Thallus Form Research Articles

How to build a lichen: from metabolite release to symbiotic interplay.

Exposing their vegetative bodies to the light, lichens are outstanding amongst other fungal symbioses. Not requiring a pre-established host, 'lichenized fungi' build an entirely new structure together with microbial photosynthetic partners that neither can form alone. The signals involved in the transition of a fungus and a compatible photosynthetic partner from a free-living to a symbiotic state culminating in thallus formation, termed 'lichenization', and in the maintenance of the symbiosis, are poorly understood. Here, we synthesise the puzzle pieces of the scarce knowledge available into an updated concept of signalling involved in lichenization, comprising five main stages: (1) the 'pre-contact stage', (2) the 'contact stage', (3) 'envelopment' of algal cells by the fungus, (4) their 'incorporation' into a pre-thallus and (5) 'differentiation' into a complex thallus. Considering the involvement of extracellularly released metabolites in each phase, we propose that compounds such as fungal lectins and algal cyclic peptides elicit early contact between the symbionts-to-be, whereas phytohormone signalling, antioxidant protection and carbon exchange through sugars and sugar alcohols are of continued importance throughout all stages. In the fully formed lichen thallus, secondary lichen metabolites and mineral nutrition are suggested to stabilize the functionalities of the thallus, including the associated microbiota.

Morphology of filamentous thalli of Analipus japonicus (Ralfsiales), temperature and irradiance effects on their formation and growth, and erect thallus formation in the field

Free-living filamentous thalli derived from an Analipus japonicus erect thallus were established in laboratory culture. The filamentous thalli were composed of slender filaments and densely pigmented oval or sub-oval cell masses. Temperature (5–20 °C) and irradiance (25–200 μmol photons m−2 s−1) effects on formation and growth of the thalli from three explant groups (isolated filaments, small cell masses, and large cell masses) were investigated for 15 days. These explants survived over the examined range. The isolated filaments had a high ability to form the thalli, and the isolated and new filaments formed filaments and cell masses. Meanwhile, the isolated and new cell masses formed only filaments, and their ability to grow and form the thalli decreased as they grew in size. Optimum culture conditions for formation and growth of the thalli from the isolated filaments, and the two cell mass growths were 15–20 °C and 100–200 μmol photons m−2 s−1. To produce erect thalli, the filamentous thalli were cultured on seedling threads for 6 weeks at 15 °C, 60 μmol photons m−2 s−1 and 20 cm s−1 water flow velocity. Erect axis density increased with increasing coverage of the filamentous thalli, and their cell masses (primordia of erect thallus holdfast) with mean area of about 140 × 103 μm2 began to produce erect axes. After 4 months of field cultivation, they grew into full-grown erect thalli of about 20 cm long. These findings show that the free-living filamentous thalli were applicable for stable seedling production of A. japonicus to be used in aquaculture.

Lessons from culturing lichen soredia

Vegetative propagules play various important roles in lichen biology. We cultured soredia of Cladonia lichens in vitro and obtained three noteworthy results. Firstly, soredia are a beneficial source for the isolation of lichen symbionts. The mycobiont was obtained from 66% and the photobiont from 67% of the cultured soredia that were not contaminated. Secondly, the development of soredia followed a previously recognized pattern, arachnoid stage – soredium field – primordium, but a thalline structure was not achieved. We suggest that thallus formation in vitro is a question of favourable environmental factors, not partners compatibility. Finally, we discovered that fungi, other than the mycobiont, as well as airborne contaminants are dispersed together with lichen soredia. This is the first-ever report of such a phenomenon. The possible ecological consequences are discussed. Cystobasidiomycete yeasts were found among these fungi. We isolated representatives of three different lineages from a single thallus suggesting a low specificity for this association.

