| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
|
First published online July 10, 2009; doi:10.3732/ajb.0800258 American Journal of Botany 96: 1409-1418 (2009) © 2009 Botanical Society of America, Inc. |
What's this? |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Bryology and Lichenology |
2 Department of Botany, The Field Museum, 1400 South Lake Shore Drive, Chicago, Illinois 60605-2496 USA 3 Department of Environmental Science and Policy, George Mason University, Fairfax, Virginia 22030-4444 USA 4 Laboratorio de Hongos, Instituto Nacional de Biodiversidad (INBio), Apdo. 22-3100, Santo Domingo de Heredia, Costa Rica 5 Botanisches Museum Berlin Dahlem, Königin-Luise-Strasse 6-8, D-14191 Berlin, Germany 6 Botany Department, Charles Darwin Foundation (AISBL), Puerto Ayora, Santa Cruz, Galápagos, Ecuador
Received for publication 24 July 2008. Accepted for publication 31 March 2009.
ABSTRACT
Phylogenetic diversity of lichen photobionts is low compared to that of fungal counterparts. Most lichen fungi are thought to be associated with just four photobiont genera, among them the cyanobacteria Nostoc and Scytonema, two of the most important nitrogen fixers in humid ecosystems. Although many Nostoc photobionts have been identified using isolated cultures and sequences, the identity of Scytonema photobionts has never been confirmed by culturing or sequencing. We investigated the phylogenetic placement of presumed Scytonema photobionts and unicellular morphotypes previously assigned to Chroococcus, from tropical Dictyonema, Acantholichen, Coccocarpia, and Stereocaulon lichens. While we confirm that filamentous and unicellular photobiont morphotypes belong to a single clade, this clade does not cluster with Scytonema but represents a novel, previously unrecognized, highly diverse, exclusively lichenized lineage, for which the name Rhizonema is available. The phylogenetic structure observed in this novel lineage suggests absence of coevolution with associated mycobionts at the species or clade level. Instead, highly efficient photobiont strains appear to have evolved through photobiont sharing between unrelated, but ecologically similar, coexisting lineages of lichenized fungi ("lichen guilds"), via the selection of particular photobiont strains through and subsequent horizontal transfer among unrelated mycobionts, a phenomenon not unlike crop domestication.
Key Words: Acantholichen • Coccocarpia • cyanobacteria • Dictyonema • lichens • Nostoc • rDNA sequence analyses • Scytonema • symbiosis • neotropics
Lichens are among the most intriguing symbioses known, having evolved morphological and physiological features not known in their individual components (Ahmadjian, 1993
; Purvis, 2000; Brodo et al., 2001; Sanders, 2001). They form important and sometimes dominant components of terrestrial ecosystems. With the advent of molecular methods, the study of lichen mycobionts has significantly changed our knowledge of fungal evolution (Lutzoni et al., 2004; Miadlikowska et al., 2006; Lumbsch and Huhndorf, 2007). On the other hand, comparatively few lichen photobionts have been studied in detail (Friedl and Rokitta, 1997; Paulsrud et al., 1998; Helms et al., 2001; Piercey-Normore and DePriest, 2001; Schultz et al., 2001; Rikkinen et al., 2002; Schultz and Büdel, 2002; Lohtander et al., 2003; López-Bautista and Chapman, 2003; Rikkinen, 2003; Baloch and Grube, 2006; Nyati et al., 2007). Yet, lichen photobionts contribute significantly to the evolutionary diversity of photosynthetic bacteria and green algae. As this paper demonstrates, their study promises discoveries as exciting as those of their fungal counterparts.
Current classifications of lichen photobionts have not changed much since the second half of the last century (Ahmadjian, 1967; Tschermak-Woess, 1988). While lichen fungi number more than 15000 species in over 1000 genera at the world level (Kirk et al., 2001), only little more than 100 species and 50 genera of lichen photobionts have been reported (Ahmadjian, 1967; Büdel and Henssen, 1983; Tschermak-Woess, 1988; Nyati et al., 2007). Of these, four genera are thought to occur in the vast majority of lichens: the chlorophytes Trebouxia and Trentepohlia and the cyanobacteria Nostoc and Scytonema (Ahmadjian, 1967; Tschermak-Woess, 1988; Büdel, 1992). Culturing and sequencing has confirmed the identity of many of these photobionts, but for the diverse scytonematoid forms inhabiting tropical cyanolichens, no such information has yet been obtained. Cyanolichens make up 10% of all lichens and are widely dispersed among both the Ascomycota and Basidiomycota (Büdel, 1992; Rambold et al., 1998; Rikkinen et al., 2002; Herrera-Campos et al., 2005; Lücking, 2008): in the Ascomycota, they are found in three major classes (Lichinomycetes, Eurotiomycetes, and Lecanoromycetes), at least five orders (Lichinales, Chaetothyriales, Agyriales, Peltigerales, Lecanorales), and in the Basidiomycota in the class Agaricomycetes (Agaricales).
