Reproductive and dispersal strategies shape the diversity of mycobiont-photobiont association in Cladonia lichens

The application of haplotypes instead of species-level ranks modifies the interpretation of ecological preferences in lichen symbiont interactions in Parmelia

The analysis of the interaction between main bionts (mycobiont and photobiont) in the lichen symbiosis delivers substantial information about their preferences in the selection of symbiotic partners, and their ecological preferences. The selectivity in the Parmelia genus has been defined as strong so far. However, data on this lichen genus, which includes several widely distributed species, are biogeographically limited. Therefore, using specialization indicators and extended sampling, in this study, we estimated the interactions between the main bionts of selected Parmelia spp., using two levels of estimation (species/OTU and haplotype). A comparison of mycobiont-photobiont interactions at different levels showed that considering only mycobiont species and Trebouxia OTUs, greater specialization is found, while Parmelia species studied in this work present a more generalistic strategy in photobiont choice when haplotypes are considered. Despite the uneven sampling of Parmelia species, the interpretation of specialization within species and individuals of the genus leads to a more precise and accurate interpretation of their adaptation strategies. Furthermore, the data from P. sulcata indicate the existence of a different pool of compatible haplotypes in some geographical regions compared to neighboring areas. This observation suggests the potential influence of climatic factors.

Diversity of lichen mycobionts and photobionts and their relationships in the Ny-Ålesund region (Svalbard, High Arctic)

Lichens are dual organisms, with one major mycobiont and one major photobiont in each lichen symbiosis, which can survive extreme environmental conditions in the Arctic. However, the diversity and distribution of lichen photobionts in the Arctic remain poorly understood compared to their mycobiont partners. This study explored the diversity of lichen mycobionts and photobionts in 197 lichen samples collected from the Ny-Ålesund region (Svalbard, High Arctic). The nuclear ribosomal internal transcribed spacer (ITS) regions were sequenced and phylogenetic analyses were performed. The relationships between mycobionts and photobionts, as well as the association patterns, were also investigated. A total of 48 species of lichen mycobionts (16 families, nine orders) and 31 species/lineages of photobionts were identified. These 31 photobiont species belonged to one class (Trebouxiophyceae) and five genera, including 22 species of Trebouxia, five species of Asterochloris, two species of Chloroidium, one species of Symbiochloris, and one species of Coccomyxa. The results indicated that most analyzed lichen mycobionts could associate with multiple photobiont species, and the photobionts also exhibited a similar pattern. The results provided an important reference dataset for characterizing the diversity of lichen mycobionts and photobionts in the High Arctic region.

The co-dispersal strategy of Endocarpon (Verrucariaceae) shapes an unusual lichen population structure

The reproduction and dispersal strategies of lichens play a major role in shaping their population structure and photobiont diversity. Sexual reproduction, which is common, leads to high lichen genetic diversity and low photobiont selectivity. However, the lichen genus Endocarpon adopts a special co-dispersal model in which algal cells from the photobiont and ascospores from the mycobiont are released together into the environment. To explore the dispersal strategy impact on population structures, a total of 62 Endocarpon individuals and 12 related Verrucariaceae genera individuals, representing co-dispersal strategy and conventional independent dispersal mode were studied. Phylogenetic analysis revealed that Endocarpon, with a large-scale geographical distribution, showed an extremely high specificity of symbiotic associations with their photobiont. Furthermore, three types of group I intron at 1769 site have been found in most Endocarpon mycobionts, which showed a high variety of group I intron in the same insertion site even in the same species collected from one location. This study suggested that the ascospore-alga co-dispersal mode of Endocarpon resulted in this unusual mycobiont-photobiont relationship; also provided an evidence for the horizontal transfer of group I intron that may suggest the origin of the complexity and diversity of lichen symbiotic associations.

