ULTRASTRUCTURE OF THE LICHEN COENOGONIUM INTERPLEXUM NYL.

Coenogonium interplexum Nyl. is a green to yellow-orange filamentous lichen commonly found on tree bark, rocks, and soil. The mycobiont is the ascomycetous fungus Coenogonium. The ultrastructure of the lichenized phycobiont, Trentepohlia, closely resembles that of the non-lichenized form, a filamentous subaerial green alga. The mycobiont has a typical fungal ultrastructure, and the cell wall sometimes appears thinner at points of contact with the phycobiont wall. Several branched fungal hyphae are usually randomly arranged around a Trentepohlia filament, and may in some cases completely ensheath the alga. Although no haustoria were observed, this relationship may still be termed a lichen since there is some modification of the alga and the lichen is structurally distinct from the two symbionts.     

Proteins from the liehenXanthoria parietina which bind to phycobiont cell walls Localization in the intact liehen and cultured mycobiont

Summary A protein fraction, previously isolated from the lichenXanthoria parietina and known to bind to the appropriate culturedTrebouxia phycobiont, was visualized in the intact lichen thallus and cultured mycobiont by an indirect immunoperoxidase assay. The protein was localized in both the upper and lower cortices of the lichen thallus; it was also present in the cell walls of the mycobiont culturedin vitro. The possible role of this protein in the recognition, or initial interaction, between separated lichen symbionts is discussed.     

Ultrastructural and Biogeochemical Features of Microbiotic Soil Crusts and their Implications in a Semi-Arid Habitat

This investigation was designed to explore the relationships between lichen symbionts (phycobiont and mycobiont) and the substrate on which they grow by examining the chemical and ultrastructural features of the lichen-soil interface. These lichens form an integral part of microbiotic soil crusts. Fragments of three different lichen biotypes growing over gypsum crystals and marls were fixed and embedded in resin. The lichen-substratum interface was then examined by scanning electron microscopy with backscattered electron imaging. In situ observation, microanalytical (EDS), and FT-Raman plus infrared spectroscopy of the lichen-substratum interface indicated that different ultrastructural features of the mycobiont were related to biogeochemical processes and Ca 2+ distribution in the soil crust. Phycobionts were observed to make direct contact with the substratum and to be surrounded by a nondifferentiated thallus structure. These observations suggest that they can grow outside the thallus in the early stages of lichen development in the semi-arid conditions of their habitat. The particular ultrastructural features of the lichen thallus and of the lichen-substratum interface appear to have marked effects on runoff phenomena and ponding generation of the surface.     

OBSERVATIONS ON LICHENIZED AND FREE-LIVING PHYSOLINUM (CHLOROPHYTA,TRENTEPOHLIACEAE)1

Lichenized Physolinum Printz and free-living Physolinum from a dimly lit cave were studied from fresh collections and cultures, preserved specimens fixed in situ, and cultures that had persisted for 5 years in an environmental chamber. The branched filamentous association consists of a phycobiont and a characteristic ascomycetous mycobiont of one layer that completely ensheathes the algal partner. Epiphytic blue-green algae commonly occur attached to the mycobiont. The phycobiont, Physolinum monilia (De Wildem.) Printz, produces thick-walled, green spiny cells, some of which enlarge and contact the sheathing mycobiont cells; the phycobiont and mycobiont may then develop into new lichenized filaments. The hyaline mycobiont cells extend haustoria bound by the fungus wall deeply into the phycobiont chloroplasts. The epiphytes, Synechocystis-like colonies, are firmly attached to the outer walls of the mycobiont and are associated with several-celled extensions of the fungus beyond the apical phycobiont cells. Free-living Physolinum monilia filaments are branched and moniliform; the search-containing uninucleate cells are spherical to pyriform and have walls of cellulose. Each cell has a single massive chloroplast with plastoglobuli among tightly packed thylakoids. Except for their larger cells, P. monilia filaments appear to be identical to the phycobiont of lichenized Physolinum.     