The taxonomy of cyanobacterial photobionts and their free-living counterparts is traditionally based on morphological characters, as well as cell anatomy and cell division (De Toni, 1907; Fogg et al., 1973; Stanier and Cohen-Bazire, 1977; Rippka et al., 1979; Boone and Castenholtz, 2001; Cavalier-Smith, 2002; Garrity et al., 2004). The group contains five orders, two of which are unicellular, whereas the others are filamentous. The filamentous Nostocales (with false branching) and Stigonematales (with true branching) feature specialized heterocytes capable of fixing atmospheric nitrogen. Both orders have been confirmed as monophyletic in molecular studies (Ishida et al., 1997; Honda et al., 1999; Turner et al., 2001; Henson et al., 2004). Lichenized cyanobacteria were mostly assigned to either Chroococcales (unicellular morphotypes) or Nostocales (filamentous morphotypes) (Ahmadjian, 1967; Parmasto, 1978; Arvidsson, 1982; Tschermak-Woess, 1988); the photobiont of the well-known tropical montane lichen Dictyonema glabratum was originally identified as Chroococcus and that of the closely related Dictyonema sericeum as Scytonema, although it was later suspected that both belong to the same genus (Parmasto, 1978; Tschermak-Woess, 1988; Chaves et al., 2004). The main difference between the two commonly identified filamentous cyanobacteria in lichens, Nostoc and Scytonema, is their colonial morphology: Nostoc forms gelatinous, amorphous matrices in which the filaments are loosely dispersed, whereas Scytonema is truly aerial and forms felt-like mats in which the filaments have solid gelatinous sheaths. Phylogenetic studies support the monophyly of Nostoc, including both lichenized and nonlichenized forms (Paulsrud et al., 1998; Schultz et al., 2001; Rikkinen et al., 2002; Schultz and Büdel, 2002; Rikkinen, 2003, but see O’Brien et al., 2005). However, no molecular data have been collected for lichenized Scytonema species until the current study.
To assess the phylogenetic identity of tropical cyanolichen photobionts hitherto identified as Scytonema and the unicellular morphotypes believed to represent Chroococcus, we analyzed 16S rDNA sequences representing three major lineages of cyanolichen fungi in the Ascomycota and Basidiomycota. Our objectives were (1) to determine the phylogenetic position of lichenized scytonematoid photobionts and (2) to determine whether different photobiont morphotypes found in foliose and filamentous lichens are phylogenetically distinct. In this paper, we report on the surprising finding that the lichenized photobionts supposed to represent Scytonema actually do not belong to that genus and instead form a novel, previously unrecognized lineage within the filamentous cyanobacteria with heterocytes.
MATERIALS AND METHODS
Taxon sampling
The data set consisted of 68 terminal units, including new sequences obtained from photobionts of 20 specimens of Dictyonema, Acantholichen, and Coccocarpia lichens collected from 13 tropical localities. One specimen of a free-living Scytonema sp., collected at the Organization for Tropical Studies Las Cruces Biological Station in Costa Rica, was also included (Table 1). We added GenBank sequences from 23 specimens representing free-living and lichenized members of Nostoc and Scytonema (Nostocales), including unidentified cyanobacteria reported from cephalodia of the genus Stereocaulon, 14 sequences representing nonlichenized members of the Nostocales, and 10 sequences representing basal cyanobacterial lineages according to various sources (Ishida et al., 1997; Honda et al., 1999; Schultz et al., 2001; Turner et al., 2001; Rikkinen et al., 2002; Schultz and Büdel, 2002; Rikkinen, 2003; Henson et al., 2004).
|
, and the photographs in Chaves et al., 2004), whereas in the remaining Dictyonema species the photobiont is filamentous (e.g., depicted in Lücking, 2008). Before sequencing, we prepared thin hand-sections of all specimens to confirm the presence of a particular photobiont morphotype. The sections were studied in water without staining, using a Zeiss (Jena, Germany) Axioscop 2 compound microscope connected to a Dage (Michigan City, Indiana, USA) MTI DC-330 3CCD digital color camera. Macrophotographs of the lichens were taken with a Nikon (Tokyo, Japan) F301 SLR camera, a Nikon Coolpix 5400 digital camera, and a Canon (Lake Success, New York, USA) Powershot A640 digital camera.
DNA extraction, amplification, and sequencing
Genomic DNA was extracted from lichenized tissue using a FastDNA Spin Kit (Qbiogen, Illkirch, France), and the 16S rRNA gene was amplified and sequenced using a combination of universal bacterial primers and cyanobacteria-specific primers obtained from (Nübel et al., 1997). We used L27F (5'-AGAGTTTGATCMTGGCTCAG-3') and CYA781R (5'-GACTACWGGGGTATCTAATCCCWTT-3') for amplification, and then sequenced the PCR products using L27F and 355R (5'-ACTCCTACGGGAGGCAGC-3') for one portion, CYA106F (5'-CGGACGGGTGAGTAACGCGTGA-3') and CYA781R for another, then combined the contigs. Ambiguities caused by multiple operons (Boyer et al., 2001; Yeager et al., 2007) or multiple organisms in the lichen were resolved by cloning short PCR products using the TOPO TA Cloning Kit (Invitrogen, Carlsbad, California, USA). The purified PCR products from clones were used in standard sequencing reactions and run on a SCE-9610 capillary machine (SpectruMedix, Reedsville, Pennsylvania, USA). The data collected were analyzed using BASESPECTRUM software (SpectruMedix) and about 600–700 bases were collected for each primer used. Sequence fragments were assembled with the program SEQUENCHER version 4.7 (Gene Codes Corp., Ann Arbor, Michigan, USA). Sequences were subjected to BLAST searches to verify their closest relatives and to detect potential contaminations (Altschul et al., 1997).
Sequence alignment and phylogenetic analysis
Sequence positions representing primer sites and missing data at the terminal ends of sequences were trimmed, and ambiguous regions were excluded to produce an approximately 660-bp alignment. The data set was prealigned using the program CLUSTAL_W 2 (Larkin et al., 2007) and corrected manually in the program MacClade version 4.08 (Maddison and Maddison, 2005) and submitted to the database TreeBase (http://TreeBase.org, accession no. SN3949).