Bacterial communities associated with an island radiation of lichen-forming fungi

Evolutionary radiations are one of the most striking processes biologists have studied in islands. A radiation is often sparked by the appearance of ecological opportunity, which can originate in processes like trophic niche segregation or the evolution of key innovations. Another recently proposed mechanism is facilitation mediated by the bacterial communities associated with the radiating species. Here we explore the role of the bacterial communities in a radiation of lichen-forming fungi endemic to Macaronesia. Bacterial diversity was quantified by high throughput sequencing of the V1-V2 hyper-variable region of 172 specimens. We characterized the taxonomic and phylogenetic diversity of the bacterial communities associated with the different species, tested for compositional differences between these communities, carried out a functional prediction, explored the relative importance of different factors in bacterial community structure, searched for phylosymbiosis and tried to identify the origin of this pattern. The species of the radiation differed in the composition of their bacterial communities, which were mostly comprised of Alphaproteobacteria and Acidobacteriia, but not in the functionality of those communities. A phylosimbiotic pattern was detected, but it was probably caused by environmental filtering. These findings are congruent with the combined effect of secondary chemistry and mycobiont identity being the main driver of bacterial community structure. Altogether, our results suggest that the associated bacterial communities are not the radiation's main driver. There is one possible exception, however, a species that has an abnormally diverse core microbiome and whose bacterial communities could be subject to a specific environmental filter at the functional level.

Myrmecia, Not Asterochloris, Is the Main Photobiont of Cladonia subturgida (Cladoniaceae, Lecanoromycetes)

This study explores the diversity of photobionts associated with the Mediterranean lichen-forming fungus Cladonia subturgida. For this purpose, we sequenced the whole ITS rDNA region by Sanger using a metabarcoding method for ITS2. A total of 41 specimens from Greece, Italy, France, Portugal, and Spain were studied. Additionally, two specimens from Spain were used to generate four cultures. Our molecular studies showed that the genus Myrmecia is the main photobiont of C. subturgida throughout its geographic distribution. This result contrasts with previous studies, which indicated that the main photobiont for most Cladonia species is Asterochloris. The identity of Myrmecia was also confirmed by ultrastructural studies of photobionts within the lichen thalli and cultures. Photobiont cells showed a parietal chloroplast lacking a pyrenoid, which characterizes the species in this genus. Phylogenetic analyses indicate hidden diversity within this genus. The results of amplicon sequencing showed the presence of multiple ASVs in 58.3% of the specimens studied.

Fungal-Algal Association Drives Lichens' Mutualistic Symbiosis: A Case Study with Trebouxia-Related Lichens

Biotic and abiotic factors influence the formation of fungal-algal pairings in lichen symbiosis. However, the specific determinants of these associations, particularly when distantly related fungi are involved, remain poorly understood. In this study, we investigated the impact of different drivers on the association patterns between taxonomically diverse lichenized fungi and their trebouxioid symbiotic partners. We collected 200 samples from four biomes and identified 41 species of lichenized fungi, associating them with 16 species of trebouxioid green algae, of which 62% were previously unreported. The species identity of both the fungal and algal partners had the most significant effect on the outcome of the symbiosis, compared to abiotic factors like climatic variables and geographic distance. Some obviously specific associations were observed in the temperate zone; however, the nestedness value was lower in arid regions than in cold, polar, and temperate regions according to interaction network analysis. Cophylogenetic analyses revealed congruent phylogenies between trebouxioid algae and associated fungi, indicating a tendency to reject random associations. The main evolutionary mechanisms contributing to the observed phylogenetic patterns were "loss" and "failure to diverge" of the algal partners. This study broadens our knowledge of fungal-algal symbiotic patterns in view of Trebouxia-associated fungi.

