles. Many species distribute across climate zones, suggesting population-level adaptations to conditions such as freezing and drought. Here, we aim to understand how climate affects population genomic patterns in lichenized fungi. We analysed population structure along elevational gradients in closely related Umbilicaria phaea (North American; two gradients) and Umbilicaria pustulata (European; three gradients). All gradients showed clear genomic breaks splitting populations into low-elevation (Mediterranean zone) and high-elevation (cold temperate zone). A total of 3301 SNPs in U. phaea and 138 SNPs in U. pustulata were driven to fixation between the two ends of the gradients. The difference between the species is likely due to differences in recombination rate: the sexually reproducing U. phaea has a higher recombination rate than the primarily asexually reproducing U. pustulata. Cline analysis revealed allele frequency transitions along all gradients at approximately 0°C, coinciding with the transition between the Mediterranean and cold temperate zones, suggesting freezing is a strong driver of population differentiation. Genomic scans further confirmed temperature-related selection targets. Both species showed similar differentiation patterns overall, but different selected alleles indicate convergent adaptation to freezing. Our results enrich our knowledge of fungal genomic functions related to temperature and climate, fungal population genomics, and species responses to environmental heterogeneity.

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.

A direct PCR approach with low-biomass insert opens new horizons for molecular sciences on cryptogam communities

Molecular sequence data have transformed research on cryptogams (e.g., lichens, microalgae, fungi, and symbionts thereof) but methods are still strongly hampered by the small size and intermingled growth of the target organisms, poor cultivability and detrimental effects of their secondary metabolites. Here, we aim to showcase examples on which a modified direct PCR approach for diverse aspects of molecular work on environmental samples concerning biocrusts, biofilms, and cryptogams gives new options for the research community. Unlike traditional approaches, this methodology only requires biomass equivalent to colonies and fragments of 0.2 mm in diameter, which can be picked directly from the environmental sample, and includes a quick DNA lysis followed by a standardized PCR cycle that allows co-cycling of various organisms/target regions in the same run. We demonstrate that this modified method can (i) amplify the most widely used taxonomic gene regions and those used for applied and environmental sciences from single colonies and filaments of free-living cyanobacteria, bryophytes, fungi, and lichens, including their mycobionts, chlorobionts, and cyanobionts from both isolates and in situ material during co-cycling; (ii) act as a tool to confirm that the dominant lichen photobiont was isolated from the original sample; and (iii) optionally remove inhibitory secondary lichen substances. Our results represent examples which highlight the method's potential for future applications covering mycology, phycology, biocrusts, and lichenology, in particular.IMPORTANCECyanobacteria, green algae, lichens, and other cryptogams play crucial roles in complex microbial systems such as biological soil crusts of arid biomes or biofilms in caves. Molecular investigations on environmental samples or isolates of these microorganisms are often hampered by their dense aggregation, small size, or metabolism products which complicate DNA extraction and subsequent PCRs. Our work presents various examples of how a direct DNA extraction and PCR method relying on low biomass inserts can overcome these common problems and discusses additional applications of the workflow including adaptations.

News from 'black belt' masters of symbiosis

This article is a Commentary on Chrismas et al. (2024), 241: 2243–2257.

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.