Maximum likelihood searches were performed in the program GARLI v0.95 (Zwickl, 2006). Each run used random starting trees and the auto-terminate setting and default parameters (10000 generations without improving topology required for termination, 0.01 lnL increase required for a new topology to be considered, 0.05 score improvement threshold). Nonparametric bootstrap replicates (significant at 70%) were used to calculate a majority rule consensus tree in GARLI to assess clade support. Bayesian analyses used the Metropolis-coupled Markov chain Monte Carlo (MCMCMC) method in the program MrBayes version 3.0b4 (Ronquist and Huelsenbeck, 2003). Analyses were run under the GTR model (estimated with MODELTEST version 3.7, Posada and Crandall, 1998) using a gamma-distributed rate parameter and a proportion of invariable sites. Two parallel MCMCMC runs were performed each using four chains and 5000000 generations, sampling trees every 100th generation. The proportion of burn-in trees sampled before reaching equilibrium was estimated by plotting likelihood scores as a function of the number of generations. Posterior probabilities (PP) were determined by calculating a majority-rule consensus tree in PAUP* with the proportion of trees gathered after convergence of likelihood scores was reached, and clades with PP 
0.95 were considered to be significantly supported.
RESULTS
Phylogenetic placement of the new lineage
Maximum likelihood analysis resulted in one optimal tree (Fig. 1) with a score of –lnL = 5011.525 using the GTR+
+I model with nucleotide frequencies estimated (A = 0.2598, C = 0.1981, G = 0.3332, T = 0.2089), a rate matrix of substitutions (A-C = 0.9053, A-G = 2.0264, A-T = 1.4046, C-G = 0.4587, C-T = 3.4507, G-T = 1.0), proportion of invariable sites = 0.3352, and 
= 0.5118. Bayesian runs converged after 750000 generations and 87700 trees were used to compute posterior probability values. The topology of the Bayesian consensus tree (not shown) was fully consistent with that of the ML tree.
|
Morphotypes of the novel scytonematoid photobiont lineage do not represent phylogenetically distinct clades
The novel cyanobacterial lineage was found to associate with at least 12 species and four genera of lichen mycobionts (Acantholichen, Coccocarpia, Dictyonema, Stereocaulon) that represent a wide range of morphological variation (Fig. 2) and systematic affinities. In the basidiolichen fungus Dictyonema glabratum (Fig. 2A), there are nearly unicellular or pseudocolonial forms reminescent of Chroococcus (Fig. 2B), whereas D. schenkianum (Fig. 2C), D. aeruginosulum, D. hernandezii, D. interruptum, D. phyllogenum, and D. sericeum, associate with distinctly filamentous forms resembling Scytonema sensu stricto (Fig. 2D). The related Acantholichen pannarioides (Fig. 2E) and the ascolichen fungi Coccocarpia stellata (Fig. 2F), C. filiformis, C. palmicola, as well as the cephalodia of Stereocaulon fronduliferum and S. ramulosum, feature photobiont morphotypes identical to those found in D. glabratum. Our phylogeny indicates that these morphotypes do not form a monophyletic group but are dispersed over the entire clade: several well-supported subclades include both the filamentous forms found typically in D. schenckianum and also pseudocolonial forms found in D. glabratum and Coccocarpia palmicola (Fig. 1). Whether these subclades actually represent different species cannot be stated at present but is likely considering the species concept assigned to the genetic variation found in the Nostoc clade (Fig. 1).
|
A novel lineage of filamentous cyanobacteria found only in lichen symbioses
Lichen photobionts assumed to be members of the well-known cyanobacterial genus Scytonema represent a previously unrecognized, entirely lichenized lineage of filamentous cyanobacteria with heterocytes. One species of this novel lineage was originally described as Calothrix interrupta (De Toni, 1907; Parmasto, 1978), and for the lineage itself, the generic name Rhizonema is available, as discussed in the following section. Although filamentous morphotypes are similar to Scytonema, the novel clade is phylogenetically more closely related to Nostoc, Anabaena, Fischerella, and Hapalosiphon and related genera, from which it can be distinguished by the scytonematoid habit: filaments of broad, subrectangular cells enclosed in a well-defined, gelatinous sheath. All representatives of this novel clade are members of a lichen-forming association, which also denotes its defining characteristic when compared to Scytonema s.str. The photobiont associated with filamentous species of Dictyonema (in which nonlichenized filaments connected to lichenized filaments are frequently observed) is very similar in cell shape, cell arrangement, cell division, and nature of the gelatinous sheath to free-living representatives of the core group of Scytonema, S. hoffmanii, and S. javanicum (Komárek and Anagnostidis, 2005). Lichenized Rhizonema and nonlichenized Scytonema are often found growing side by side in the same microhabitats, e.g., on leaf surfaces in tropical rainforests (Freiberg, 1998; Lücking, 2008). This phenomenon is exemplified by the free-living Scytonema (R09) collected in the same habitat on the same substrate as the lichenized Rhizonema associated with Dictyonema schenkianum (R07). While cultures are needed to establish the exact morphological variation in Rhizonema, we have to assume that, other than lichenization, differences between Rhizonema and Scytonema are to be found at the level of microanatomy and physiology, rather than morphology, a situation not uncommon in prokaryotes. This problem has important implications for ecosystem research because a large proportion of nitrogen-fixing cyanobacteria previously believed to occur both lichenized and free-living (Forman, 1975; Becker, 1980; Green et al., 1980; Fritz-Sheridan, 1988; Antoine, 2004; Cornelissen et al., 2007) appear to be restricted to lichen symbioses.