Biodiversity of Basidiomycetous Yeasts Associated with Cladonia rei Lichen in Japan, with a Description of Microsporomyces cladoniophilus sp. nov

For more than a century, lichens have been used as an example of dual-partner symbiosis. Recently, this has been challenged by the discovery of various basidiomycetous yeasts that coexist in multiple lichen species, among which Cladonia lichens from Europe and the United States were discovered to be highly specifically associated with the basidiomycetous yeast of the family Microsporomycetaceae. To verify this highly specific relationship, we investigated the diversity of basidiomycetous yeasts associated with Cladonia rei, a widely distributed lichen in Japan, by applying two approaches: yeast isolation from the lichen thalli and meta-barcoding analysis. We obtained 42 cultures of Cystobasidiomycetous yeast which were grouped into six lineages within the family Microsporomycetaceae. Unexpectedly, although the cystobasidiomycetes-specific primer was used, not only the cystobasidiomycetous yeasts but species from other classes were also detected via the meta-barcoding dataset; in particular, pucciniomycetous yeasts were found at a high frequency in some samples. Further, Halobasidium xiangyangense, which was detected in every sample with high abundance, is highly likely a generalist epiphytic fungus that has the ability to associate with C. rei. In the pucciniomycetous group, most of the detected species belong to the scale insect-associated yeast Septobasidium genus. In conclusion, even though Microsporomyces species are not the only yeast group associated with Cladonia lichen, our study demonstrated that the thalli of Cladonia rei lichen could be a suitable habit for them.

A call to reconceptualize lichen symbioses

Several decades of research across disciplines have overturned historical perspectives of symbioses dominated by binary characterizations of highly specific species-species interactions. This paradigm shift has unlocked the previously underappreciated and overlooked dynamism of fungal mutualisms such as mycorrhizae. Lichens are another example of important fungal mutualisms where reconceptualization is urgently needed to realize their potential as model systems. This reconceptualization requires both an objective synthesis of new data and envisioning a revised integrative approach that unifies the spectrum of ecology and evolution. We propose a ten-theme framework that if pursued would propel lichens to the vanguard of symbiotic theory.

Photobiont Diversity in Lichen Symbioses From Extreme Environments

Fungal–algal relationships—both across evolutionary and ecological scales—are finely modulated by the presence of the symbionts in the environments and by the degree of selectivity and specificity that either symbiont develop reciprocally. In lichens, the green algal genus Trebouxia Puymaly is one of the most frequently recovered chlorobionts. Trebouxia species-level lineages have been recognized on the basis of their morphological and phylogenetic diversity, while their ecological preferences and distribution are still only partially unknown. We selected two cosmopolitan species complexes of lichen-forming fungi as reference models, i.e., Rhizoplaca melanophthalma and Tephromela atra, to investigate the diversity of their associated Trebouxia spp. in montane habitats across their distributional range worldwide. The greatest diversity of Trebouxia species-level lineages was recovered in the altitudinal range 1,000–2,500 m a.s.l. A total of 10 distinct Trebouxia species-level lineages were found to associate with either mycobiont, for which new photobionts are reported. One previously unrecognized Trebouxia species-level lineage was identified and is here provisionally named Trebouxia “A52.” Analyses of cell morphology and ultrastructure were performed on axenically isolated strains to fully characterize the new Trebouxia “A52” and three other previously recognized lineages, i.e., Trebouxia “A02,” T. vagua “A04,” and T. vagua “A10,” which were successfully isolated in culture during this study. The species-level diversity of Trebouxia associating with the two lichen-forming fungi in extreme habitats helps elucidate the evolutionary pathways that this lichen photobiont genus traversed to occupy varied climatic and vegetative regimes.

