Joint Dispersal Does Not Imply Maintenance of Partnerships in Lichen Symbioses

Dispersal of symbiotic partners by joint propagules is considered as an efficient strategy to maintain successful associations and to circumvent low symbiont availability. Joint dispersal is widespread in diverse symbioses and a particularly common reproductive mode in lichens. We were interested in the implications of joint symbiont dispersal on population genetic structure and investigated patterns of symbiont association in populations of two closely related lichen species in the genus Physconia, with similar range of compatible algal partners. One of the lichen species is characterized by joint dispersal of both symbionts, whereas the other species propagates by meiotic fungal spores alone. The latter species must re-establish the symbiotic stage with appropriate algae sampled from the environment. Both fungal species have specialized on photobionts representing a monophyletic lineage of the algal genus Trebouxia. The results indicate no correlated association of symbiont genotypes in the species with joint symbiont dispersal. We rather show that algal gene diversity in populations of lichenized fungi with different propagation strategies is not necessarily different. The association with algae that differ from the co-dispersed genotypes during the vegetative development of the thalli is the most likely explanation for the observed pattern. Maintenance of symbiotic associations is an option but not a strict consequence of joint symbiont dispersal in lichens.     

Drought-induced structural alterations at the mycobiont-photobiont interface in a range of foliose macrolichens

Summary Cryotechniques, such as low temperature scanning electron microscopy (LTSEM) and freeze-substitution for transmission electron microscopy (TEM), were applied to two cyanobacterial and three green algal macrolichens in order to locate free water and to visualize drought-induced structural alterations at the mycobiont—photobiont interface. The following species were examined:Peltigera canina/Nostoc punctiforme, Sticta sylvatica/Nostoc sp. (both Peltigerales),Parmelia sulcata/Trebouxia impressa, Hypogymnia physodes/Trebouxia sp. (both Lecanorales), andXanthoria parietina/Trebouxia arboricola (Teloschistales). In all species free water was confined to the symplast and the apoplast. No intercellular water reservoirs were found in the gas-filled thallus interior. Thalline fluctuations in water content reflect fluctuations in apoplastic and symplastic water. All the taxonomically diverse lichen photobionts have access to water and dissolved nutrients via the fungal apoplast only. Drought stress (i.e., water content 20%/dw and below) caused dramatic shrinkage and deformation in all cell types. At any level of hydration the fungal and algal protoplast maintained close contact with the cell wall. This applied to the cyanobacterial photobionts and their murein sacculus and gelatinous sheath too. Although the cytoplasm of both partners was strongly condensed in desiccated lichens the cellular membrane systems, usually negatively contrasted, were very well preserved. The significance of these data is discussed with regard to the functioning of the symbiotic relationship.     

High photobiont diversity associated with the euryoecious lichen-forming ascomycete Lecanora rupicola (Lecanoraceae, Ascomycota)

The diversity and phylogenetic position of photobionts in the widespread saxicolous, crustose lichen-forming ascomycete Lecanora rupicola s.l. is presented. The algal partners of this lichen species complex belong to diverse and unrelated lineages in the genus Trebouxia . Specimens were sampled from different habitats and geographical origins. Either whole thallus DNA extractions or minute fragments of the algal layer of the lichen thallus were subjected to polymerase chain reaction, using primers that specifically amplify internal transcribed spacer rDNA of the photobionts. No correlations between different chemical races of L. rupicola with particular lineages of Trebouxia spp. were found. Irrespective of the different algal partners, all lichen thalli abundantly developed ascomata. L. rupicola apparently maintains full fecundity with a low degree of selectivity for photobionts, which promotes the occurrence of this lichen-forming species in ample ecological situations.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 283–293.     

Photobiont selectivity leads to ecological tolerance and evolutionary divergence in a polymorphic complex of lichenized fungi

Background and Aims

The integrity and evolution of lichen symbioses depend on a fine-tuned combination of algal and fungal genotypes. Geographically widespread species complexes of lichenized fungi can occur in habitats with slightly varying ecological conditions, and it remains unclear how this variation correlates with symbiont selectivity patterns in lichens. In an attempt to address this question, >300 samples were taken of the globally distributed and ecologically variable lichen-forming species complex Tephromela atra, together with closely allied species, in order to study genetic diversity and the selectivity patterns of their photobionts.