We predict that more tropical cyanolichens reported to feature Scytonema photobionts harbor members of the novel Rhizonema lineage. Thus far, only one scytonematoid photobiont was identified as Scytonema on the basis of cultures: S. hofmannii in the lichen Heppia lutosa (Wetmore, 1970). However, identifications of cultures in the absence of sequences are doubtful: during the course of this study, we found at least one deposited Scytonema UTEX culture to actually belong in the Stigonematales (J. D. Lawrey et al., unpublished data). Even if there are genuine Scytonema species among cyanolichens not yet sequenced, it will not affect the finding that the novel clade discovered here is phylogenetically distinct from Scytonema s.str. The cyanobacterial photobionts reported in other genera of tropical lichenized fungi (Santesson, 1952; Herrera-Campos et al., 2005; Lücking, 2008) are morphologically identical to those found in Dictyonema and Coccocarpia, suggesting that the novel lineage may also occur in members of the Pannariaceae as well as the genera Pyrenothrix (Chaetothyriales) and Bacidina p.p. (Lecanorales), and as accessory photobiont in cephalodia of Pilocarpaceae (Calopadia, Calopadiopsis, Lasioloma, Sporopodium).
A name for the novel clade
Reviews on lichen photobionts by Ahmadjian (1967) and Tschermak-Woess (1988) suggested that none of the photobionts belonging to the novel clade had been formally described. Yet, several names treated as lichen fungi in the monograph of Dictyonema by Parmasto (1978) were originally established as blue-green "algae," including Calothrix interrupta Carmich. ex Hook. However, the International Code of Botanical Nomenclature (ICBN) states that if a name is based on material consisting of more than one organism, it has to be applied to the component that was originally intended in the description. Calothrix interrupta was originally described as blue-green "alga," without noting its lichenized condition. The description of this species as a blue-green alga was overlooked by other workers (Parmasto, 1978; Coppins and James, 1979; Purvis et al., 1992; Etayo et al., 1995) and requires correction: the name Calothrix interrupta applies to the cyanobacterial photobiont of the lichen fungus known as "Dictyonema" interruptum, and not the fungal component, which requires a new name. Upon further investigation, we found that a separate genus, Rhizonema, had been established by Thwaites in Smith and Sowerby (1849) for Calothrix interrupta. Thus, Rhizonema is the valid generic name for the novel cyanobacterial clade discovered here. Our nomenclatural conclusions are supported by morphological and molecular evidence: the morphological data of "Dictyonema" interruptum published and illustrated by Parmasto (1978), Coppins and James (1979), Etayo et al. (1995), and Purvis et al. (1992) and the fact that the sequence of the studied photobiont sample of "Dictyonema" interruptum falls into the novel clade (see Fig. 1).
Morphological variation suggests morphogenetic effects in the lichen symbiosis
Morphologically, the lichenized Rhizonema photobionts are more variable than free-living members of the genus Scytonema, including both filamentous forms and unicellular, pseudo-colonial forms. Our phylogeny confirms that filamentous and unicellular forms of Rhizonema belong to the same genus and possibly even to the same species in individual cases (Roskin, 1970; Parmasto, 1978; Tschermak-Woess, 1988; Chaves et al., 2004). The pseudocolonial forms are dispersed over the entire clade and do not form a monophyletic lineage, suggesting that the mycobionts of these lichens associate with photobiont strains that are filamentous in origin but broken down into unicellular clusters in the lichen symbiosis. Cultures are required to ultimately demonstrate this phenomenon, but morphogenetic effects of lichen mycobionts on photobionts are well documented, especially in the related cyanobacterial genus Nostoc, where the pseudocolonial forms found in Pannaria, Peltigera, and Pseudocyphellaria cluster undifferentially with the gelatinous-filamentous, free-living forms (Koriem and Ahmadjian, 1986; Boissiere et al., 1987; O’Brien et al., 2005; see also Fig. 1). The opposite case has been demonstrated for the green algal photobionts of the lichen fungus Psoroglaena, which are transformed from unicellular into pseudofilamentous forms (Nyati et al., 2007). Yet another pattern occurs in photobionts of the green algal family Trentepohliaceae, where molecular studies suggest that disparate morphotypes found in different lichen associations actually represent different species (López-Bautista and Chapman, 2003; Baloch and Grube, 2006).
High specificity at the genus level but low specificity at the infrageneric level
Rambold et al. (1998) define specificity of lichen symbionts as "the range and taxonomic relatedness of acceptable partners" and selectivity as the "preferential interaction of the organisms." The distinction between specificity and selectivity is not clear cut, and we consider the principal difference to be phylogenetic. Specificity means selectivity to the point where the selected photobionts represent a monophyletic clade (or paraphyletic grade), implying that the character providing compatibility between mycobionts and photobionts represents a synapomorphy of the photobiont. Selectivity should be used when the selected photobionts are unrelated and polyphyletic at the corresponding taxonomic level (and thus the compatibility does not represent a photobiont synapomorphy). Lichen fungi are usually specific toward their photobionts at the genus level, i.e., a particular lichen fungus associates with photobionts of a particular genus. Within that genus, the lichen fungus is selective rather than specific, meaning that it associates with different species or strains that are not necessarily closely related, as has been demonstrated for trebouxioid photobionts (Kroken and Taylor, 2000; Piercey-Normore and DePriest, 2001; Beck et al., 2002; Piercey-Normore, 2004; Blaha et al., 2006; Hauck et al., 2007), as well as for trentepohlioid lineages (Baloch and Grube, 2006). Cyanolichens appear also to be specific at the genus level, although lack of accurate identifications of cyanobionts makes the assessment of specificity more difficult for these associations. Our results indicate strong genus-level specificity of the involved mycobionts toward Rhizonema photobionts. However, the phylogenetic dispersion of photobionts originating from the same mycobiont associations (e.g., from Dictyonema glabratum or Coccocarpia palmicola), as well as the fact that closely related photobiont strains are found in phylogenetically unrelated mycobiont associations (e.g., in D. schenkianum and C. palmicola), suggests absence of specificity at the species or strain level.