Endogenous Phytohormones of Fern Polystichum aculeatum (L.) Roth Gametophytes at Different Stages of Morphogenesis in vitro Culture

The content of indole-3-acetic (IAA), gibberellic (GA3), abscisic acids (ABA), and cytokinins—zeatin (Z), zeatin glucoside (ZG), zeatin riboside (ZP), isopentenyladenin (iР), and isopentenyladenosine (iРА)—was determined using the method of high-performance liquid chromatography in fern Polystichum aculeatum gametophytes at different stages of morphogenesis in vitro culture. It was shown that, at the stage of spatulate prothallium development, gametophytes contained significant amounts of GA3 and IAA, while the content of ABA and cytokinins was low. The inactive ZG and active iP were dominant. At the stage of cordiform thallus formation, the level of GA3 increased by 1.6, IAA by 2.4, and ABA by 1.7 times. The content of inactive iPA increased twice, significantly increased the level of active ZR and iP. At the development stage of sporophyte on the surface of gametophyte thallus, the levels of IAA, ABA, and GA3 decreased. The accumulation of inactive ZG and decrease in the content of active zeatin and zeatin riboside were documented. At all stages of gametophytes’ morphogenesis, GA3 was dominant, and its highest content was detected during the development of archegonia and antheridia. The pattern of gibberellic acid and zeatin riboside accumulation has shown the key role of these hormones in the regulation of growth processes and the development of reproductive structures of P. aculeatum gametophytes.

Actin guides filamentous rhizoid growth and morphogenesis in the zoosporic fungus Chytriomyces hyalinus

ABSTRACTThe advantage of filamentous growth to the fungal lifestyle is so great that it arose multiple times. Most zoosporic fungi from phylum Chytridiomycota exhibit a monocentric thallus form consisting of anucleate filamentous rhizoids that anchor reproductive sporangia to substrata and absorb nutrients. Actin function during polarized growth and cytokinesis is well documented across eukaryotes, but its role in sculpting nonhyphal, nonyeast fungal cells is unknown. We sought to provide a basis for comparing actin organization among major fungal lineages and to investigate the effects of actin disruption on morphogenesis in a monocentric thallus. Using fluorescence microscopy, we observed fixed, rhodamine phalloidin–stained actin in chemically fixed Chytriomyces hyalinus, exemplifying monocentric thallus development within the diverse, zoosporic phylum Chytridiomycota. We also compared rhizoid lengths and rhizoid branching of thalli incubated with the actin inhibitor latrunculin B to determine the effects of actin disruption on morphology. Actin was concentrated at the tips of growing rhizoids. Actin cables typically formed cortical, parallel arrays in hyphae, but in mature sporangia they were concentrated in a funnel-shaped array in the central region. Thalli treated with latrunculin B had shorter rhizoids with fewer branches than controls. In both hyphae and monocentric thalli, actin localization coincides with active, polarized growth and cytokinesis. Specific actin localization patterns are largely shared between monocentric species but differ significantly from patterns observed in hyphae. Actin integrity is critical for sustaining filamentous growth in all fungi.

Phenotypic plasticity and morphological adaptation of Halimeda opuntia (Bryopsidales, Chlorophyta) to light intensity

ABSTRACTPlasticity gives plants the ability to adapt their morphology and growth to various environments. Environmental factors can lead to changes in the growth forms of algae. Halimeda opuntia was used as a model organism to determine the effects of highly irradiated and shade conditions in the field and its responses to low light conditions in the laboratory, which are known to be some of the main factors driving its morphological plasticity. To investigate the morphological and anatomical characteristics of the species, a total of 61 characteristics and 54 samples from Thailand, Japan and Indonesia were examined. In addition, we also tested the response under different light conditions in the laboratory. A phylogenetic tree was constructed using the tufA marker to confirm the species identification. We found that the thallus forms, branching patterns and segment shapes and sizes responded to the different light conditions. A brittle thallus network was found under shade conditions with low light intensity, while a compact thallus form was found under highly irradiated conditions. The H. opuntia segment shape was reniform under highly irradiated conditions, but its segments were adapted to be deeply trilobed in the lower thallus and tripartite shape in the upper thallus under shade conditions. The segment size also adapted; the segments in shaded areas were longer than those in sunny areas, which might have helped them increase their thallus height to ensure exposure to the high light habitat. This morphological variation and plasticity expanded our understanding of the range of H. opuntia plasticity present in the field and could help explain why this species is common and successfully colonizes various habitats worldwide.