Differenze Ultrastrutturali Di Alcune Specie Di Trebouxia Poste in Condizioni Di Illuminazione Differenti

Abstract

Ultrastructural changes in some species of « Trebouxia » under different light conditions. — Some species of the phycobiont alga Trebouxia (Tr. decolorans and Tr. albulescens), both isolated and grown on synthetic medium and still in the lichen, were examined in order to study the effect of light on the plastid ultrastructures. The species isolated from Buellia punctata and Xanthoria parietina were very sensitive to light condition and lost their chlorophyll content quickly. Striking ultrastructural changes were found in the algae grown under small light intensities and those which become achloric owing to strong light. In the latter, modifications of the Iamellar System were observed. The disappearance of Chlorophyll pigments was followed by a reduced electron density of the whole Iamellar system, as if were lacking the Iipidic compounds which are usually present and absorb fixators and dyers, thus allowing a good view. On the contrary, normal light conditions did not affect cultures of Trebouxia humicola, a free living alga. In the chloroplasts of the phycobiont species, unlike in the free living alga, grana were very close and sometimes formed very thick masses towards the edge of the chloroplast. It could not be ascertained whether such changes corresponded to a different composition of the lipoproteic compounds of the lamellar system.

Xanthoria parietina could grow in very lighted environments with no damage of the algae present in its thallus. The lichen thalluses, under different light conditions, showed very different colourings: the overlighted ones were rusty-red and the shadowed ones deep green. The chlorophyll content of the lichen thalluses with various shades (table 1) were very similar. The ultrastructural changes induced by strong light intensities in the phycobiont algae, kept in the lichen, were very small in respect of those observed in the same algae isolated and grown on synthetic medium and concerned the Iamellar system and the pyrenoid, above all. The rusty-red lichen showed a great number of stromatic lamellae, often with a parallel trend, so as to simulate a Iamellar system not organized in grana and often presented groups of lamellae concentrically arranged. In the pyrenoid of the algae from rusty-red thalluses, compared with the green ones, a much greater number of electron dense masses was observed, which are very thick and occupy the whole stromatic portion of the pyrenoid. But the Chlorophyll content did not decrease. Unlike the results of PEVELING, we noted that the electron dense masses (cited by the Author as « osmiophilic plastoglobules) were visible even after fixation with permanganate; the different numbers of these globules might depend on environmental factors. The phycobiont alga, when in the lichen thallus, could perhaps support strong light intensities, because pigments or compounds formed with the mycobiont or by it alone prevented the photooxidation of chlorophyll. Hypothetically a relationship might exist between the sensitivity of the phycobiont algae to light intensities and the content in antraquinonic pigments in the lichen thallus. But also using filters with absorption maxima similar to those of these pigments, the « in vitro » cultures of the phycobiont algae became achloric in the same time as the control ones.

Some Authors had found in Trebouxia humicola a different relationship between Chlorophyll pigments and carotinoids from that observed in the phycobiont species and had ascribed to it the greater resistence to strong light of the free living alga. Pigments or other substances present in the mycobiont can have a protective action on the Chlorophyll content and on the ultrastructures. In the phycobiont algae the resistence to strong light might be explained by an exchange of compounds between mycobiont and phycobiont, ending with the structural changes of the pyrenoid.     

The ultrastructure of the symbionts ofRhizocarpon geographicum,Parmelia conspersa andUmbilicaria pustulata growing under dryness conditions

Summary The dryness-induced ultrastructural changes of both myco- and phycobiont of three lichen species (R. geographicum, P. conspersa, andU. pustulata) have been studied over three months and half, period of time. During this time other ecological factors, such as rock substratum, temperature, light and gas interchange were unaltered compared to the natural conditions. A large number of ultrastructural changes were observed in the mycobiont as well as in the phycobiont (Trebouxia) and often, cells showed a highly disorganized morphology. The most important ultrastructural modifications were: 1. pyrenoglobuli of the algae were peripheral, 2. new and unknown structures were observed in the phycobionts of bothR. geographicum andU. pustulata as well as in the mycobiont of the latter species.     

