Population genetics of lichen symbiosis
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Congruent Genetic Structure in the Lichen-Forming Fungus Lobaria pulmonaria and Its Green-Algal Photobiont

Congruent Genetic Structure in the Lichen-Forming Fungus Lobaria pulmonaria and Its Green-Algal Photobiont | Population genetics of lichen symbiosis | Scoop.it

The extent of codispersal of symbionts is one of the key factors shaping genetic structures of symbiotic organisms. Concordant patterns of genetic structure are expected in vertically transmitted symbioses, whereas horizontal transmission generally uncouples genetic structures unless the partners are coadapted. Here, we compared the genetic structures of mutualists, the lichen-forming fungus Lobaria pulmonaria and its primary green-algal photobiont, Dictyochloropsis reticulata. We performed analysis of molecular variance and variogram analysis to compare genetic structures between symbiosis partners. We simulated the expected number of multilocus-genotype recurrences to reveal whether the distribution of multilocus genotypes of either species was concordant with panmixia. Simulations and tests of linkage disequilibrium provided compelling evidence for the codispersal of mutualists. To test whether genotype associations between symbionts were consistent with randomness, as expected under horizontal transmission, we simulated the recurrence of fungal-algal multilocus genotype associations expected by chance. Our data showed nonrandom associations of fungal and algal genotypes. Either vertical transmission or horizontal transmission coupled with coadaptation between symbiont genotypes may have created these nonrandom associations. This study is among the first to show codispersal and highly congruent genetic structures in the partners of a lichen mutualism.

 

Werth S, Scheidegger C, 2012. Congruent genetic structure in the lichen-forming fungus Lobaria pulmonaria and its green-algal photobiont. Molecular Plant-Microbe Interactions 25, 220-230.

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Population genetics of lichen-forming fungi – a review

Population genetics of lichen-forming fungi – a review | Population genetics of lichen symbiosis | Scoop.it

Population genetics investigates the distribution of genetic variation in natural populations and the genetic differentiation among populations. Lichen-forming fungi are exciting subjects for population genetic studies due to their obligate symbiosis with a green-algal and/or cyanobacterial photobiont, and because their different reproductive strategies could influence fungal genetic structures in various ways. In this review, first, I briefly summarize the results from studies of chemotype variation in populations of lichen-forming fungi. Second, I compare and evaluate the DNA-based molecular tools available for population genetics of lichen-forming fungi. Third, I review the literature available on the genetic structure of lichen fungi to show general trends. I discuss some fascinating examples, and point out directions for future research.

 

Werth S, 2010. Population genetics of lichen-forming fungi – a review. Lichenologist 42, 499-519.

 

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Ecophysiology and genetic structure of polar versus temperate populations of the lichen Cetraria aculeata

Ecophysiology and genetic structure of polar versus temperate populations of the lichen Cetraria aculeata | Population genetics of lichen symbiosis | Scoop.it

We studied polar and temperate samples of the lichen Cetraria aculeata to investigate whether genetical differences between photobionts are correlated with physiological properties of the lichen holobiont. Net photosynthesis and dark respiration (DR) at different temperatures (from 0 to 30 °C) and photon flux densities (from 0 to 1,200 μmol m−2 s−1) were studied for four populations of Cetraria aculeata. Samples were collected from maritime Antarctica, Svalbard, Germany and Spain, representing different climatic situations. Sequencing of the photobiont showed that the investigated samples fall in the polar and temperate clade described in Fernández-Mendoza et al. (Mol Ecol 20:1208–1232, 2011). Lichens with photobionts from these clades differ in their temperature optimum for photosynthesis, maximal net photosynthesis, maximal DR and chlorophyll content. Maximal net photosynthesis was much lower in Antarctica and Svalbard than in Germany and Spain. The difference was smaller when rates were expressed by chlorophyll content. The same is true for the temperature optima of polar (11 °C) and temperate (15 and 17 °C) lichens. Our results indicate that lichen mycobionts may adapt or acclimate to local environmental conditions either by selecting algae from regional pools or by regulating algal cell numbers (chlorophyll content) within the thallus.