Phylogeny and Ecology of Trebouxia Photobionts From Bolivian Lichens

In the past few years, new phylogenetic lineages in Trebouxia were detected as a result of molecular approaches. These studies included symbiont selectivity in lichen communities, transects along altitudinal gradients at local and global scales and the photobiont diversity in local populations of lichen-forming fungal species. In most of these studies, phylogenetic and haplotype analyses based on the internal transcribed spacer (ITS) locus have continuously allowed the recognition of new monophyletic lineages, which suggests that still numerous undiscovered Trebouxia lineages can be hidden in lichens from unexplored areas, especially in the tropics. Here, we estimated the biodiversity of photobionts in Bolivian Andean vegetation and assessed their specificity. About 403 lichen samples representing 42 genera, e.g., Haematomma, Heterodermia, Hypotrachyna, Lecanora, Lepra, Leucodermia, Parmotrema, Pertusaria, Polyblastidium, and Usnea, containing Trebouxia photobionts, were analyzed. ITS ribosomal DNA (rDNA) and rbcL markers were used. We obtained Trebouxia sequences from Bolivian samples belonging to already described clades A, C, I, and S. Thirty-nine Trebouxia lineages were distinguished within these clades, while 16 were new. To reveal the structure of the community of Bolivian photobionts and their relationships with mycobionts, the comparative effects of climate, altitude, geographical distances, substrate, and habitat type, as well as functional traits of lichens such as growth forms, propagation mode and secondary metabolites, were analyzed. Furthermore, new Bolivian records were included in analysis on a global scale. In our study, the mycobiont genus or even species are the most important factors correlated with photobiont identity. Moreover, we revealed that the community of Bolivian photobionts is shaped by altitude.

An Exception to the Rule? Could Photobiont Identity Be a Better Predictor of Lichen Phenotype than Mycobiont Identity?

With rare exceptions, the shape and appearance of lichen thalli are determined by the fungal partner; thus, mycobiont identity is normally used for lichen identification. However, it has repeatedly been shown in recent decades that phenotypic data often does not correspond with fungal gene evolution. Here, we report such a case in a three-species complex of red-fruited Cladonia lichens, two of which clearly differ morphologically, chemically, ecologically and in distribution range. We analysed 64 specimens of C. bellidiflora, C. polydactyla and C. umbricola, mainly collected in Europe, using five variable mycobiont-specific and two photobiont-specific molecular markers. All mycobiont markers exhibited very low variability and failed to separate the species. In comparison, photobiont identity corresponded better with lichen phenotype and separated esorediate C. bellidiflora from the two sorediate taxa. These results can be interpreted either as an unusual case of lichen photomorphs or as an example of recent speciation, in which phenotypic differentiation precedes the separation of the molecular markers. We hypothesise that association with different photobionts, which is probably related to habitat differentiation, may have triggered speciation in the mycobiont species.

Promiscuity in Lichens Follows Clear Rules: Partner Switching in Cladonia Is Regulated by Climatic Factors and Soil Chemistry

Climatic factors, soil chemistry and geography are considered as major factors affecting lichen distribution and diversity. To determine how these factors limit or support the associations between the symbiotic partners, we revise the lichen symbiosis as a network of relationships here. More than one thousand thalli of terricolous Cladonia lichens were collected at sites with a wide range of soil chemical properties from seven biogeographical regions of Europe. A total of 18 OTUs of the algal genus Asterochloris and 181 OTUs of Cladonia mycobiont were identified. We displayed all realized pairwise mycobiont-photobiont relationships and performed modularity analysis. It revealed four virtually separated modules of cooperating OTUs. The modules differed in mean annual temperature, isothermality, precipitation, evapotranspiration, soil pH, nitrogen, and carbon contents. Photobiont switching was strictly limited to algae from one module, i.e., algae of similar ecological preferences, and only few mycobionts were able to cooperate with photobionts from different modules. Thus, Cladonia mycobionts generally cannot widen their ecological niches through photobiont switching. The modules also differed in the functional traits of the mycobionts, e.g., sexual reproduction rate, presence of soredia, and thallus type. These traits may represent adaptations to the environmental conditions that drive the differentiation of the modules. In conclusion, the promiscuity in Cladonia mycobionts is strictly limited by climatic factors and soil chemistry.