Methods

Lichen thalli of T. atra and of closely related species T. grumosa, T. nashii and T. atrocaesia were collected from six continents, across 24 countries and 62 localities representing a wide range of habitats. Analyses of genetic diversity and phylogenetic relationships were carried out both for photobionts amplified directly from the lichen thalli and from those isolated in axenic cultures. Morphological and anatomical traits were studied with light and transmission electron microscopy in the isolated algal strains.

Key Results

Tephromela fungal species were found to associate with 12 lineages of Trebouxia. Five new clades demonstrate the still-unrecognized genetic diversity of lichen algae. Culturable, undescribed lineages were also characterized by phenotypic traits. Strong selectivity of the mycobionts for the photobionts was observed in six monophyletic Tephromela clades. Seven Trebouxia lineages were detected in the poorly resolved lineage T. atra sensu lato, where co-occurrence of multiple photobiont lineages in single thalli was repeatedly observed.

Conclusions

Low selectivity apparently allows widespread lichen-forming fungi to establish successful symbioses with locally adapted photobionts in a broader range of habitats. This flexibility might correlate with both lower phylogenetic resolution and evolutionary divergence in species complexes of crustose lichen-forming fungi.     

Fungal specificity and selectivity for algae play a major role in determining lichen partnerships across diverse ecogeographic regions in the lichen‐forming family Parmeliaceae (Ascomycota)

Microbial symbionts are instrumental to the ecological and long‐term evolutionary success of their hosts, and the central role of symbiotic interactions is increasingly recognized across the vast majority of life. Lichens provide an iconic group for investigating patterns in species interactions; however, relationships among lichen symbionts are often masked by uncertain species boundaries or an inability to reliably identify symbionts. The species‐rich lichen‐forming fungal family Parmeliaceae provides a diverse group for assessing patterns of interactions of algal symbionts, and our study addresses patterns of lichen symbiont interactions at the largest geographic and taxonomic scales attempted to date. We analysed a total of 2356 algal internal transcribed spacer (ITS) region sequences collected from lichens representing ten mycobiont genera in Parmeliaceae, two genera in Lecanoraceae and 26 cultured Trebouxia strains. Algal ITS sequences were grouped into operational taxonomic units (OTUs); we attempted to validate the evolutionary independence of a subset of the inferred OTUs using chloroplast and mitochondrial loci. We explored the patterns of symbiont interactions in these lichens based on ecogeographic distributions and mycobiont taxonomy. We found high levels of undescribed diversity in Trebouxia, broad distributions across distinct ecoregions for many photobiont OTUs and varying levels of mycobiont selectivity and specificity towards the photobiont. Based on these results, we conclude that fungal specificity and selectivity for algal partners play a major role in determining lichen partnerships, potentially superseding ecology, at least at the ecogeographic scale investigated here. To facilitate effective communication and consistency across future studies, we propose a provisional naming system for Trebouxia photobionts and provide representative sequences for each OTU circumscribed in this study.     

SOME NEW AND INTERESTING SPECIES OF TREBOUXIA,A GENUS OF LICHENIZED ALGAE

Ahmadjian , Vernon . (Clark U., Worcester, Mass.) Some new and interesting species of Trebouxia, a genus of lichenized algae. Amer. Jour. Bot. 47 (8): 677–683. Illus. 1960.—Four species of the phycobiont Trebouxia, 3 of which are new, have been described in this paper. On the basis of this study, the algal genus has been divided into 2 main groups depending on the position that the chromatophore assumes during its bipartitions and on the shape of the cells. Growth rates have been calculated and growth curves drawn for the various species. In some cases, a physiological difference was shown between morphologically identical forms. In several instances, the same algal symbiont was isolated from widely separated lichen species, a fact which is contrary to earlier claims of the high specificity of the lichen fungus to its algal component.     