 

Domaschke S, Vivas M, Sancho LG, Printzen C, 2013. Ecophysiology and genetic structure of polar versus temperate populations of the lichen Cetraria aculeata. Oecologia, 1-11.

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Phylogenetic Species, Reproductive Mode, and Specificity of the Green Alga Trebouxia Forming Lichens with the Fungal Genus Letharia

Phylogenetic Species, Reproductive Mode, and Specificity of the Green Alga Trebouxia Forming Lichens with the Fungal Genus Letharia | Population genetics of lichen symbiosis | Scoop.it

The green algal symbionts of the lichenized fungus Letharia are identified as members of the morphospecies Trebouxia jamesii, based on ITS sequences

 

that place them in a molecularly diverse clade with other lichen algae also identified as T. jamesii. However, based on additional sequence from a second locus, actin, T. jamesii is seen to consist of many phylogenetic species. A clade of six to seven closely related phylogenetic species of T. jamesii are symbiotic with five of the six phylogenetic species of Letharia. Within this broad level of specificity, there are also narrower patterns of specificity. For example, three cryptic species of T. jamesii were found only in two species of Letharia in California, a pattern that may be due either to biogeography, or to Letharia phylogenetically tracking Trebouxia. Allopatric Letharia vulpina has switched to a genetically distant phylogenetic species within the T. jamesii morphospecies, suggesting that habitat plays a role in specificity. One phylogenetic species of T. jamesii was found to have a recombining population structure, as two alleles of two loci were found in all four pairwise combinations among the haploid genotypes of that species. This result suggests that Trebouxia, a genus in which sexual structures have never been observed, is nevertheless recombining.

 

Scott Kroken and John W. Taylor (2000) Phylogenetic Species, Reproductive Mode, and Specificity of the Green Alga Trebouxia Forming Lichens with the Fungal Genus Letharia. The Bryologist: Vol. 103, No.

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Identity and genetic structure of the photobiont of the epiphytic lichen Ramalina menziesii on three oak species in southern California

Identity and genetic structure of the photobiont of the epiphytic lichen Ramalina menziesii on three oak species in southern California | Population genetics of lichen symbiosis | Scoop.it

Lichens, a classic example of an obligate symbiosis between fungi and photobionts (which could be algae or cyanobacteria), are abundant in many terrestrial ecosystems. The genetic structure of the photobiont population found in association with a lichen-forming fungal species could be affected by fungal reproductive mode and by the spatial extent of gene flow in the photobiont. Using DNA sequences from one nuclear ribosomal and two chloroplast loci, we analyzed the genetic structure of the photobiont associated with the fungus Ramalina menziesii at an oak woodland study site in southern California. We had previously shown that the fungus exhibited no genetic structure among four local sites or three phorophyte species. Our goals were to identify the photobiont species and assess its genetic structure. We found that R. menziesii was highly specific in its photobiont choice and associated with one alga, Trebouxia decolorans. In contrast to the fungal population, we found significant differentiation among the algae sampled on three oak species and little genetic structure among the sites for two of the three algal loci. We hypothesize that R. menziesii is locally adapted to the phorophyte species through habitat specialization in the algal partner of the symbiosis.

 

Werth S, Sork VL, 2010. Identity and genetic structure of the photobiont of the epiphytic lichen Ramalina menziesii on three oak species in southern California. American Journal of Botany 97, 821-830

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Algal switching among lichen symbioses

Lichens are intimate and long-term symbioses of algae and fungi. Such intimate associations are often hypothesized to have undergone long periods of symbiotic interdependence and coevolution. However, coevolution has not been rigorously tested for lichen associations. In the present study we compared the nuclear internal transcribed spacer (ITS) phylogenies of algal and fungal partners from 33 natural lichen associations to test two aspects of coevolution, cospeciation and parallel cladogenesis. Since statistically significant incongruence between symbiont phylogenies rejected parallel cladogenesis and minimized cospeciation events, we conclude that switching of highly selected algal genotypes occurs repeatedly among these symbiotic lichen associations.

 

Piercey-Normore MD, DePriest PT, 2001. Algal switching among lichen symbioses. American Journal of Botany 88, 1490–1498.