Despite the absence of species-level specificity, and cospeciation, in most lichen associations, there is abundant evidence for photobiont selectivitiy in both chlorolichens and cyanolichens (Rambold et al., 1998; Beck, 2002). Apparently, only certain compatible strains of photobionts are used by particular mycobionts to form successful lichen associations. For Nostoc cyanolichens, studies indicate that compatible strains are obtained from either free-living or lichenized populations (Paulsrud et al., 2000; Oksanen et al., 2002; Rikkinen et al., 2002; O’Brien et al., 2005). In situations where little selectivity is observed, there is usually an obvious lack of available photobionts that causes this low selectivity, such as in particularly harsh environments in Antarctica (Wirtz et al., 2003). Whether selectivity applies to the lichen fungi associated with cyanobacteria of the novel clade cannot be said with the data set at hand. More sequences especially from common lichen associations such as D. glabratum and C. palmicola will have to be analyzed to clarify this question.
Unrelated lichen mycobionts select photobiont strains through photobiont sharing
The wide range of fungi that associate with the novel Rhizonema clade, belonging to four orders and three classes in the Ascomycota and Basidiomycota, implies that photobiont sharing took place frequently among phylogenetically unrelated but ecologically equivalent taxa. The involved cyanolichens (e.g., in the genera Coccocarpia and Dictyonema) exhibit strong ecological coherence, being characteristic elements of tropical montane to alpine rain and cloud forest and paramo vegetation (Sipman, 1999; Büdel et al., 2000; Chaves et al., 2004; Lücking et al., 2007). Instead of cospeciation, which implies reciprocal monophyly of the interacting organisms (Futuyma and Slatkin, 1983; Thompson, 1994), these lichen guilds (Rikkinen et al., 2002) display an alternative strategy involving repeated horizontal photobiont transfer, a phenomenon also found to a certain extent in chlorolichens associated with Phycopeltis or Trebouxia photobionts (Piercey-Normore and DePriest, 2001; Rikkinen, 2003; Gorbushina et al., 2005; Baloch and Grube, 2006; Nelsen and Gargas, 2008).
We envision a scenario in which photobiont strains are selected based on their compatibility with lichen mycobionts (mycobiont selection) and their ability to contribute successfully to establishment and growth of the lichen thallus (environmental selection). Differential environmental selection acting upon the lichen association will increase the frequency of particular photobiont–mycobiont associations, and thus particular photobiont strains, in the lichen populations. Increased frequency, in turn, will increase the likelihood of the photobiont strains being available for other lichen associations. If such strains are shared among unrelated mycobionts, by means of resymbiosis with sexually produced asco- or basidiospores or by mycobionts "stealing" photobionts away from other lichens or photobionts accidentally escaping from lichen symbioses, it follows that unrelated lichen mycobionts inadvertently "cooperate" in the selection, "domestication," and distribution of photobionts in the lichen community by proliferating particular strains that increase the success of the lichen association. This phenomenon is not unlike human crop domestication, in which farmers develop improved crop strains which are then shared with, and enhanced by, other farmers, leading to higher yields for the farmers and proliferation of the most widely used varieties of crops. This analogy underlines the notion of lichen symbioses as "fungi that have discovered agriculture" (Goward, 1994, p. 14). It also shows that environmental selection acting upon symbiotic systems does not necessarily have consequences at the species level only, but can also be effective across a pool of unrelated species, resulting in one component of the symbiosis forming a polyphyletic assemblage (Law, 1985; Piercey-Normore and DePriest, 2001).
One important precondition for this scenario is genetic variation among the associated photobionts, which requires genetic recombination (Law and Lewis, 1983). In cyanobacteria, genetic variation is accomplished by mechanisms such as transformation (Lorenz and Wackernagel, 1994; Rudi et al., 1998; Gugger et al., 2005). If the Rhizonema clade stems from an already lichenized ancestor, it means that considerable genetic differentiation took place after an initial lichenization event, suggesting that the mechanisms for genetic recombination in cyanobacteria continue to function in the lichenized condition or in the brief nonlichenized stages "in-between lichens."
The notion of fungi cultivating photobionts invites comparison with leaf-cutter ants, in which the protagonists have long been regarded as "farmers" that cultivate fungi (Mueller and Gerardo, 2002; Mueller et al., 2005). Leaf-cutter ants form one of the most complex symbioses known, involving at least four components that not only mimic farming but even biological pest control against fungal parasites (Currie et al., 1999, 2003). One of the main differences with the lichen symbiosis is the high level of coevolution between the involved partners, which is not seen in lichens. In analogy with agriculture, this pattern would correspond to all farmers coming from the same family. And while the lichen symbiosis is less specific, it surpasses the leaf-cutter ant symbiosis in complexity. Many species of fungi are known to colonize and parasitize lichen thalli (Richardson, 1999; Lawrey and Diederich, 2003; Hawksworth, 2004), and recent studies suggest that lichen thalli harbor an unexpected diversity of endolichenic fungi and bacteria with unique chemical properties (Cardinale et al., 2006; Liba et al., 2006; Arnold, 2007).
Our study demonstrates that phylogenetic approaches, in combination with inventories of understudied organisms in little explored ecogeographical regions, provide opportunities to discover new microbial communities, reconstruct evolutionary histories, and explore novel ecologies and functional roles of microbes in the environment. Cyanobacteria represent one of the most ancient and ecologically diverse groups of microorganisms, and the discovery of a diverse, novel lineage of lichenized cyanobacteria reminds us that the microbial world is still largely a terra incognita (Curtis and Sloan, 2005).