Episternus onthophagi: a new monotypic genus of epizoic fungus found on Onthophagus beetles (Scarabaeoidea)

During research on the biological diversity of coprophagous Scarabaeoidea in the Polish Carpathians, a particular morphotype of epizoic fungus was found on several species of coprophages of the Onthophagus genus. The fungus occurs on the mesosternum and legs of the beetles. Detailed morphological observations and genetic analyses proved this epizoic fungus to be distinct from any other known species, it is new to science. It is very characteristic in terms of morphological features, as it grows in the form of thallus closely adhered to the insect exoskeleton with only the outermost layer of cells viable. The shape of thallus itself is determined by its location on insect’s body. Only vegetative structures and conidial reproduction were observed. In the presented study, we describe and establish this newly found morphotype in the rank of new monotypic genus and document its distinctness with eleven barcode sequences. Three gene based, LSU plus SSU plus ITS, phylogenetic positioning places the new genus firmly nested within Lasiosphaeriaceae (Sordariales) with Zygospermella insignis (Mouton) Cain as its closest relative. No epizoic colonization of dung beetles was described in Lasiosphaeriaceae to date nor Lasiosphaeriaceae species with similar morphology. The potential role of the described epizoic fungus in the living environment of the Onthophagus dung beetles is discussed.

Zopfochytrium is a new genus in the Chytridiales with distinct zoospore ultrastructure

While surveying chytrid diversity in lakes and streams, we found on cellulosic bait a chytrid that had both monocentric and polycentric thallus forms. We brought this chytrid into axenic culture from three sites in eastern North America, studied its thallus development and zoospore ultrastructure, and compared its 28S rDNA sequence with those of other members of the Chytridiomycota. Thallus morphology matched that described for the rare chytrid, Cladochytrium polystomum Zopf. Sporangia were spherical and produced numerous long discharge tubes. After discharge, zoospores remained in spherical clusters at the tips of the inoperculate openings of discharge tubes. After 10–30 min zoospores either swam away or encysted in place. Zoospore ultrastructural features included a cell coat, flagellar plug, and paracrystalline inclusion, features typical of members of the Chytridiales. However, the flagellar apparatus structure and organellar organization differed from that of zoospores previously described. Based on its molecular phylogeny and its zoospore ultrastructural features, we classify C. polystomum as a member of the Chytridiaceae in the Chytridiales. Because its thallus development and its ribosomal DNA sequences diverged decidedly from those of Cladochytrium tenue Nowak, the type species of Cladochytrium, we erected Zopfochytrium as a new genus for this chytrid.

Modifications in the structure of the lichen Cladonia thallus in the aftermath of habitat contamination and implications for its heavy-metal accumulation capacity

Phenotypic traits of lichens can be greatly modified by environmental factors. Granulose thalli on soil and podetia, densely covered with granules, referring to common and widespread lichen Cladonia cervicornis subsp. verticillata were found near zinc smelter. The granules are stratified, filled with fungal medulla and heavily encrusted with calcium oxalate weddellite crystals, not observed on regularly developed thalli of the species. Phylogenetic analysis revealed that deformed granulose forms belong to this taxon, showing that the phenotypic plasticity of the lichens of Cladonia can lead to the emergence of features that do not coincide with the taxonomic definition of the species. The heavy-metal accumulation capacity of both granulose and regular form of primary and secondary lichen thallus, in relation to the element content in corresponding substrate, was determined. Granulose-modified thalli accumulate greater amounts of heavy metals than regular ones, meaning that the bioaccumulation property of a given species may be greatly affected by morphological modifications. The granulose forms are also characterised by considerably higher ratios of Cd, Pb and As concentrations in lichen samples in relation to the corresponding substrates than regular ones. This means that collection of variously formed thalli should be avoided in biomonitoring sampling procedures. The results indicate that a substantial part of the element load, in particular zinc, in the examined lichen thalli collected near the smelter originates from atmospheric fallout.