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Strong fungal specificity and selectivity for algal symbionts in Florida scrub Cladonia lichens

Strong fungal specificity and selectivity for algal symbionts in Florida scrub Cladonia lichens | Population genetics of lichen symbiosis | Scoop.it

Symbiosis is a major theme in the history of life and can be an important force driving evolution. However, across symbioses, it is difficult to tease apart the mechanisms that structure the interactions among potential partners. We used genetic similarity and frequency-based methods to qualitatively and quantitatively examine the patterns of association among several co-occurring Cladonia lichen fungi and their algal photobionts in six disjunct Florida scrub sites. The patterns of association were described by the degree of specificity, i.e. the phylogenetic range of associated partners, and of selectivity, i.e. the frequency of association among partners. Six fungal species associated with only one algal internal transcribed spacer clade, with the remaining two fungi being associated with two algal clades. In all cases, the fungi associated in unequal frequencies with the observed algal photobiont genotypes within those clades — suggesting that both specificity and selectivity were higher than expected. Fungal species can be grouped into three significantly different specificity classes: photobiont specialists, intermediates and generalists. In contrast to the pronounced specificity for photobionts among fungal species, the different Florida scrub sites do not harbour distinct photobiont pools, and differential photobiont availability cannot explain the patterning of lichen associations at this spatial scale. Therefore, we conclude that fungal specificity and selectivity for algal photobionts are major factors in determining the local composition of symbiotic partnerships.

 

Yahr R, Vilgalys R, DePriest PT, 2004. Strong fungal specificity and selectivity for algal symbionts in Florida scrub Cladonia lichens. Molecular Ecology 13, 3367-3378.

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Landscape-level gene flow in Lobaria pulmonaria, an epiphytic lichen

Landscape-level gene flow in Lobaria pulmonaria, an epiphytic lichen | Population genetics of lichen symbiosis | Scoop.it

Epiphytes are strongly affected by the population dynamics of their host trees. Owing to the spatio-temporal dynamics of host tree populations, substantial dispersal rates--corresponding to high levels of gene flow--are needed for populations to persist in a landscape. However, several epiphytic lichens have been suggested to be dispersal-limited, which leads to the expectation of low gene flow at the landscape scale. Here, we study landscape-level genetic structure and gene flow of a putatively dispersal-limited epiphytic lichen, Lobaria pulmonaria. The genetic structure of L. pulmonaria was quantified at three hierarchical levels, based on 923 thalli collected from 41 plots situated within a pasture-woodland landscape and genotyped at six fungal microsatellite loci. We found significant isolation by distance, and significant genetic differentiation both among sampling plots and among trees. Landscape configuration, i.e. the effect of a large open area separating two forested regions, did not leave a traceable pattern in genetic structure, as assessed with partial Mantel tests and analysis of molecular variance. Gene pools were spatially intermingled in the pasture-woodland landscape, as determined by Bayesian analysis of population structure. Evidence for local gene flow was found in a disturbed area that was mainly colonized from nearby sources. Our analyses indicated high rates of gene flow of L. pulmonaria among forest patches, which may reflect the historical connectedness of the landscape through gene movement. These results support the conclusion that dispersal in L. pulmonaria is rather effective, but not spatially unrestricted.

 

Werth S, Gugerli F, Holderegger R, Wagner HH, Csencsics D, Scheidegger C, 2007. Landscape-level gene flow in Lobaria pulmonaria, an epiphytic lichen. Molecular Ecology 16, 2807-2815.

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European phylogeography of the epiphytic lichen fungus Lobaria pulmonaria and its green algal symbiont

European phylogeography of the epiphytic lichen fungus Lobaria pulmonaria and its green algal symbiont | Population genetics of lichen symbiosis | Scoop.it

In lichen symbiosis, fungal and algal partners form close associations, often codispersed by vegetative propagules. Due to the particular interdependence, processes such as colonization, dispersal or genetic drift are expected to result in congruent patterns of genetic structure in the symbionts. To study the population structure of an obligate symbiotic system in Europe, we genotyped the fungal and algal symbionts of the epiphytic lichen Lobaria pulmonaria at eight and seven microsatellite loci, respectively, and analysed about 4300 L. pulmonaria thalli from 142 populations from the species' European distribution range. Based on a centroid approach, which localizes centres of genetic differentiation with a high frequency of geographically restricted alleles, we identified the South Italy–Balkan region as the primary glacial refugial area of the lichen symbiosis. Procrustean rotation analysis and a distance congruence test between the fungal and algal population graphs indicated general concordance between the phylogeographies of the symbionts. The incongruent patterns found in areas of postglacial recolonization may show the presence of an additional refugial area for the fungal symbiont, and the impact that horizontal photobiont transmission and different mutation rates of the symbionts have on their genotypic associations at a continental scale.