FOOTNOTES
1 Material used in this study was collected in Guatemala, Costa Rica, Peru, Bolivia, and The Philippines, in the framework of three NSF grants to The Field Museum: DEB 0206125 (PI R.L.), DEB 0516116 (PI Thorsten Lumbsch, Co-PI R.L.), and DEB 0715660 (PI R.L.), and with support by the OTS for a tropical lichen workshop in Las Cruces, Costa Rica. The Costa Rican MINAE and SINAC, as well as INBio, are thanked for their support and cooperation in obtaining permits for collecting and DNA studies. The Galápagos National Park Service provided specimens permits for analysis. F.B. is particularly grateful to National Park Director E. Muñoz and Technical Director W. Tapia for continued support of the Galápagos lichen inventory. Specimens were collected in the Galápagos Islands with support from a National Geographic Research Grant 8162–07 to F.B. This is a Charles Darwin Research Station publication with CDRS no. 1088. F. Barrie is kindly thanked for nomenclatural advice. 
7 Author for correspondence (e-mail: rlucking{at}fieldmuseum.org)
LITERATURE CITED
Ahmadjian, V. 1967. A guide to the algae occurring as lichen symbionts: Isolation, culture, cultural physiology, and identification. Phycologia 6: 127–160.
Ahmadjian, V. 1993. The lichen symbiosis. John Wiley, New York, New York, USA.
Altschul, S. F., T. L. Madden, A. A. Schäffer, J. Zhang, Z. Zhang, W. Miller, AND D. J. Lipman. 1997. Gapped BLAST and PSI-BLAST: A new generation of protein database search programs. Nucleic Acids Research 25: 3389–3402.
Antoine, M. E. 2004. An ecophysiological approach to quantifying nitrogen fixation by Lobaria oregana. Bryologist 107: 82–87.[CrossRef]
Arnold, A. E. 2007. Understanding the diversity of foliar endophytic fungi: Progress, challenges, and frontiers, a review. Fungal Biology Reviews 21: 51–66.[CrossRef]
Arvidsson, L. 1982. A monograph of the lichen genus Coccocarpia. Opera Botanica 67: 1–86.
Baloch, E., AND M. Grube. 2006. Evolution and phylogenetic relationships within Porinaceae (Ostropomycetidae), focusing on foliicolous species. Mycological Research 110: 125–136.[CrossRef][Web of Science][Medline]
Beck, A. 2002. Photobionts: Diversity and selectivity in lichen symbioses. International Lichenological Newsletter 35: 18–24.
Beck, A., T. Kasalicky, AND G. Rambold. 2002. Myco-photobiontal selection in a Mediterranean cryptogam community with Fulgensia fulgida. New Phytologist 153: 317–326.[CrossRef][Web of Science]
Becker, V. E. 1980. Nitrogen fixing lichens in forests of the southern Appalachian Mountains of North Carolina. Bryologist 83: 29–39.[CrossRef]
Blaha, J., E. Baloch, AND M. Grube. 2006. High photobiont diversity associated with the euryoecious lichen-forming ascomycete Lecanora rupicola (Lecanoraceae, Ascomycota). Biological Journal of the Linnean Society 88: 283–293.[CrossRef][Web of Science]
Boissiere, J. C., M. C. Boissiere, P. Champion-Arnaud, R. Lallement, AND J. Wagner. 1987. Le cycle des Nostoc des genres Peltigera et Collema en cultures in vitro et dans le thalle lichenique. Canadian Journal of Botany 65: 1468–1477.[CrossRef]
Boone, D. R., AND R. W. Castenholz. [eds.]. 2001. Bergey’s manual of systematic bacteriology, vol. 1, The Archaea and the deeply branching and phototrophic bacteria, 2nd ed. Springer, New York, New York, USA.
Boyer, S. L., V. R. Flechtner, AND J. R. Johansen. 2001. Is the 16S-23S rRNA internal transcribed spacer region a good tool for use in molecular systematics and population genetics? A case study in cyanobacteria. Molecular Biology and Evolution 18: 1057–1069.
Brodo, I. M., S. Duran Sharnoff, AND S. Sharnoff. 2001. Lichens of North America. Yale University Press, New Haven, Connecticut, USA.
Büdel, B. 1992. Taxonomy of lichenized procaryotic blue-green algae. In W. Reisser [ed.], Algae and symbioses, 301–324. Biopress, Bristol, UK.
Büdel, B., AND A. Henssen. 1983. Chroococcidiopsis (Cyanophyceae), a phycobiont in the lichen family Lichinaceae. Phycologia 22: 367–375.[Web of Science]
Büdel, B., A. Meyer, N. Salazar, H. Zellner, G. Zotz, AND O. L. Lange. 2000. Macrolichens of montane rain forests in Panama, Province Chiriqui. Lichenologist 32: 539–551.[CrossRef][Web of Science]
Cardinale, M., A. M. Puglia, AND M. Grube. 2006. Molecular analysis of lichen-associated bacterial communities. FEMS Microbiology Ecology 57: 484–495.[CrossRef][Medline]
Cavalier-Smith, T. 2002. The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification. International Journal of Systematic and Evolutionary Microbiology 52: 7–76.[Abstract]
Chaves, J. L., R. Lücking, H. J. M. Sipman, L. Umaña, AND E. Navarro. 2004. A first assessment of the ticolichen biodiversity inventory in Costa Rica: The genus Dictyonema (Polyporales: Atheliaceae). Bryologist 107: 242–249.[CrossRef]
Coppins, B. J., AND P. W. James. 1979. A British species of Dictyonema. Lichenologist 11: 103–108.[CrossRef][Web of Science]
Cornelissen, J. H. C., S. I. Lang, N. A. Soudzilovskaia, AND H. J. During. 2007. Comparative cryptogam ecology: A review of bryophyte and lichen traits that drive biogeochemistry. Annals of Botany 99: 987–1001.