Laju Pertumbuhan Rumput Laut Gracilaria sp Melalui Budidaya IMTA (Integrated Multi Trophic Aquaculture) di Pantai Geger, Nusa Dua, Kabupaten Badung, Bali

Gracilaria sp is red alga which belong to class rhodophyceae. It can grow in shallow salt water with a general characteristic is having a cylindrical and branched thallus form. Seaweed cultivation can be performed through IMTA (Integrated Multi Trophic Aquaculture) method. The basic principle of cultivation through the IMTA method is utilizing service of the low thropic level species in marine ecosystems, such as shells and seaweed. This method is believed to be able to overcome the environmental problems caused by cultivation activities, such as feces and uneaten feed. This research was perfomed 45 days, from February 25 to April 11, 2017. It was held at Geger Beach, Nusa Dua, Badung Regency, Bali, using a complete randomized design method that consist of three treatments and three repetition for each treatment. Treatment control (T1) consisted of 10 bunches of seaweed 100 grams without the abalone, treatement 2 (T2) consisted of 10 bunches of seaweed 100 grams and 20 abalones and treatement 3 (T3) consisted of 10 bunches of seaweed 100 grams and 40 abalones. The result showed that specific growth rate in Gracilaria sp. was highest in treatment T1, compared to treatment T2 and T3. This was caused by the cage construction of abalone amd the present of small mesh size net to cover the seaweed planting, which made the growth was not optimal. However, the use of a nets system in seaweed cultivation could minimize the loss of thallus. It was proved by the survival rate of seaweed in first, second and third, that is averagely high, ranging from 90% to 100%. Physical and chemical water quality parameters are also measured, and It showed that the water quality was suitable for the cultivation of both, seaweed and abalone.

A new genus and family for the misclassified chytrid, Rhizophlyctis harderi

A chytrid first discovered in Mediterranean sands and called Rhizophlyctis harderi was classified in the genus Rhizophlyctis based on its interbiotic vegetative thalli with multiple rhizoidal axes and resting thalli with tufts of rhizoid-like appendages. Developmental, electron microscopic and molecular analyses, however, have brought into question the proper placement of this chytrid. Because its original description was in German and not Latin, the name R. harderi is not validly published. We found that this chytrid produces three thallus forms that could place it in three different morpho-genera: Rhizophydium, Phlyctochytrium or Rhizophlyctis. The ultrastructural architecture of its zoospore is different from that of zoospores of Rhizophlyctis rosea, the type species for Rhizophlyctis, and shares zoospore ultrastructural characteristics with the Rhizophydiales. Zoospores of this chytrid exhibit a distinctive kinetosome-associated structure (KAS), a curved shield bridged to two of the kinetosomal triplets and a layered cap anterior to the kinetosome. Phylogenetic analyses of nuc rDNA also support the placement of this chytrid in the Rhizophydiales and not in the Rhizophlyctidales. Given its molecularly based phylogenetic placement and its distinctive zoospore architecture, we describe this chytrid in a new genus, Uebelmesseromyces, in the Rhizophydiales and erect Uebelmesseromycetaceae as a new family to accommodate it.

Recognition- and defense-related gene expression at 3 resynthesis stages in lichen symbionts.