 

Widmer I, Dal Grande F, Excoffier L, Holderegger R, Keller C, Mikryukov VS, Scheidegger C, 2012. European phylogeography of the epiphytic lichen fungus Lobaria pulmonaria and its green algal symbiont. Molecular Ecology 21, 5827–5844.

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PLOS ONE: Genetic Basis of Self-Incompatibility in the Lichen-Forming Fungus Lobaria pulmonaria and Skewed Frequency Distribution of Mating-Type Idiomorphs: Implications for Conservation

PLOS ONE: Genetic Basis of Self-Incompatibility in the Lichen-Forming Fungus Lobaria pulmonaria and Skewed Frequency Distribution of Mating-Type Idiomorphs: Implications for Conservation | Population genetics of lichen symbiosis | Scoop.it

Fungal populations that reproduce sexually are likely to be genetically more diverse and have a higher adaptive potential than asexually reproducing populations. Our study provides direct evidence of self-incompatibility (heterothallism) in L. pulmonaria. It can thus be hypothesized that sexual reproduction in small populations might be limited by an unbalanced distribution of mating-type genes. We therefore assessed neutral genetic diversity (using microsatellites) and mating-type ratio in 27 lichen populations (933 individuals). We found significant differences in the frequency of the two mating types in 13 populations, indicating a lower likelihood of sexual reproduction in these populations. This suggests that conservation translocation activities aiming at maximizing genetic heterogeneity in threatened and declining populations should take into account not only presence of fruiting bodies in transplanted individuals, but also the identity and balanced representation of mating-type genes.

 

Singh G, Dal Grande F, Cornejo C, Schmitt I, Scheidegger C, 2012. Genetic basis of self-incompatibility in the lichen-forming fungus Lobaria pulmonaria and skewed frequency distribution of mating-type idiomorphs: implications for conservation. PLoS ONE 7, e51402.

 

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Outcrossing and Recombination in the Lichenized Fungus Letharia

Outcrossing and Recombination in the Lichenized Fungus Letharia | Population genetics of lichen symbiosis | Scoop.it

We report evidence for recombination in lichenized fungi based on sequenced nuclear DNA markers, judging from the incongruence of their gene genealogies. Recombining population structures were found in two phylogenetic species of Letharia, one species that is observed in nature to produce abundant sexual structures (ascomata) and another species that produces abundant clonal reproductive structures (soredia) and only rarely produces ascomata. To determine whether sexual reproduction was the cause of recombination in both species, we compared several variable loci in the ascomata and maternal tissue for evidence of outcrossing. All ascomata of both species were heterozygous for at least one locus, as would be expected to result from outcrossing and not from selling. Therefore, it appears that even in the sorediate species, rare sexual reproduction results in recombination.

 

Kroken S, Taylor JW, 2001. Outcrossing and recombination in the lichenized fungus Letharia. Fungal Genetics and Biology 34, 83-92.

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Genetic combinations of symbionts in a vegetatively reproducing lichen, Parmotrema tinctorum, based on ITS rDNA sequences

Genetic combinations of symbionts in a vegetatively reproducing lichen, Parmotrema tinctorum, based on ITS rDNA sequences | Population genetics of lichen symbiosis | Scoop.it