Currie, C. R., J. A. Scott, R. C. Summerbell, AND D. Malloch. 1999. Fungus-growing ants use antibiotic-producing bacteria to control garden parasites. Nature 398: 701–704.[CrossRef][Web of Science]
Currie, C. R., B. Wong, A. E. Stuart, T. R. Schultz, S. A. Rehner, U. G. Mueller, G.-H. Sung, AND J. W. Spatafora. 2003. Ancient tripartite coevolution in the attine ant–microbe symbiosis. Science 299: 386–388.
Curtis, T. P., AND W. T. Sloan. 2005. Exploring microbial diversity—A vast below. Science 309: 1331–1333.
De Toni, J. B. [ed.]. 1907. Sylloge algarum, vol. 5, Myxophyceae. [Publisher unknown], Padua, Italy.
Etayo, J., P. Diederich, AND E. Sérusiaux. 1995. Dictyonema interruptum, new for the Pyrenees. Graphis Scripta 7: 5–6.
Fogg, G. E., W. D. P. Stewart, P. Fay, AND A. E. Walsby. 1973. The blue-green algae. Academic Press, New York, New York, USA.
Forman, R. T. T. 1975. Canopy lichens with blue-green algae: A nitrogen source in a Colombian rain forest. Ecology 56: 1176–1184.[CrossRef][Web of Science]
Freiberg, E. 1998. Microclimatic parameters influencing nitrogen fixation in the phyllosphere in a Costa Rican premontane rain forest. Oecologia 117: 9–18.[CrossRef][Web of Science]
Friedl, T., AND C. Rokitta. 1997. Species relationships in the lichen alga Trebouxia (Chlorophyta, Trebouxiophyceae): Molecular phylogenetic analyses of nuclear-encoded large subunit rRNA gene sequences. Symbiosis 23: 125–148.[Web of Science]
Fritz-Sheridan, R. P. 1988. Nitrogen fixation on a tropical volcano, La Soufriere: Nitrogen fixation by the pioneer lichen Dictyonema glabratum. Lichenologist 20: 96–100.[CrossRef][Web of Science]
Futuyma, D. J., AND M. Slatkin. [eds.]. 1983. Coevolution. Sinauer, Sunderland, Massachusetts, USA.
Garrity, G. M., J. A. Bell, AND T. G. Lilburn. 2004. Taxonomic outline of the prokaryotes. Bergey’s Manual of systematic bacteriology, 2nd ed., release 5.0 [online document]. Springer, New York, New York, USA. Website http://www.bergeys.org/outlines/bergeysoutline_5_2004.pdf [accessed 12 August 2008].
Gorbushina, A. A., A. Beck, AND A. Schulte. 2005. Microcolonial rock inhabiting fungi and lichen photobionts: Evidence for mutualistic interactions. Mycological Research 109: 1288–1296.[CrossRef][Web of Science][Medline]
Goward, T. 1994. Living antiquities. Nature Canada Summer 1994: 14–21.
Green, T. G. A., J. Horstmann, H. Bonnett, A. Wilkins, AND W. B. Silvester. 1980. Nitrogen fixation by members of the Stictaceae (Lichenes) of New Zealand. New Phytologist 84: 339–348.[CrossRef][Web of Science]
Gugger, M., R. Molica, B. Le Berre, P. Dufour, C. Bernard, AND J.-F. Humbert. 2005. Genetic diversity of Cylindrospermopsis strains (Cyanobacteria) isolated from four continents. Applied and Environmental Microbiology 71: 1097–1100.
Hauck, M., G. Helms, AND T. Friedl. 2007. Photobiont selectivity in the epiphytic lichens Hypogymnia physodes and Lecanora conizaeoides. Lichenologist 39: 195–204.[CrossRef][Web of Science]
Hawksworth, D. L. 2004. Fungi living on lichens: A source of unexplored diversity. British Wildlife 15: 192–199.
Helms, G., T. Friedl, G. Rambold, AND H. Mayrhofer. 2001. Identification of photobionts from the lichen family Physciaceae using algal-specific ITS rDNA sequencing. Lichenologist 33: 73–86.[CrossRef][Web of Science]
Henson, B., S. M. Hesselbrock, L. E. Watson, AND S. R. Barnum. 2004. Molecular phylogeny of the heterocystous cyanobacteria (subsections IV and V) based on nifD. International Journal of Systematic and Evolutionary Microbiology 54: 493–497.
Herrera-Campos, M. A., S. Huhndorf, AND R. Lücking. 2005. The foliicolous lichen flora of Mexico IV: A new, foliicolous species of Pyrenothrix (Chaetothyriales: Pyrenotrichaceae). Mycologia 97: 356–361.
Honda, D., A. Yokota, AND J. Sugiyama. 1999. Detection of seven major evolutionary lineages in cyanobacteria based on the 16S rRNA gene sequence analysis with new sequences of five marine Synechococcus strains. Journal of Molecular Evolution 48: 723–739.[CrossRef][Web of Science][Medline]
Ishida, T., A. Yokota, AND J. Sugiyama. 1997. Phylogenetic relationships of filamentous cyanobacterial taxa inferred from 16S rRNA sequence divergence. Journal of General and Applied Microbiology 43: 237–241.[CrossRef][Web of Science][Medline]
Kirk, P. M., P. F. Cannon, J. C. David, AND J. A. Stalpers. 2001. Ainsworth and Bisby’s dictionary of the Fungi, 9th ed. CAB International, Egham, UK.
Komárek, J., AND K. Anagnostidis. 2005. Cyanoprokaryota 2. Teil/part 2, Oscillatoriales. In B. Büdel, L. Krienitz, G. Gärtner, and M. Schagerl. [eds.], Süsswasserflora von Mitteleuropa, vol. 19, part 2. Elsevier/Spektrum, Heidelberg, Germany.