Influence of long-term exposure to copper on the lichen photobiont <Emphasis Type="Italic">Trebouxia erici</Emphasis> and the free-living algae <Emphasis Type="Italic">Scenedesmus quadricauda</Emphasis>

In the present work, the long-term effect (14 days) of copper on the levels of intracellular and total copper accumulation, growth, assimilation pigment composition, chlorophyll a fluorescence, soluble protein content and oxidative status (production of hydrogen peroxide and superoxide) in two algal species (Scenedesmus quadricauda and Trebouxia erici) was assessed. Scenedesmus quadricauda is a free-living alga while Trebouxia erici is the photobiont of a lichen. The presence of copper negatively affected growth, assimilation pigments, chlorophyll a fluorescence, soluble protein content and oxidative status in both the algae. However, Scenedesmus was much more sensitive compared to Trebouxia.     

Identification of photobionts from the lichen family Physciaceae using algal-specific ITS rDNA sequencing

Abstract:The identity of photobionts from 20 species of the Physciaceae from different habitats and geographical regions has been determined by ITS rDNA sequence comparisons in order to estimate the diversity of photobionts within that lichen group, to detect patterns of specificity of mycobionts towards their photobionts and as a part of an ongoing study to investigate possible parallel cladogenesis of both symbionts. Algal-specific PCR primers have been used to determine the ITS rDNA sequences from DNA extractions of dried lichens that were up to 5 years old. Direct comparisons and phylogenetic analyses allowed the assignment of Physciaceae photobionts to four distinct clades in the photobiont ITS rDNA phylogeny. The results indicate a diversity within the genus Trebouxia Puymaly and Physciaceae photobionts that is higher than expected on the basis of morphology alone. Physciaceae photobionts belonged to 12 different ITS lineages of which nine could unambiguously be assigned to six morphospecies of Trebouxia. The identity of the remaining three sequences was not clarified; they may represent new species. Specificity at the generic level was low as a whole range of photobiont species were found within a genus of Physciaceae and different ranges were detected. The photobionts ofPhyscia (Schreb.) Michaux were closely related and represented one morphospecies of Trebouxia, whereas the algal partners of Buellia De Not and Rinodina (Ach.) S. Gray were in distant lineages of the ITS phylogeny and from several Trebouxia morphospecies. Photobiont variation within a genus of Physciaceae may be due to phylogeny, geographical distance or because photobionts from neighbouring lichens were taken (‘algal sharing’). At the species level Physciaceae mycobionts seem to be rather selective and contained photobionts that were very closely related within one morphospecies of Trebouxia.     

The complexity of symbiotic interactions influences the ecological amplitude of the host: A case study in Stereocaulon (lichenized Ascomycota)

Symbiosis plays a fundamental role in nature. Lichens are among the best known, globally distributed symbiotic systems whose ecology is shaped by the requirements of all symbionts forming the holobiont. The widespread lichen‐forming fungal genus Stereocaulon provides a suitable model to study the ecology of microscopic green algal symbionts (i.e., phycobionts) within the lichen symbiosis. We analysed 282 Stereocaulon specimens, collected in diverse habitats worldwide, using the algal ITS rDNA and actin gene sequences and fungal ITS rDNA sequences. Phylogenetic analyses revealed a great diversity among the predominant phycobionts. The algal genus Asterochloris (Trebouxiophyceae) was recovered in most sampled thalli, but two additional genera, Vulcanochloris and Chloroidium, were also found. We used variation‐partitioning analyses to investigate the effects of climatic conditions, substrate/habitat characteristic, spatial distribution and mycobionts on phycobiont distribution. Based on an analogy, we examined the effects of climate, substrate/habitat, spatial distribution and phycobionts on mycobiont distribution. According to our analyses, the distribution of phycobionts is primarily driven by mycobionts and vice versa. Specificity and selectivity of both partners, as well as their ecological requirements and the width of their niches, vary significantly among the species‐level lineages. We demonstrated that species‐level lineages, which accept more symbiotic partners, have wider climatic niches, overlapping with the niches of their partners. Furthermore, the survival of lichens on substrates with high concentrations of heavy metals appears to be supported by their association with toxicity‐tolerant phycobionts. In general, low specificity towards phycobionts allows the host to associate with ecologically diversified algae, thereby broadening its ecological amplitude.     