The genetic combinations between mycobionts and photobionts in Parmotrema tinctorum collected from ca. 60 km(2) of the Shimizu district, of Shizuoka City in Japan was investigated based on ITS rDNA sequences. This lichen produces apothecia quite rarely, and in principle propagates vegetatively by isidia. The genetic diversity of the mycobiont comprised four types, while that of the photobiont comprised 21 types. There were 28 different combinations between mycobiont and photobiont. All the photobionts were identified as Trebouxia corticola (s. lat.), based on both molecular phylogenetic results and morphological observation of culture strains obtained in this study. Therefore, P. tinctorum is considered to be highly selective toward the photobiont. The 28 combinations from the small area represent an unexpectedly high diversity, because P. tinctorum is thought to propagate vegetatively. Four possible mechanisms to account the high genetic combinations are suggested: i.e., photobiont exchange, fusion of thalli, and long-distance dispersal of isidia or ascospores. The genetic diversity of photobionts was poor in the urban area, but rich in suburbs and mountainsides. This might be caused by a bottleneck or founder effect in the population recovering from former damage by heavy air pollution, or variable selectivity of P. tinctorum depending on the environments.

 

Ohmura Y, Kawachi M, Kasai F, Watanabe MM, Takeshita S, 2006. Genetic combinations of symbionts in a vegetatively reproducing lichen, Parmotrema tinctorum, based on ITS rDNA sequences. Bryologist 109, 43-59.

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Low genetic diversity in Antarctic populations of the lichenforming ascomycete Cetraria aculeata and its photobiont

Low genetic diversity in Antarctic populations of the lichenforming ascomycete Cetraria aculeata and its photobiont | Population genetics of lichen symbiosis | Scoop.it

Lichens, symbiotic associations of fungi (mycobionts) and green algae or cyanobacteria (photobionts), are poikilohydric organisms that are particularly well adapted to withstand adverse environmental conditions. Terrestrial ecosystems of the Antarctic are therefore largely dominated by lichens. The effects of global climate change are especially pronounced in the maritime Antarctic and it may be assumed that the lichen vegetation will profoundly change in the future. The genetic diversity of populations is closely correlated to their ability to adapt to changing environmental conditions and to their future evolutionary potential. In this study, we present evidence for low genetic diversity in Antarctic mycobiont and photobiont populations of the widespread lichen Cetraria aculeata. The mycobiont diversity is highest in Arctic populations, while the photobionts are most diverse in temperate regions. Photobiont diversity decreases significantly towards the Antarctic but less markedly towards the Arctic, indicating that ecological factors play a minor role in determining the diversity of Antarctic photobiont populations. Richness estimators calculated for the four geographical regions suggest that the low genetic diversity of Antarctic populations is not a sampling artefact. Cetraria aculeata appears to have diversified in the Arctic and subsequently expanded its range into the Southern Hemisphere. The reduced genetic diversity in the Antarctic is most likely due to founder effects during long-distance colonization.

 

Domaschke S, Fernandez-Mendoza F, Garcia MA, Martin MP, Printzen C, 2012. Low genetic diversity in Antarctic populations of the lichen-forming ascomycete Cetraria aculeata and its photobiont. Polar Research 31.

 

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Population structure of mycobionts and photobionts of the widespread lichen Cetraria aculeata

Population structure of mycobionts and photobionts of the widespread lichen Cetraria aculeata | Population genetics of lichen symbiosis | Scoop.it

Lichens are symbioses between fungi (mycobionts) and photoautotrophic green algae or cyanobacteria (photobionts). Many lichens occupy large distributional ranges covering several climatic zones. So far, little is known about the large-scale phylogeography of lichen photobionts and their role in shaping the distributional ranges of lichens. We studied south polar, temperate and north polar populations of the widely distributed fruticose lichen Cetraria aculeata. Based on the DNA sequences from three loci for each symbiont, we compared the genetic structure of mycobionts and photobionts. Phylogenetic reconstructions and Bayesian clustering methods divided the mycobiont and photobiont data sets into three groups. An amova shows that the genetic variance of the photobiont is best explained by differentiation between temperate and polar regions and that of the mycobiont by an interaction of climatic and geographical factors. By partialling out the relative contribution of climate, geography and codispersal, we found that the most relevant factors shaping the genetic structure of the photobiont are climate and a history of codispersal. Mycobionts in the temperate region are consistently associated with a specific photobiont lineage. We therefore conclude that a photobiont switch in the past enabled C. aculeata to colonize temperate as well as polar habitats. Rare photobiont switches may increase the geographical range and ecological niche of lichen mycobionts by associating them with locally adapted photobionts in climatically different regions and, together with isolation by distance, may lead to genetic isolation between populations and thus drive the evolution of lichens.