Effects of starvation and desiccation on energy metabolism in desert and mesic Drosophila

Energy availability can limit the ability of organisms to survive under stressful conditions. In Drosophila, laboratory experiments have revealed that energy storage patterns differ between populations selected for desiccation and starvation. This suggests that flies may use different sources of energy when exposed to these stresses, but the actual substrates used have not been examined. We measured lipid, carbohydrate, and protein content in 16 Drosophila species from arid and mesic habitats. In five species, we measured the rate at which each substrate was metabolized under starvation or desiccation stress. Rates of lipid and protein metabolism were similar during starvation and desiccation, but carbohydrate metabolism was several-fold higher during desiccation. Thus, total energy consumption was lower in starved flies than desiccated ones. Cactophilic Drosophila did not have greater initial amounts of reserves than mesic species, but may have lower metabolic rates that contribute to stress resistance.     

Effect of low water potential on photosynthesis in intact lichens and their liberated algal components

Earlier experiments (T.D. Brock 1975, Planta124, 13–23) addressed the question whether the fungus of the lichen thallus might enable the algal component to function when moisture stress is such that the algal component would be unable to function under free-living conditions. It was concluded that the liberated phycobiont in ground lichen thalli could not photosynthesize at water potentials as low as those at which the same alga could when it was present within the thallus. However, our experience with lichen photosynthesis has not substantiated this finding. Using instrumentation developed since the mid-1970's to measure photosynthesis and control humidity, we repeated Brock's experiments. When applying “matric” water stress (equilibrium with air of constant relative humidity) we were unable to confirm the earlier results for three lichen species including one of the species,Letharia vulpina, had also been used by Brock. We found no difference between the effects of low water potential on intact lichens and their liberated algal components (ground thallus material and isolated algae) and no indication that the fungal component of the lichen symbiosis protects the phycobiont from the adverse effects of desiccation once equilibrium conditions are reached. The photosynthetic apparatus of the phycobiont alone proved to be highly adapted to water stress as it possesses not only the capability of functioning under extremely low degrees of hydration but also of becoming reactivated solely by water vapor uptake.     

Protective mechanism of desiccation tolerance in <Emphasis Type="Italic">Reaumuria soongorica</Emphasis>: Leaf abscission and sucrose accumulation in the stem

Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub, is widely found in semi-arid areas in northwestern China and can survive severe desiccation of its vegetative organs. In order to study the protective mechanism of desiccation tolerance in R. soongorica, diurnal patterns of net photosynthetic rate (Pn), water use efficiency (WUE) and chlorophyll fluorescence parameters of Photosystem II (PSII), and sugar content in the source leaf and stem were investigated in 6-year-old plants during progressive soil drought imposed by the cessation of watering. The results showed that R. soongorica was characterized by very low leaf water potential, high WUE, photosynthesis and high accumulation of sucrose in the stem and leaf abscission under desiccation. The maximum Pn increased at first and then declined during drought, but intrinsic WUE increased remarkably in the morning with increasing drought stress. The maximal photochemical efficiency of PSII (Fv/Fm) and the quantum efficiency of noncyclic electric transport of PSII(Φ_PSII) decreased significantly under water stress and exhibited an obvious phenomenon of photoinhibition at noon. Drought stressed plants maintained a higher capacity of dissipation of the excitation energy (measured as NPQ) with the increasing intensity of stress. Conditions of progressive drought promoted sucrose and starch accumulation in the stems but not in the leaves. However, when leaf water potential was less than −21.3 MPa, the plant leaves died and then abscised. But the stem photosynthesis remained and, afterward the plants entered the dormant state. Upon rewatering, the shoots reactivated and the plants developed new leaves. Therefore, R. soongorica has the ability to reduce water loss through leaf abscission and maintain the vigor of the stem cells to survive desiccation.     

ADAPTATION TO DESICCATION FAILS TO GENERATE PRE‐ AND POSTMATING ISOLATION IN REPLICATE DROSOPHILA MELANOGASTER LABORATORY POPULATIONS