High photobiont diversity in the common European soil crust lichen Psora decipiens

The genetic diversity of green algal photobionts (chlorobionts) in soil crust forming lichens was studied as part of the SCIN-project (Soil Crust InterNational). A total of 64 lichen samples were collected from four different sites along latitudinal and altitudinal gradients in Europe (Tabernas/Spain; Hochtor-Großglockner/Austria; Gynge Alvar/Sweden; Ruine Homburg/Germany). The dominant lichen species at all four sites was Psora decipiens, often occurring with Buellia elegans, Fulgensia bracteata, F. fulgens and Peltigera rufescens. Genetic identification of chlorobionts was carried out using the nuclear marker (nrITS) and a chloroplast marker (psbL-J). We found P. decipiens to be associated with several different species of Trebouxia and Asterochloris, although previously described to only have Asterochloris sp. The phylogenetic analyses revealed a high chlorobiont diversity with 12 well supported clades, including Trebouxia asymmetrica, T. jamesii, T. impressa and other, as yet taxonomically unidentified clades (Trebouxia sp. URa1-4, T. sp. URa6, T. sp. URa7-13). Additionally, five clades of Asterochloris were identified (A. magna, A. sp. URa14 -17). Most of the chlorobiont species appeared to be cosmopolitan, but five clades were unevenly distributed between the sampling sites with only Trebouxia being found in the warm and dry Spanish habitats and combinations of Trebouxia and Asterochloris in the cooler and more humid habitats. The wide range of chlorobiont species might contribute to the observed domination of P. decipiens at all four research sites of the SCIN project which range from a desert in Spain to an alpine site in the Alps of Austria.     

Simple method for fractionation of <Emphasis Type="Italic">Parmelia sulcata</Emphasis> lichen thalli

Three fractions were isolated from Parmelia sulcata lichen thalli: mycobiont-enriched fraction, highly purified preparation of Trebouxia sp. alga, and a photobiont-enriched fraction, in which the fungal component and symbiotic relations between the bionts were retained. The ratio between the fungal and algal components in all fractions was estimated from the photosynthesis/respiration and chlorophyll/dry weight ratios. Simple methods for lichen thallus fractionation are suggested to study symbiotic relationships of bionts.Translated from Fiziologiya Rastenii, Vol. 52, No. 2, 2005, pp. 306–310.Original Russian Text Copyright © 2005 by Maslov, Pavlova.This revised version was published online in April 2005 with a corrected cover date.     

Pectin localization in the Mediterranean orchid Limodorum abortivum reveals modulation of the plant interface in response to different mycorrhizal fungi

In most mycorrhizal symbioses, phylogenetically distinct fungi colonize simultaneously the roots of individual host plants. A matter of debate is whether plants can distinguish among these fungal partners and differentiate their cellular responses. We have addressed this question in the orchid mycorrhizal symbiosis, where individual roots of the Mediterranean species Limodorum abortivum can be colonized by a dominant unculturable fungal symbiont belonging to the genus Russula and by more sporadic mycelia in the genus Ceratobasidium (form-genus Rhizoctonia). The phylogenetic position of the Ceratobasidium symbionts was further investigated in this work. Both Russula and Ceratobasidium symbionts form intracellular coils in the cortical roots of L. abortivum, but hyphae are very different in size and morphology, making the two fungi easily distinguishable. We have used John Innes Monoclonal 5, a widely used monoclonal antibody against pectin, to investigate the composition of the symbiotic plant interface around the intracellular coils formed by the two fungal partners. Immunolabelling experiments showed that pectin is exclusively found in the interface formed around the Ceratobasidium, and not around the Russula symbiont. These data indicate that the plant responses towards distinct mycorrhizal fungal partners can vary at a cellular level.     

THE ULTRASTRUCTURE OF LICHENS. I. A GENERAL SURVEY

The fine structure of 10 lichens was examined. A comparison was made of the storage products of the algal symbiont (Trebouxia) in situ in the desiccated and hydrated states of the lichens. All the Trebouxia phycobionts, with the exception of that in Usnea strigosa, had lipid-containing globules in the pyrenoid. The globules were present in both the hydrated and desiccated conditions. Trebouxia in the hydrated condition contained starch granules in the chloroplast as well as the lipid-containing globules in the pyrenoid. The cell wall of Trebouxia consists of an outer electron-dense layer and an inner electron-light layer. Fungal haustoria (in Lecanora rubina) rupture the outer layer of the algal cell wall and invaginate the inner layer. A thick polysaccharide fibrillar material surrounds the fungal cells. Many bacteria were observed within this material. Septa and lomasomes are described. Ellipsoidal bodies, which appear to be an integral and unique part of the lichen fungal ultrastructure, were observed associated with membrane profiles.     