 

Fernández-Mendoza F, Domaschke S, García MA, Jordan P, Martín MP, Printzen C, 2011. Population structure of mycobionts and photobionts of the widespread lichen Cetraria aculeata. Molecular Ecology 20, 1208-1232.

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Local genetic structure in a North American epiphytic lichen, Ramalina menziesii (Ramalinaceae)

Local genetic structure in a North American epiphytic lichen, Ramalina menziesii (Ramalinaceae) | Population genetics of lichen symbiosis | Scoop.it

Epiphytic lichens possess unique life history traits that can have conflicting effects on genetic structure: symbiotic mutualism between a fungus with its algal or cyanobacterial photobiont, association with a host plant, and ability to reproduce sexually and asexually. Our study species, Ramalina menziesii, has small ascospores that can facilitate long-distance gene movement, and it is capable of clonal reproduction. The goals of this study are to test whether different haplotypes were differentially distributed across host plant species, to look for evidence of asexual vs. sexual reproduction, and to assess the local genetic structure of the population. We sampled individuals from multiple trees of three oak species in four lichen subpopulations within a savanna ecosystem. Using DNA sequence data from four fungal nuclear loci, we found no tendency for host specialization. Alleles were randomly distributed across subpopulations. The frequency of multilocus genotypes was consistent with a randomly mating population. Sexual reproduction involving relichenization appeared to be the predominant mode of reproduction of R. menziesii at this study site. We found no significant local genetic structure suggesting widespread gene flow at the local scale. The genetic structure of this lichen is comparable to that of widely distributed epiphytic plants.

 

Werth S, Sork VL, 2008. Local genetic structure in a North American epiphytic lichen, Ramalina menziesii (Ramalinaceae). American Journal of Botany 95, 568-576.

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Symbiont flexibility in Thamnolia vermicularis (Pertusariales: Icmadophilaceae)

Symbiont flexibility in Thamnolia vermicularis (Pertusariales: Icmadophilaceae) | Population genetics of lichen symbiosis | Scoop.it

Algal and fungal symbionts of the lichenized genus Thamnolia typically co-disperse through thallus fragmentation, which may be expected to lead to fungal associations with a restricted range of algal symbionts. Here we examine the range of algae that associate with the fungus Thamnolia vermicularis. Phylogenetic analyses of internal transcribed spacer rDNA (ITS) sequences suggest that Trebouxia algae associated with T. vermicularis are not monophyletic. Algal and fungal phylogenies were compared, and although some congruence was found, a Mantel test found no significant correlation between fungal and algal genetic distances. An AMOVA suggested that ecogeographic factors play a stronger role than fungal genotype in structuring photobiont diversity. Additionally, as a species, T. vermicularis associates with a range of algae equal to or greater than that of many other fungal taxa.

 

Matthew P. Nelsen and Andrea Gargas (2009) Symbiont flexibility in Thamnolia vermicularis (Pertusariales: Icmadophilaceae) The Bryologist: Vol. 112, No. 2, pp. 404-417.

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The lichen-forming ascomycete Evernia mesomorpha associates with multiple genotypes of Trebouxia jamesii

The lichen-forming ascomycete Evernia mesomorpha associates with multiple genotypes of Trebouxia jamesii | Population genetics of lichen symbiosis | Scoop.it

The epiphyte Evernia mesomorpha forms a lichen association with green algae in the genus Trebouxia. Little is known about the population structure of E. mesomorpha. Here, population structure of the algal and fungal symbionts was examined for 290 lichen thalli on 29 jack pine (Pinus banksiana) trees in Manitoba.Through phylogenetic analysis of internal transcribed spacer (ITS) nuclear ribosomal DNA (rDNA) sequences, five algal genotypes were detected that were nested within T. jamesii. Two fungal genotypes were detected that formed a clade with two other Evernia species. The genus Evernia was paraphyletic with E. prunastri, sister to Parmelia saxatilis. Restriction fragment length polymorphism (RFLP) of ITS rDNA showed multiple algal genotypes in 45% of the 290 lichen thalli collected, whereas all thalli only contained one fungal genotype.Low population subdivision of algal and fungal genotypes among trees suggested that the algal symbiont was being dispersed in the lichen soredium.Low fungal specificity for multiple algal genotypes and a hypothesized algal switch may be important life history strategies for E. mesomorpha to adapt to changing environmental conditions.