Choosing the Right Life Partner: Ecological Drivers of Lichen Symbiosis

Lichens are an iconic example of symbiotic systems whose ecology is shaped by the requirements of the symbionts. Previous studies suggest that fungal (mycobionts) as well as photosynthesizing (phycobionts or cyanobionts) partners have a specific range of acceptable symbionts that can be chosen according to specific environmental conditions. This study aimed to investigate the effects of climatic conditions and mycobiont identity on phycobiont distribution within the lichen genera Stereocaulon, Cladonia, and Lepraria. The study area comprised the Canary Islands, Madeira, Sicily, and the Aeolian Islands, spanning a wide range of climatic conditions. These islands are known for their unique and diverse fauna and flora; however, lichen phycobionts have remained unstudied in most of these areas. In total, we genetically analyzed 339 lichen samples. The phycobiont pool differed significantly from that outside the studied area. Asterochloris mediterranea was identified as the most abundant phycobiont. However, its distribution was limited by climatic constraints. Other species of Asterochloris and representatives of the genera Chloroidium, Vulcanochloris, and Myrmecia were also recovered as phycobionts. The selection of symbiotic partners from the local phycobiont pool was driven by mycobiont specificity (i.e., the taxonomic range of acceptable partners) and the environmental conditions, mainly temperature. Interestingly, the dominant fungal species responded differently in their selection of algal symbionts along the environmental gradients. Cladonia rangiformis associated with its phycobiont A. mediterranea in a broader range of temperatures than Stereocaulon azoreum, which favors other Asterochloris species along most of the temperature gradient. Stereocaulon vesuvianum associated with Chloroidium spp., which also differed in their temperature optima. Finally, we described Stereocaulon canariense as a new endemic species ecologically distinct from the other Stereocaulon species on the Canary Islands.

Biodiversity Patterns and Ecological Preferences of the Photobionts Associated With the Lichen-Forming Genus Parmelia

The worldwide, ecologically relevant lichen-forming genus Parmelia currently includes 41 accepted species, of which the Parmelia sulcata group (PSULgp) and the Parmelia saxatilis group (PSAXgp) have received considerable attention over recent decades; however, phycobiont diversity is poorly known in Parmelia s. lat. Here, we studied the diversity of Trebouxia microalgae associated with 159 thalli collected from 30 locations, including nine Parmelia spp.: P. barrenoae, P. encryptata, P. ernstiae, P. mayi, P. omphalodes, P. saxatilis, P. serrana, P. submontana, and P. sulcata. The mycobionts were studied by carrying out phylogenetic analyses of the nrITS. Microalgae genetic diversity was examined by using both nrITS and LSU rDNA markers. To evaluate putative species boundaries, three DNA species delimitation analyses were performed on Trebouxia and Parmelia. All analyses clustered the mycobionts into two main groups: PSULgp and PSAXgp. Species delimitation identified 13 fungal and 15 algal species-level lineages. To identify patterns in specificity and selectivity, the diversity and abundance of the phycobionts were identified for each Parmelia species. High specificity of each Parmelia group for a given Trebouxia clade was observed; PSULgp associated only with clade I and PSAXgp with clade S. However, the degree of specificity is different within each group, since the PSAXgp mycobionts were less specific and associated with 12 Trebouxia spp., meanwhile those of PSULgp interacted only with three Trebouxia spp. Variation-partitioning analyses were conducted to detect the relative contributions of climate, geography, and symbiotic partner to phycobiont and mycobiont distribution patterns. Both analyses explained unexpectedly high portions of variability (99 and 98%) and revealed strong correlations between the fungal and algal diversity. Network analysis discriminated seven ecological clusters. Even though climatic conditions explained the largest proportion of the variation among these clusters, they seemed to show indifference relative to climatic parameters. However, the cluster formed by P. saxatilis A/P. saxatilis B/Trebouxia sp. 2/Trebouxia sp. S02/Trebouxia sp. 3A was identified to prefer cold-temperate as well as humid summer environments.