Many laboratory speciation experiments have raised allopatric populations in different environments to determine whether reproductive isolation evolves as a byproduct of adaptation (a form of ecological speciation). Few, however, have addressed the evolution of both pre‐ and postmating isolation or investigated the conditions affecting the process. We present results of an evolution experiment in which 12 lines of Drosophila melanogaster were derived from a common population and then independently evolved for more than 57 generations under alternative selection treatments. Six “desiccation” populations had access to food and water removed during a period of their adult lives generating strong desiccation selection, and six “starvation” populations had access to food but not water removed for the same period, generating a mild starvation stress. Substantial divergence of cuticular hydrocarbons occurred between the desiccation and starvation populations, key traits that have been implicated in sexual isolation in Drosophila. Despite this divergence, there was no detectable premating isolation between desiccation and starvation populations and postmating isolation was asymmetrical: the fitness of F1 hybrids was reduced in the desiccation but not the starvation environment. This asymmetry was likely caused by the absence of divergent selection: adaptation to desiccation appears to have come at no cost to performance in the starvation environment. Novel environments are thought to promote the evolution of reproductive isolation. Understanding the conditions that favor or hamper this remains a key challenge for speciation research.     

Plasticity and cross‐tolerance to heterogeneous environments: divergent stress responses co‐evolved in an African fruit fly

Plastic adjustments of physiological tolerance to a particular stressor can result in fitness benefits for resistance that might manifest not only in that same environment but also be advantageous when faced with alternative environmental stressors, a phenomenon termed ‘cross‐tolerance’. The nature and magnitude of cross‐tolerance responses can provide important insights into the underlying genetic architecture, potential constraints on or versatility of an organism's stress responses. In this study, we tested for cross‐tolerance to a suite of abiotic factors that likely contribute to setting insect population dynamics and geographic range limits: heat, cold, desiccation and starvation resistance in adult Ceratitis rosa following acclimation to all these isolated individual conditions prior to stress assays. Traits of stress resistance scored included critical thermal (activity) limits, chill coma recovery time (CCRT), heat knockdown time (HKDT), desiccation and starvation resistance. In agreement with other studies, we found that acclimation to one stress typically increased resistance for that same stress experienced later in life. A more novel outcome, however, is that here we also found substantial evidence for cross‐tolerance. For example, we found an improvement in heat tolerance (critical thermal maxima, CT_max) following starvation or desiccation hardening and improved desiccation resistance following cold acclimation, indicating pronounced cross‐tolerance to these environmental stressors for the traits examined. We also found that two different traits of the same stress resistance differed in their responsiveness to the same stress conditions (e.g. HKDT was less cross‐resistant than CT_max). The results of this study have two major implications that are of broader importance: (i) that these traits likely co‐evolved to cope with diverse or simultaneous stressors, and (ii) that a set of common underlying physiological mechanisms might exist between apparently divergent stress responses in this species. This species may prove to be a valuable model for future work on the evolutionary and mechanistic basis of cross‐tolerance.     

Desiccation tolerance in <Emphasis Type="Italic">Staphylococcus aureus</Emphasis>

Staphylococcus aureus is a multidrug-resistant pathogen that not only causes a diverse array of human diseases, but also is able to survive in potentially dry and stressful environments, such as the human nose, on skin and on inanimate surfaces such as clothing and surfaces. This study investigated parameters governing desiccation tolerance of S. aureus and identified several components involved in the process. Initially, the role of environmental parameters such as temperature, growth phase, cell density, desiccation time and protectants in desiccation tolerance were determined. This established a robust model of desiccation tolerance in which S. aureus has the ability to survive on dry plastic surfaces for more than 1,097 days. Using a combination of a random screen and defined mutants, clpX, sigB and yjbH were identified as being required for desiccation tolerance. ClpX is a part of the ATP-dependent ClpXP protease, important for protein turnover, and YjbH has a proposed linked function. SigB is an accessory sigma factor with a role in generalized stress resistance. Understanding the molecular mechanisms that govern desiccation tolerance may determine the break points to be exploited to prevent the spread of this dangerous pathogen in hospitals and communities.     