Recognition and defense responses are early events in plant-pathogen interactions and between lichen symbionts. The effect of elicitors on responses between lichen symbionts is not well understood. The objective of this study was to compare the difference in recognition- and defense-related gene expression as a result of culture extracts (containing secreted water-soluble elicitors) from compatible and incompatible interactions at each of 3 resynthesis stages in the symbionts of Cladonia rangiferina. This study investigated gene expression by quantitative PCR in cultures of C. rangiferina and its algal partner, Asterochloris glomerata/irregularis, after incubation with liquid extracts from cultures of compatible and incompatible interactions at 3 early resynthesis stages. Recognition-related genes were significantly upregulated only after physical contact, demonstrating symbiont recognition in later resynthesis stages than expected. One of 3 defense-related genes, chit, showed significant downregulation in early resynthesis stages and upregulation in the third resynthesis stage, demonstrating a need for the absence of chitinase early in thallus formation and a need for its presence in later stages as an algal defense reaction. This study revealed that recognition- and defense-related genes are triggered by components in culture extracts at 3 stages of resynthesis, and some defense-related genes may be induced throughout thallus growth. The parasitic nature of the interaction shows parallels between lichen symbionts and plant pathogenic systems.

Colonisation and growth strategies in two Codium species (Bryopsidales, Chlorophyta) with different thallus forms

:In clonal macroalgae, evidence of guerrilla and phalanx growth strategies has been related to a differential response due to a heterogeneous habitat. However, some species of the green algal genus Codium may exhibit different growth strategies at different times of their development. Since the crustose species C. bernabei and the erect C. fragile had different thallus forms as well as ecological and geographic distributions, we used them to test the idea that despite morphological and growth differences, both species exhibited a similar propagation strategy. We cultured, under controlled conditions, individuals of both species to determine, first, if isolated utricles can function as propagation units; second, the type of growth at different stages of development; and, third, if species show differences in specific growth rate. Our results indicated that isolated utricles could be used for propagation because they had the ability to regenerate young mat-forming thalli in both species. Thallus regeneration implied morphological modifications of utricles, which combined guerrilla and phalanx strategies. In C. bernabei, the dynamic of vegetative propagation was independent of water movement in restoring a young thallus, which combined guerrilla and phalanx growth to colonise all available substrate. In C. fragile, water movement was required to stimulate the phalanx growth strategy and to regenerate the upright thallus. However, the guerrilla phase of C. fragile grew twice as fast as that of C. bernabei. We suggest that guerrilla-type growth is required to generate the prostrate filamentous network that first colonises new substratum. Later, the phalanx-type growth develops, generating the young mat-forming thalli in both studied species. Therefore, in these species the guerrilla strategy is not only a response to environmental constraints but also an obligate early ontogenetic stage of development. Further studies are needed to explore whether both growth strategies also occur in other species, thereby conferring them adaptive plasticity in heterogeneous environments.

Molecular phylogeny of the genus Dichotomaria (Galaxauraceae, Rhodophyta) from the Indo-Pacific region, including a new species D. hommersandii from South Africa

Members of the Galaxauraceae are all calcified and have a wide distribution from tropical to temperate regions. Of the four genera of Galaxauraceae, only Dichotomaria contains two distinct thallus forms, which are (1) a flattened form with either narrow (e.g. D. marginata) or wide (e.g. D. diesingiana) branches, and (2) a terete form with either slender (e.g. D. papillata) or robust (e.g. D. obtusata) branches. In this study, we present a molecular phylogeny of new Dichotomaria samples from the Indo-Pacific region and describe a new species, D. hommersandii S.L. Liu & S.M. Lin, from South Africa, based on rbcL sequence analysis and morphological evidence. Dichotomaria hommersandii is characterized by a thallus consisting of terete smooth branches (0.5–1.5 mm in diameter), a moniliform branching pattern with constrictions at the nodes, ramification of young branches derived from the damaged terminal regions of the old branches, and cystocarps containing a conspicuous fusion cell. Our phylogenetic analyses indicate that the terete thallus morphology could be the ancestral character state in Dichotomaria.