 

Piercey-Normore MD, 2006. The lichen-forming ascomycete Evernia mesomorpha associates with multiple genotypes of Trebouxia jamesii. New Phytologist 169, 331-344.

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Geographic variation in algal partners of Cladonia subtenuis (Cladoniaceae) highlights the dynamic nature of a lichen symbiosis

Geographic variation in algal partners of Cladonia subtenuis (Cladoniaceae) highlights the dynamic nature of a lichen symbiosis | Population genetics of lichen symbiosis | Scoop.it

Multiple interacting factors may explain variation present in symbiotic associations, including fungal specificity, algal availability, mode of transmission and fungal selectivity. To separate these factors, we sampled the lichenized Cladonia subtenuis and associated Asterochloris algae across a broad geographic range.We sampled 87 thalli across 11 sites using sequence data to test for fungal specificity (phylogenetic range of association) and selectivity (frequency of association), fungal reproductive mode, and geographic structure among populations. Permutation tests were used to examine symbiont transmission.Four associated algal clades were found. Analysis of molecular variation (amova) and partial Mantel tests suggested that the frequency of associated algal genotypes was significantly different among sites and habitats, but at random with respect to fungal genotype and clade. The apparent specificity for Clade II algae in the fungal species as a whole did not scale down to further within-species lineage-dependent specificity for particular algae. Fungal genotypes were not structured according to site and appeared to be recombining.We suggest that ecological specialization exists for a specific lichen partnership and a site, and that this selectivity is dynamic and environment-dependent. We present a working model combining algal availability, fungal specificity and selectivity, which maintains variation in symbiotic composition across landscapes.

 

Yahr R, Vilgalys R, DePriest PT, 2006. Geographic variation in algal partners of Cladonia subtenuis (Cladoniaceae) highlights the dynamic nature of a lichen symbiosis. New Phytologist 171, 847-860.

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Quantifying dispersal and establishment limitation in a population of an epiphytic lichen

Quantifying dispersal and establishment limitation in a population of an epiphytic lichen | Population genetics of lichen symbiosis | Scoop.it

 

Dispersal is a process critical for the dynamics and persistence of metapopulations, but it is difficult to quantify. It has been suggested that the old-forest lichen Lobaria pulmonaria is limited by insufficient dispersal ability. We analyzed 240 DNA extracts derived from snow samples by a L. pulmonaria-specific real-time PCR (polymerase chain reaction) assay of the ITS (internal transcribed spacer) region allowing for the discrimination among propagules originating from a single, isolated source tree or propagules originating from other locations. Samples that were detected as positives by real-time PCR were additionally genotyped for five L. pulmonaria microsatellite loci. Both molecular approaches demonstrated substantial dispersal from other than local sources. In a landscape approach, we additionally analyzed 240 snow samples with real-time PCR of ITS and detected propagules not only in forests where L. pulmonaria was present, but also in large unforested pasture areas and in forest patches where L. pulmonaria was not found. Monitoring of soredia of L. pulmonaria transplanted to maple bark after two vegetation periods showed high variance in growth among forest stands, but no significant differences among different transplantation treatments. Hence, it is probably not dispersal limitation that hinders colonization in the old-forest lichen L. pulmonaria, but ecological constraints at the stand level that can result in establishment limitation. Our study exemplifies that care has to be taken to adequately separate the effects of dispersal limitation from a limitation of establishment.

 

Werth S, Wagner HH, Gugerli F, Holderegger R, Csencsics D, Kalwij JM, Scheidegger C, 2006. Quantifying dispersal and establishment limitation in a population of an epiphytic lichen. Ecology 87, 2037-2046.