Global Biodiversity Patterns of the Photobionts Associated with the Genus Cladonia (Lecanorales, Ascomycota)

The diversity of lichen photobionts is not fully known. We studied here the diversity of the photobionts associated with Cladonia, a sub-cosmopolitan genus ecologically important, whose photobionts belong to the green algae genus Asterochloris. The genetic diversity of Asterochloris was screened by using the ITS rDNA and actin type I regions in 223 specimens and 135 species of Cladonia collected all over the world. These data, added to those available in GenBank, were compiled in a dataset of altogether 545 Asterochloris sequences occurring in 172 species of Cladonia. A high diversity of Asterochloris associated with Cladonia was found. The commonest photobiont lineages associated with this genus are A. glomerata, A. italiana, and A. mediterranea. Analyses of partitioned variation were carried out in order to elucidate the relative influence on the photobiont genetic variation of the following factors: mycobiont identity, geographic distribution, climate, and mycobiont phylogeny. The mycobiont identity and climate were found to be the main drivers for the genetic variation of Asterochloris. The geographical distribution of the different Asterochloris lineages was described. Some lineages showed a clear dominance in one or several climatic regions. In addition, the specificity and the selectivity were studied for 18 species of Cladonia. Potentially specialist and generalist species of Cladonia were identified. A correlation was found between the sexual reproduction frequency of the host and the frequency of certain Asterochloris OTUs. Some Asterochloris lineages co-occur with higher frequency than randomly expected in the Cladonia species.

Thallus Growth Stage and Geographic Origin Shape Microalgal Diversity in Ramalina farinacea Lichen Holobionts

Lichen symbioses are microecosystems hosting many other living organisms besides the two major lichen symbionts (i.e., lichenized fungi [the mycobiont] and green microalgae or cyanobacteria [the photobiont]). Recent investigations evidenced that other fungi, non-photosynthetic bacteria, and microalgae co-inhabit within the lichen thalli, but their diversity and their roles are still underinvestigated. Here we present an ad hoc stratified sampling design and in-depth Illumina paired-end metabarcoding approach to explore microalgal diversity in lichen thalli of the model species Ramalina farinacea from different ecologies. Lichen thalli were surveyed according to three different sizes, and different thallus parts were considered for molecular, bioinformatics, and community diversity analyses. The results revealed that microalgal diversity strongly depends on the growth stage of the thalli, the geographic area, and the habitat type. The results also show that microalgal diversity does not vary along the thallus branches (lacinias)-that is, it does not correlate with the apical growth and founder effects-and that there is no balanced co-presence of two main photobionts as previously established in R. farinacea. The sampling design performed here minimizes bias in the assessment of photobiont diversity in lichens and is proposed to be reliable and applicable to further study microalgal diversity in lichen symbioses.

New lineages of photobionts in Bolivian lichens expand our knowledge on habitat preferences and distribution of Asterochloris algae

We studied the biodiversity of Asterochloris photobionts found in Bolivian lichens to better understand their global spatial distribution and adaptation strategies in the context of a worldwide phylogeny of the genus. Based on nuclear ITS rDNA, the chloroplast rbcL gene and the actin type I gene we reconstructed a phylogenetic tree that recovered nine new Asterochloris lineages, while 32 Bolivian photobiont samples were assigned to 12 previously recognized Asterochloris lineages. We also show that some previously discovered Asterochloris photobiont species and lineages may occur in a broader spectrum of climatic conditions, and mycobiont species and photobionts may show different preferences along an altitude gradient. To reveal general patterns of of mycobiont specificity towards the photobiont in Asterochloris, we tested the influence of climate, altitude, geographical distance and effects of symbiotic partner (mycobiont) at the species level of three genera of lichen forming fungi: Stereocaulon, Cladonia and Lepraria. Further, we compared the specificity of mycobionts towards Asterochloris photobionts in cosmopolitan, Neotropical, and Pantropical lichen forming fungi. Interestingly, cosmopolitan species showed the lowest specificity to their photobionts, but also the lowest haplotype diversity. Neotropical and Paleotropical mycobionts, however, were more specific.