Photobiont selectivity for lichens and evidence for a possible glacial refugium in the Ross Sea Region, Antarctica

Lichens are a symbiosis consisting of heterotrophic, fungal (mycobiont) and photosynthetic algal or cyanobacterial (photobiont) components. We examined photobiont sequences from lichens in the Ross Sea Region of Antarctica using the internal transcribed spacer region of ribosomal DNA and tested the hypothesis that lichens from this extreme environment would demonstrate low selectivity in their choice of photobionts. Sequence data from three targeted lichen species (Buellia frigida, Umbilicaria aprina and Umbilicaria decussata) showed that all three were associated with a common algal haplotype (an unnamed Trebouxia species) which was present in all taxa and at all sites, suggesting lower selectivity. However, there was also association with unique, local photobionts as well as evidence for species-specific selection. For example, the cosmopolitan U. decussata was associated with two photobiont species, Trebouxia jamesii and an unnamed species. The most commonly collected lichen (B. frigida) had its highest photobiont haplotype diversity in the Dry Valley region, which may have served as a refugium during glacial periods. We conclude that even in these extreme environments, photobiont selectivity still has an influence on the successful colonisation of lichens. However, the level of selectivity is variable among species and may be related to the ability of some (e.g. B. frigida) to colonise a wider range of habitats.     

Lichen acclimation to changing environments: Photobiont switching vs. climate‐specific uniqueness in Psora decipiens

Unraveling the complex relationship between lichen fungal and algal partners has been crucial in understanding lichen dispersal capacity, evolutionary processes, and responses in the face of environmental change. However, lichen symbiosis remains enigmatic, including the ability of a single fungal partner to associate with various algal partners. Psora decipiens is a characteristic lichen of biological soil crusts (BSCs), across semi‐arid, temperate, and alpine biomes, which are particularly susceptible to habitat loss and climate change. The high levels of morphological variation found across the range of Psora decipiens may contribute to its ability to withstand environmental change. To investigate Psora decipiens acclimation potential, individuals were transplanted between four climatically distinct sites across a European latitudinal gradient for 2 years. The effect of treatment was investigated through a morphological examination using light and SEM microscopy; 26S rDNA and rbcL gene analysis assessed site‐specific relationships and lichen acclimation through photobiont switching. Initial analysis revealed that many samples had lost their algal layers. Although new growth was often determined, the algae were frequently found to have died without evidence of a new photobiont being incorporated into the thallus. Mycobiont analysis investigated diversity and determined that new growth was a part of the transplant, thus, revealing that four distinct fungal clades, closely linked to site, exist. Additionally, P. decipiens was found to associate with the green algal genus Myrmecia, with only two genetically distinct clades between the four sites. Our investigation has suggested that P. decipiens cannot acclimate to the substantial climatic variability across its environmental range. Additionally, the different geographical areas are home to genetically distinct and unique populations. The variation found within the genotypic and morpho‐physiological traits of P. decipiens appears to have a climatic determinant, but this is not always reflected by the algal partner. Although photobiont switching occurs on an evolutionary scale, there is little evidence to suggest an active environmentally induced response. These results suggest that this species, and therefore, other lichen species, and BSC ecosystems themselves may be significantly vulnerable to climate change and habitat loss.     