Hymenenopsis heterophylla gen. et sp. nov. (Delesseriaceae, Rhodophyta) from New Zealand, based on a red alga previously known as Hymenena palmata f. marginata sensu Kylin, with emphasis on its cystocarp development

Lin S.-M., Nelson W.A. and Hommersand M.H. 2012. Hymenenopsis heterophylla gen. et sp. nov. (Delesseriaceae, Rhodophyta) from New Zealand, based on a red alga previously known as Hymenena palmata f. marginata sensu Kylin, with emphasis on its cystocarp development. Phycologia 51: 62–73. DOI: 10.2216/11-38.1The alga known as Hymenena palmata f. marginata (Harvey) Kylin has been thought to be confined to the northern part of the South Island and the southern part of the North Island of New Zealand and is represented by a variety of thallus forms. The pattern of its apical growth is similar to that of Hymenena palmata (Harvey) Kylin, a poorly known species that is said to be widespread throughout New Zealand. As a consequence, H. palmata f. marginata has been misplaced under Hymenena for a long time. Unlike many species of Hymenena, the thallus branches entirely from the margins and lacks both macroscopic nerves and microscopic veins. Phylogenetic analysis of rbcL sequence data shows that, while H. palmata f. marginata is a member of the subfamily Phycodryoideae, it belongs in the Phycodryeae rather than the Cryptopleureae, a tribe that includes Hymenena, where it might otherwise be placed based on its gross morphology and growth by means of a marginal meristem with many obliquely dividing marginal cells. Procarp and cystocarp development are similar to that of the Phycodryeae, and the carposporangia are formed in chains rather than terminally as in Hymenena. The cystocarp of H. palmata f. marginata is unusual in that cells derived from the cystocarp floor form secondary pit connections with a large central fusion cell. We propose the name Hymenenopsis heterophylla S.-M. Lin, W.A. Nelson & Hommersand gen. et sp. nov. for this New Zealand red alga.

<p><strong>Lichens of the Gorgona Island National Natural Park (Cauca, Colombia)</strong><br /><strong>Líquenes del Parque Nacional Natural Isla de Gorgona (Cauca, Colombia)</strong></p>

As part of the Gorgona Island Natural National Park Scientific Expedition, an inventory of lichens was carried out. 83 lichen species, included in 34 genera and 14 families, were found. All of the reported species are registered for the first time for the Gorgona Island Natural National Park; 31 species are registered for the first time for Colombia and 61 for Cauca. The genus Eugeniella is registered for the first time for Colombia. The genera with the most number of species were Pyrenula (7 species), Tricharia (6), Strigula , Herpothallon (6) and Graphis (5), whereas 24 genera were represented by only one or two species. The Families Gomphillaceae and Graphidaceae were the best represented with 18 and 17 species respectively. 53% of species were corticolous, 45% were foliicolous and 3% were saxicolous. The majority of corticolous lichen species were found in the road leading to Mirador (14 species). The vast majority of lichens (93%) presented crustase thallus, whereas foliose, gelatinous and filamentous forms where least abundant (4, 3 and 1% respectively). Neither fruticose nor dimorphic thallus lichens were found. Ampliotrema amplius (Nyl.) Kalb, which is frequent in primary or less disturbed forests was found.

The free-living Hormosira banksii in Botany Bay, Australia: twenty years on.

Hormosira Banksii is the most common fucoid alga on rocky shores in temperate Australasia. It is the only fucoid alga in the region which also exists in free-living populations, and in this is it comparable with the unattached saltmarsh fucoids occurring in the northern hemisphere. The unattached populations of Hormosira are most often associated with mangroves, and the characteristics of such plants have been described from New Zealand (moore 1950, Bergquist 1959) and south eastern Australia (Clarke and Womersley 1981, King 1981). The population that grows amongst the pneumatophores in the Avicennia mangrove communities of southern Botany Bay, Australia, has been described by King (1981). It appears to have been derived from adjacent populations on rocky shores. The characteristics that distinguish the thalli from those of rocky shore populations include the lack of holdfasts, the absence of sexual reproduction, and the compact and highly branched thallus form. Other features regarded as characteristic of unattached fucoids include dwarfed and twisted thalli often with yellowish tips (Norton and Mathieson 1983). King (1981) noted that small thalli were common in the free-living Hormosira but also noted that someplants were about four times larger than those found on rocky shores.