Photobiont Selectivity and Interspecific Interactions in Lichen Communities. I. Culture Experiments with the Mycobiont Fulgensia bracteata

Abstract: Lichen communities are characterised by interspecific interactions that not only include interactions between different lichen species but also between the symbionts within a single lichen species. The community “Bunte Erdflechtengesellschaft”, growing on weathered calciferous rocks known as Gravel Alvar on Gotland (Baltic Sea, Sweden), shows a high complexity of inter‐ and intraspecific interactions, including Fulgensia bracteata, F. fulgens, Toninia sedifolia, Squamarina cartilaginea, Psora decipiens and Lecidea lurida. F. bracteata and F. fulgens are the dominant species of this community, showing a tendency to overgrow the other species involved and even parasitic behaviour. Culture experiments have been performed to investigate the selectivity of the mycobiont of F. bracteata towards a variety of potential photobionts. The results provide evidence for the selectivity of the mycobiont and varying compatibility of the respective symbionts that can be interpreted as a cascade of interdependent processes of specific and non‐specific reactions of the symbionts involved.     

Desiccation resistance of adult Queensland fruit flies Bactrocera tryoni decreases with age

Desiccation resistance is important for the survival of adult insects, although this key physiological trait has rarely been studied in tephritid flies. In the present study, desiccation resistance of female and male adult Queensland fruit flies Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) is determined with respect to age after adult eclosion. Resistance to acute starvation is measured over the same period to disentangle the competing roles of water loss and food deprivation. Survival of adult B. tryoni subjected to conditions of low humidity and starvation is reduced considerably compared with adults that are subjected to starvation alone. Desiccation resistance of adult female B. tryoni is generally lower than that of adult males. Desiccation resistance of adult B. tryoni declines in a continuous and regular manner over the first 20 days after adult eclosion. The regular pattern of declining resistance to desiccation with age in B. tryoni indicates that this reduction is not associated with the onset of maturity and maintenance of reproductive structures, nor with sexual activity. By contrast, resistance to starvation is similar at 0 and 6 days after adult eclosion, and declines thereafter. Survival under starvation and water stress is not related to wing length, which is a standard measure of fly size.     

Variation in adult life history and stress resistance across five species of<Emphasis Type="Italic">Drosophila</Emphasis>

Dry weight at eclosion, adult lifespan, lifetime fecundity, lipid and carbohydrate content at eclosion, and starvation and desiccation resistance at eclosion were assayed on a long-term laboratory population ofDrosophila melanogaster, and one recently wild-caught population each of four other species ofDrosophila, two from themelanogaster and two from theimmigrans species group. The relationships among trait means across the five species did not conform to expectations based on correlations among these traits inferred from selection studies onD. melanogaster. In particular, the expected positive relationships between fecundity and size/lipid content, lipid content and starvation resistance, carbohydrate (glycogen) content and desiccation resistance, and the expected negative relationship between lifespan and fecundity were not observed. Most traits were strongly positively correlated between sexes across species, except for fractional lipid content and starvation resistance per microgram lipid. For most traits, there was evidence for significant sexual dimorphism but the degree of dimorphism did not vary across species except in the case of adult lifespan, starvation resistance per microgram lipid, and desiccation resistance per microgram carbohydrate. Overall,D. nasuta nasuta andD. sulfurigaster neonasuta (immigrans group) were heavier at eclosion than themelanogaster group species, and tended to have somewhat higher absolute lipid content and starvation resistance. Yet, these twoimmigrans group species were shorter-lived and had lower average daily fecundity than themelanogaster group species. The smallest species,D. malerkotliana (melanogaster group), had relatively high daily fecundity, intermediate lifespan and high fractional lipid content, especially in females.D. ananassae (melanogaster group) had the highest absolute and fractional carbohydrate content, but its desiccation resistance per microgram carbohydrate was the lowest among the five species. In terms of overall performance, the laboratory population ofD. melanogaster was clearly superior, under laboratory conditions, to the other four species if adult lifespan, lifetime fecundity, average daily fecundity, and absolute starvation and desiccation resistance are considered. This finding is contrary to several recent reports of substantially higher adult lifespan and stress resistance in recently wild-caught flies, relative to flies maintained for a long time in discretegeneration laboratory cultures. Possible explanations for these apparent anomalies are discussed in the context of the differing selection pressures likely to be experienced byDrosophila populations in laboratory versus wild environments. This paper is dedicated to the memory of our friend and former colleague Dr Hans Raj Negi, who tragically passed away at a very young age in a road accident in November 2003.     