Heavy-metal pollution induces changes in the genetic composition and anatomical properties of photobionts in pioneer lichens colonising post-industrial habitats

Certain lichens are effective colonisers of polluted sites. However, little is known about the tolerance of photobionts and the degree of mycobiont selectivity to photobionts relative to metal pollution. The present study recognises the genetic and anatomical diversity of Asterochloris photobionts in epigeic lichens, i.e. Cladonia cariosa, C. rei, and Diploschistes muscorum, in relation to a wide spectrum of soil pollution. In accordance with phylogenetic analysis, photobionts were clustered in 7 moderately- to well-supported clades, including 19 haplotypes. The mycobionts of all studied lichens demonstrated a low level of selectivity and were capable of associating with various Asterochloris lineages. This tendency was also expressed by the frequent (~25%) occurrence of multiple algal genotypes in a single thallus. This indicates that identified Asterochloris lineages are generally tolerant to heavy-metal pollution, and the low level of selectivity of mycobionts enables them to select the most suitable and/or available partner. The trend of increasing incidence of certain Asterochloris lineages and decreasing frequency of others along with increasing soil pollution was observed. This proves the superior adaptation of some photobionts to polluted sites. Such symbiotic plasticity constitute an adaptive feature necessary for the successful colonisation. High number of haplotypes at polluted sites could be the result of multiple introduction events from different areas during the initial stages of spontaneous succession. Regardless of the genetic pattern, Asterochloris cells were considerably smaller, and the density and compaction of cells in the algal layer were higher, in lichen specimens from polluted sites, indicating that photobiont characteristics may be closely dependent on heavy-metal pollution.

Contrasting co-occurrence patterns of photobiont and cystobasidiomycete yeast associated with common epiphytic lichen species

The popular dual definition of lichen symbiosis is under question with recent findings of additional microbial partners living within the lichen body. Here we compare the distribution and co-occurrence patterns of lichen photobiont and recently described secondary fungus (Cyphobasidiales yeast) to evaluate their dependency on lichen host fungus (mycobiont). We sequenced the nuclear internal transcribed spacer (ITS) strands for mycobiont, photobiont, and yeast from six widespread northern hemisphere epiphytic lichen species collected from 25 sites in Switzerland and Estonia. Interaction network analyses and multivariate analyses were conducted on operational taxonomic units based on ITS sequence data. Our study demonstrates the frequent presence of cystobasidiomycete yeasts in studied lichens and shows that they are much less mycobiont-specific than the photobionts. Individuals of different lichen species growing on the same tree trunk consistently hosted the same or closely related mycobiont-specific Trebouxia lineage over geographic distances while the cystobasidiomycete yeasts were unevenly distributed over the study area - contrasting communities were found between Estonia and Switzerland. These results contradict previous findings of high mycobiont species specificity of Cyphobasidiales yeast at large geographic scales. Our results suggest that the yeast might not be as intimately associated with the symbiosis as is the photobiont.

Symbiotic microalgal diversity within lichenicolous lichens and crustose hosts on Iberian Peninsula gypsum biocrusts