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Image source: http://www.fs.fed.us/wildflowers/interesting/lichens/gallery/foliose/images/lobaria_pulmonaria_wet.jpg

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Vertical and horizontal photobiont transmission within populations of a lichen symbiosis

Vertical and horizontal photobiont transmission within populations of a lichen symbiosis | Population genetics of lichen symbiosis | Scoop.it

Lichens are widespread symbioses and play important roles in many terrestrial ecosystems. The genetic structure of lichens is the result of the association between fungal and algal populations constituting the lichen thallus. Using eight fungus- and seven alga-specific highly variable microsatellite markers on within-population spatial genetic data from 62 replicate populations across Europe, North America, Asia and Africa, we investigated the contributions of vertical and horizontal transmission of the photobiont to the genetic structure of the epiphytic lichen Lobaria pulmonaria. Based on pairwise comparisons of multilocus genotypes defined separately for the mycobiont and for the photobiont, we inferred the transmission mode of the photobiont and the relative contribution of somatic mutation and recombination. After constraining the analysis of one symbiont to pairs of individuals with genetically identical symbiotic partners, we found that 77% of fungal and 70% of algal pairs were represented by clones. Thus, the predominant dispersal mode was by means of symbiotic vegetative propagules (vertical transmission), which dispersed fungal and algal clones co-dependently over a short distance, thus shaping the spatial genetic structure up to distances of 20 m. Evidence for somatic mutation generating genetic diversity was found in both symbionts, accounting for 30% of pairwise comparisons in the alga and 15% in the fungus. While the alga did not show statistically significant evidence of recombination, recombination accounted for 7.7% of fungal pairs with identical algae. This implies that, even in a mostly vegetatively reproducing species, horizontal transmission plays a role in shaping the symbiotic association, as shown in many coral and other symbioses in nature.

 

Dal Grande F, Widmer I, Wagner HH, Scheidegger C, 2012. Vertical and horizontal photobiont transmission within populations of a lichen symbiosis. Molecular Ecology 21, 3159-3172.

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Microsatellites reveal regional population differentiation and isolation in Lobaria pulmonaria, an epiphytic lichen

Microsatellites reveal regional population differentiation and isolation in Lobaria pulmonaria, an epiphytic lichen | Population genetics of lichen symbiosis | Scoop.it

Many lichen species produce both sexual and asexual propagules, but, aside from being minute, these diaspores lack special adaptations for long-distance dispersal. Populations from both British Columbia and Switzerland were genetically diverse at the loci. Geographically restricted alleles, low historical gene flow, and analyses of genetic distance (upgma tree) and of differentiation (amova) indicated that populations from Vancouver Island and from the Canadian mainland were separated from each other, except for one, geographically intermediate population. This differentiation was attributed to different glacial and postglacial histories of coastal and inland populations in British Columbia. In contrast to expectations, the three investigated Swiss populations were genetically neither isolated nor differentiated from each other despite the long-lasting negative human impact on the lichen's range size in Central Europe. We propose that detailed studies integrating local landscape and regional scales are now needed to understand the processes of dispersal and gene flow in lichens.

 

Walser JC, Holderegger R, Gugerli F, Hoebee SE, Scheidegger C, 2005. Microsatellites reveal regional population differentiation and isolation in Lobaria pulmonaria, an epiphytic lichen. Molecular Ecology 14, 457-467.

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Phylogeography of Cavernularia hultenii: evidence of slow genetic drift in a widely disjunct lichen

Phylogeography of Cavernularia hultenii: evidence of slow genetic drift in a widely disjunct lichen | Population genetics of lichen symbiosis | Scoop.it

Population structure and history is poorly known in most lichenized ascomycetes. Many species display large-scale infraspecific disjunctions, which have been explained alternately by range fragmentation in species of high age and widespread long-distance dispersal. Using the lichen Cavernularia hultenii, which is widely disjunct across North America and Europe, Pleistocene and Holocene population history was inferred. The internal transcribed spacer (ITS) and part of the the intergenic spacer (IGS) region of the nuclear ribosomal DNA were sequenced in 300 individuals representing 62 populations across the range of the species. 

 

Printzen C, Ekman S, Tønsberg T, 2003. Phylogeography of Cavernularia hultenii: evidence of slow genetic drift in a widely disjunct lichen. Molecular Ecology 12, 1473-1486.

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