Genetic diversity of algal and fungal partners in four species of Umbilicaria (Lichenized Ascomycetes) along a transect of the Antarctic peninsula

Lichens from the genus Umbilicaria were collected across a 5,000-km transect through Antarctica and investigated for DNA sequence polymorphism in a region of 480-660 bp of the nuclear internal transcribed spacer region of ribosomal DNA. Sequences from both fungal (16 ascomycetes) and photosynthetic partners (22 chlorophytes from the genus Trebouxia) were determined and compared with homologs from lichens inhabiting more temperate, continental climates. The phylogenetic analyses reveal that Antarctic lichens have colonized their current habitats both through multiple independent colonization events from temperate embarkation zones and through recent long-range dispersal in the Antarctic of successful preexisting colonizers. Furthermore, the results suggest that relichenization-de novo establishment of the fungus-photosynthesizer symbiosis from nonlichenized algal and fungal cells-has occurred during the process of Antarctic lichen dispersal. Independent dispersal of algal and fungal cultures therefore can lead to a successful establishment of the lichen symbiosis even under harsh Antarctic conditions.     

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

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.     

Molecular and morphological diversity of Trebouxia microalgae in sphaerothallioid Circinaria spp. lichens1

Three vagrant (Circinaria hispida, Circinaria gyrosa, and Circinaria sp. ‘paramerae’) and one crustose (semi‐vagrant, Circinaria sp. ‘oromediterranea’) lichens growing in very continental areas in the Iberian Peninsula were selected to study the phycobiont diversity. Mycobiont identification was checked using nrITS DNA barcoding: Circinaria sp. ‘oromediterranea’ and Circinaria sp. ‘paramerae’ formed a new clade. Phycobiont diversity was analyzed in 50 thalli of Circinaria spp. using nrITS DNA and LSU rDNA, with microalgae coexistence being found in all the species analyzed by Sanger sequencing. The survey of phycobiont diversity showed up to four different Trebouxia spp. as the primary phycobiont in 20 thalli of C. hispida, in comparison with the remaining Circinaria spp., where only one Trebouxia was the primary microalga. In lichen species showing coexistence, some complementary approaches are needed (454 pyrosequencing and/or ultrastructural analyses). Five specimens were selected for high‐throughput screening (HTS) analyses: 22 Trebouxia OTUs were detected, 10 of them not previously known. TEM analyses showed three different cell morphotypes (Trebouxia sp. OTU A12, OTU S51, and T. cretacea) whose ultrastructure is described here in detail for the first time. HTS revealed a different microalgae pool in each species studied, and we cannot assume a specific pattern between these pools and the ecological and/or morphological characteristics. The mechanisms involved in the selection of the primary phycobiont and the other microalgae by the mycobiont are unknown, and require complex experimental designs. The systematics of the genus Circinaria is not yet well resolved, and more analyses are needed to establish a precise delimitation of the species.     

Zur Morphologie und Systematik desTrebouxia-Phycobionten im Thallus vonUsnea longissima (Lecanorales)

In the lichen genusUsnea different species ofTrebouxia-phycobionts as well as different haustorial types are known. The isolated and cultivated phycobiont ofUsnea longissima Ach. was studied by light- and electron microscopy and resembles in cytomorphological details the type ofTrebouxia impressa Ahmad. In addition to simple wall-to-wall contacts between the symbiotic components also intraparietal (=intrawall-)haustoria could be observed as the normal interaction type.

Frau Prof. Dr.Elisabeth Tschermak-Woess zu ihrem 70. Geburtstag gewidmet.     

The distribution of group I introns in lichen algae suggests that lichenization facilitates intron lateral transfer

The nuclear-encoded small subunit ribosomal DNA gene of many lichen-forming green algae in the genus Trebouxia contains a group I intron at Escherichia coli genic position 1512. We studied the evolutionary history of the 1512 intron in Trebouxia spp. (Trebouxiophyceae) by analyzing intron and "host" cell phylogenies. The host trees were constructed by comparing internal transcribed spacer regions of rDNA. Maximum-likelihood, maximum-parsimony, and distance analyses suggest that the 1512 intron was present in the common ancestor of the green algal classes Trebouxiophyceae, Chlorophyceae, and Ulvophyceae. The 1512 intron, however, was laterally transferred at least three times among later-diverging Trebouxia spp. that form lichen partnerships. Intron secondary structure analyses are consistent with this result. Our results support the hypothesis that lichenization may facilitate 1512 group I intron lateral transfer through the close cell-to-cell contact that occurs between the lichen algal and fungal symbionts in the developing lichen thallus.