This study analyses the interactions among crustose and lichenicolous lichens growing on gypsum biocrusts. The selected community was composed of Acarospora nodulosa, Acarospora placodiiformis, Diploschistes diacapsis, Rhizocarpon malenconianum and Diplotomma rivas-martinezii. These species represent an optimal system for investigating the strategies used to share phycobionts because Acarospora spp. are parasites of D. diacapsis during their first growth stages, while in mature stages, they can develop independently. R. malenconianum is an obligate lichenicolous lichen on D. diacapsis, and D. rivas-martinezii occurs physically close to D. diacapsis. Microalgal diversity was studied by Sanger sequencing and 454-pyrosequencing of the nrITS region, and the microalgae were characterized ultrastructurally. Mycobionts were studied by performing phylogenetic analyses. Mineralogical and macro- and micro-element patterns were analysed to evaluate their influence on the microalgal pool available in the substrate. The intrathalline coexistence of various microalgal lineages was confirmed in all mycobionts. D. diacapsis was confirmed as an algal donor, and the associated lichenicolous lichens acquired their phycobionts in two ways: maintenance of the hosts' microalgae and algal switching. Fe and Sr were the most abundant microelements in the substrates but no significant relationship was found with the microalgal diversity. The range of associated phycobionts are influenced by thallus morphology.

Cophylogenetic patterns in algal symbionts correlate with repeated symbiont switches during diversification and geographic expansion of lichen-forming fungi in the genus Sticta (Ascomycota, Peltigeraceae)

Species in the fungal genus Sticta form symbiotic associations primarily with either green algae or cyanobacteria, but tripartite associations or photosymbiodemes involving both types of photobionts occur in some species. Sticta is known to associate with green algae in the genus Symbiochloris. However, previous studies have shown that algae from other genera, such as Heveochlorella, may also be suitable partners for Sticta. We examined the diversity of green algal partners in the genus Sticta and assessed the patterns of association between the host fungus and its algal symbiont. We used multi-locus sequence data from multiple individuals collected in Australia, Cuba, Madagascar, Mauritius, New Zealand, Reunion and South America to infer phylogenies for fungal and algal partners and performed tests of congruence to assess coevolution between the partners. In addition, event-based methods were implemented to examine which cophylogenetic processes have led to the observed association patterns in Sticta and its green algal symbionts. Our results show that in addition to Symbiochloris, Sticta associates with green algae from the genera Chloroidium, Coccomyxa, Elliptochloris and Heveochlorella, the latter being the most common algal symbiont associated with Sticta in this study. Geography plays a strong role in shaping fungal-algal association patterns in Sticta as mycobionts associate with different algal lineages in different geographic locations. While fungal and algal phylogenies were mostly congruent, event-based methods did not find any evidence for cospeciation between the partners. Instead, the association patterns observed in Sticta and associated algae, were largely explained by other cophylogenetic events such as host-switches, losses of symbiont and failure of the symbiont to diverge with its host. Our results also show that tripartite associations with green algae evolved multiple times in Sticta.

Formally described species woefully underrepresent phylogenetic diversity in the common lichen photobiont genus Trebouxia (Trebouxiophyceae, Chlorophyta): An impetus for developing an integrated taxonomy

Lichens provide valuable systems for studying symbiotic interactions. In lichens, these interactions are frequently described in terms of availability, selectivity and specificity of the mycobionts and photobionts towards one another. The lichen-forming, green algal genus Trebouxia Puymaly is among the most widespread photobiont, associating with a broad range of lichen-forming fungi. To date, 29 species have been described, but studies consistently indicate that the vast majority of species-level lineages still lack formal description, and new, previously unrecognized lineages are frequently reported. To reappraise the diversity and the evolutionary relationships of species-level lineages in Trebouxia, we assembled DNA sequence data from over 1600 specimens, compiled from a range of sequences from previously published studies, axenic algal cultures, and lichens collected from poorly sampled regions. From these samples, we selected representatives of the currently known genetic diversity in the lichenized Trebouxia and inferred a phylogeny from multi-locus sequence data (ITS, rbcL, cox2). We demonstrate that the current formally described species woefully underrepresent overall species-level diversity in this important lichen-forming algal genus. We anticipate that an integrative taxonomic approach, incorporating morphological and physiological data from axenic cultures with genetic data, will be required to establish a robust, comprehensive taxonomy for Trebouxia. The data presented here provide an important impetus and reference dataset for more reliably characterizing diversity in lichenized algae and in using lichens to investigate the evolution of symbioses and holobionts.