Influence of certain herbicides and a forest fertilizer on the nitrogen fixation by the lichen Peltigera praetextata

Summary Effects of herbicides (Garlone 3A, MCPA, 2,4-D and Krenite) and nitrogen fertilizer (NH₄NO₃), commonly used in Swedish forestry, on nitrogen fixation (C₂H₂-reduction) by Peltigera praetextata (Sommerf.) Zopf. (field and laboratory) and its phycobiont Nostoc sp. (laboratory) were studied. The alga was affected by the herbicides 2,4-D and Krenite and the fertilizer, with a decrease in nitrogenase activity. Nitrogen fixation by the lichen was not affected by herbicides but treatment with NH₄NO₃ led to depression of nitrogenase activity and serious disturbance of the symbiosis, the latter effect due to the fertilizer's lethal effects on the mycobiont (electron microscopy).Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - MCPA 2-methyl-4-chlorophenoxyacetic acid - Garlone 3A triclopyre - Krenite Ammoniumethylcarbamylphosphonate     

Trade-off of energy metabolites as well as body color phenotypes for starvation and desiccation resistance in montane populations of Drosophila melanogaster

Storage of energy metabolites has been investigated in different sets of laboratory selected desiccation or starvation resistant lines but few studies have examined such changes in wild-caught populations of Drosophila melanogaster. In contrast to parallel selection of desiccation and starvation tolerance under laboratory selection experiments, opposite clines were observed in wild populations of D. melanogaster. If resistance to desiccation and starvation occurs in opposite directions under field conditions, we may expect a trade-off for energy metabolites but such correlated changes are largely unknown. We tested whether there is a trade-off for storage as well as actual utilization of carbohydrates (trehalose and glycogen), lipids and proteins in D. melanogaster populations collected from different altitudes (512-2500 m). For desiccation resistance, darker flies (> 50% body melanization) store more body water content and endure greater loss of water (higher dehydration tolerance) as compared to lighter flies (< 30% body melanization). Based on within population analysis, we found evidence for coadapted phenotypes i.e. darker flies store and actually utilize more carbohydrates to confer greater desiccation resistance. In contrast, higher starvation resistance in lighter flies is associated with storage and actual utilization of greater lipid amount. However, darker and lighter flies did not vary in the rate of utilization of carbohydrates under desiccation stress; and of lipids under starvation stress. Thus, we did not find support for the hypothesis that a lower rate of utilization of energy metabolites may contribute to greater stress resistance. Further, for increased desiccation resistance of darker flies, about two-third of total energy budget is provided by carbohydrates. By contrast, lighter flies derive about 66% of total energy content from lipids which sustain higher starvation tolerance. Our results support evolutionary trade-off for storage as well as utilization of energy metabolites for desiccation versus starvation resistance in D. melanogaster.     

Intraspecific chemical variation within the crustose lichen genus Haematomma: anthraquinone production in selected cultured mycobionts as a response to stress and nutrient supply

The mycobionts isolated from selected species of Haematomma (Haematomma africanum, Haematomma fenzlianum, Haematomma flourescens, Haematomma persoonii, Haematomma stevensiae) have been successfully cultured. The chemical profile of the mycobionts could be effectively influenced and modulated by varying the composition of the nutrient medium using alternative carbohydrates (glucose, sucrose, and polyols). Under artifical laboratory conditions and simulated environmental stress (exposure to UV light, desiccation, and lower temperatures) the mycobionts began producing typical secondary lichen metabolites after an incubation time of 5–6 months. Modified Lilly and Barnett medium (LBM) and Murashige Skoog Medium favoured the production of depsides such as sphaerophorin and isosphaeric acid. Surprisingly, the mycobiont from H. stevensiae in modified MS medium produced two anthraquinones in the mycelia, haematommone at the base and russulone in the upper parts of the mycelium. By contrast, the natural lichen only produced these anthraquinones in the reddish orange apothecia. The mycobiont from H. flourescens only produced the expected lichexanthone in LBM, enriched with the polyols, sorbitol and mannitol. Once the media requirements and environmental stress factors that trigger polyketide production in lichen mycobionts have been determined, it is possible to obtain a particular lichen product by a completely defined procedure. Using such knowledge, we should be able to study polyketide expression in mycobionts under optimized culture conditions for various genetic applications.     

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.