Ontogeny and morphogenesis of the fruticose lichen Usnea florida (L.) Weber ex F.H. Wigg

The development of the fruticose lichen Usnea florida was described in accordance with the concept of a discrete approach to the description of higher plant ontogeny proposed by T.A. Rabotnov and A.A. Uranov. The ontogeny of the U. florida thallus, represented by 4 periods and 11 ontogenetic states, includes the development of fungal mycelium from a spore, its association with alga into a rudimentary thallus, thallus morphogenesis (the life form formation and decay of the thallus), and all related morphological changes in the apothecia and other structures generated by the thallus. Based on an analysis of morphogenetic processes during thallus formation, we proposed morphological criteria for the separation of ontogenetic states. The formation of the main frame of the thallus and its intensive branching occur before the development of the apothecia because of the switching to the reproductive function. Thallus aging and decay start even before carposome formation (v2 state); later, these processes become more intensive and reach a maximum at the g3 and ss states. Changes in thallus morphogenesis can include both delay and acceleration of morphogenetic processes, representing the manifestation of adaptation mechanisms in response to different environmental factors.

Early colonization of stone by freshwater lichens of restored habitats: A case study in northern Italy

This study evaluated the effectiveness and life-strategies of freshwater lichens in colonizing newly constructed stone structures in low-elevation streams in a small nature reserve in northern Italy. Species richness, size of thalli, morphological and ontogenetic traits of the species were related to the age of restored habitats. Lichen colonization was surprisingly rapid, indicating the high potential of these organisms in colonizing restored habitats. However, the species pool found in the restored habitats was different than that found in natural sites in the same study area. The age of newly constructed habitats influenced both species richness and thallus size of the two most frequent Verrucaria species . Verrucaria aquatilis was a rapid colonizer invading the substrate by several small-sized and thin thalli which soon supported a large number of small perithecia whose development began in the earlier phase of thallus formation. V. elaeomelaena, on the contrary, developed according to a different strategy, establishing a thick thallus on which relatively large perithecia were formed much later than in V. aquatilis. As these taxa are important photoautotrophic components of freshwater ecosystems more ecological knowledge is needed to evaluate the effectiveness of different measures of river restoration on lichen communities. The main practical implication of our study is related to the value of small stone structures, such as riffles and ramps, for enhancing the establishment of pioneer freshwater lichens to rapidly colonize newly available substrata.

Asymmetric Co-evolution in the Lichen Symbiosis Caused by a Limited Capacity for Adaptation in the Photobiont

It is proposed that lichen photobionts, compared to mycobionts, have very limited capacity to evolve adaptations to lichenization, so that the symbionts in lichens do not co-evolve. This is because lichens have (a) no sequential selection of photobiont cells from one lichen into another needed for Darwinian natural selection and (b) no photobiont sexual reproduction in the thallus. Molecular studies of lichen photobionts indicate no predictable patterns of photobiont lineages that occur in lichens so supporting this proposal. Any adaptation by photobionts accumulating beneficial mutations for lichenization is probably insignificant compared to the rate of mycobiont adaptation. This proposal poses questions for research relating the photobiont sexual cycle (genetic and cellular), the fate of photobiont lineages after lichenization, whether lineages of photobionts in thalli change with time, thallus formation by from spores as well as carbohydrate movement from photobionts to mycobionts and regulation of co-development of the symbionts in the thallus.