Koriem, A. M., AND V. Ahmadjian. 1986. An ultrastructural study of lichenized and cultured Nostoc photobionts of Peltigera canina, Peltigera rufescens, and Peltigera spuria. Endocytobiosis and Cell Research 3: 65–78.[Web of Science]
Kroken, S., AND J. W. Taylor. 2000. Phylogenetic species, reproductive mode, and specificity of the green alga Trebouxia forming lichens with the fungal genus Letharia. Bryologist 103: 645–660.[CrossRef]
Larkin, M. A., G. Blackshields, N. P. Brown, R. Chenna, P. A. McGettigan, H. McWilliam, F. Valentin et al. 2007. ClustalW and ClustalX version 2. Bioinformatics 23: 2947–2948.
Law, R. 1985. Evolution in a mutualistic environment. In D. H. Boucher [ed.], The biology of mutualism: Ecology and evolution, 145–170. Oxford University Press, New York, New York, USA.
Law, R., AND D. H. Lewis. 1983. Biotic environments and the maintenance of sex—Some evidence from mutualistic symbioses. Biological Journal of the Linnean Society 20: 249–276.[CrossRef][Web of Science]
Lawrey, J. D., AND P. Diederich. 2003. Lichenicolous fungi: Interactions, evolution, and biodiversity. Bryologist 106: 80–120.[CrossRef]
Liba, C. M., F. I. S. Ferrara, G. P. Manfio, F. Fantinatti-Garboggini, R. C. Albuquerque, C. Pavan, C. Ramos et al. 2006. Nitrogen-fixing chemo-organotrophic bacteria isolated from cyanobacteria-deprived lichens and their ability to solubilize phosphate and to release amino acids and phytohormones. Journal of Applied Microbiology 101: 1076–1086.[Medline]
Lohtander, K., I. Oksanen, AND J. Rikkinen. 2003. Genetic diversity of green algal and cyanobacterial photobionts in Nephroma (Peltigerales). Lichenologist 35: 325–339.[CrossRef][Web of Science]
López-Bautista, J. M., AND R. L. Chapman. 2003. Phylogenetic affinities of the Trentepohliales inferred from small-subunit rDNA. International Journal of Systematic and Evolutionary Microbiology 53: 2099–2106.
Lorenz, M. G., AND W. Wackernagel. 1994. Bacterial gene transfer by natural genetic transformation in the environment. Microbiological Reviews 58: 563–602.
Lücking, R. 2008. Foliicolous lichenized fungi. Flora Neotropica Monograph 103: 1–867.
Lücking, R., A. Aptroot, J. L. Chaves, H. J. M. Sipman, AND L. Umaña. 2007. A first assessment of the TICOLICHEN biodiversity inventory in Costa Rica: The genus Coccocarpia (Peltigerales: Coccocarpiaceae). Bibliotheca Lichenologica 95: 429–457.
Lumbsch, H. T., AND S. M. Huhndorf. 2007. Whatever happened to the pyrenomycetes and loculoascomycetes? Mycological Research 111: 1064–1074.[CrossRef][Web of Science][Medline]
Lutzoni, F., F. Kauff, J. C. Cox, D. McLaughlin, B. Celio, M. Dentinger, D. Padamsee et al. 2004. Assembling the fungal tree of life: Progress, classification, and evolution of subcellular traits. American Journal of Botany 91: 1446–1480.
Maddison, D. R., AND W. P. Maddison. 2005. MacClade 4.08: Analysis of phylogeny and character evolution. Sinauer, Sunderland, Massachusetts, USA.
Miadlikowska, J., F. Kauff, V. Hofstetter, E. Fraker, M. Grube, J. Hafellner, V. Reeb et al. 2006. New insights from nuclear ribosomal and protein coding genes into classification and evolution of the Lecanoromycetes (Pezizomycotina, Ascomycota). Mycologia 98: 1088–1103.
Mueller, U. G., AND N. Gerardo. 2002. Fungus-farming insects: Multiple origins and diverse evolutionary histories. Proceedings of the National Academy of Sciences, USA 99: 15247–15249.
Mueller, U. G., N. M. Gerardo, D. K. Aanen, D. L. Six, AND T. R. Schultz. 2005. The evolution of agriculture in insects. Annual Review of Ecology Evolution and Systematics 36: 563–595.[CrossRef][Web of Science]
Mueller, U. G., S. A. Rehner, AND T. D. Schultz. 1998. The evolution of agriculture in ants. Science 281: 2034–2038.
Nelsen, M. P., AND A. Gargas. 2008. Dissociation and horizontal transmission of codispersing lichen symbionts in the genus Lepraria (Lecanorales: Stereocaulaceae). New Phytologist 177: 264–275.[Web of Science][Medline]
Nübel, U., F. Garcia-Pichel, AND G. Muyzer. 1997. PCR primers to amplify 16S rRNA genes from cyanobacteria. Applied and Environmental Microbiology 63: 3327–3332.
Nyati, S., A. Beck, AND R. Honegger. 2007. Fine structure and phylogeny of green algal photobionts in the microfilamentous genus Psoroglaena (Verrucariaceae, lichen-forming ascomycetes). Plant Biology 9: 390–399.[CrossRef][Medline]
O’Brien, H., J. Miadlikowska, AND F. Lutzoni. 2005. Assessing host specialization in symbiotic cyanobacteria associated with four closely related species of the lichen fungus Peltigera. European Journal of Phycology 40: 363–378.[CrossRef][Web of Science]
Oksanen, I., K. Lohtander, P. Paulsrud, AND J. Rikkinen. 2002. A molecular approach to cyanobacterial diversity in a rock-pool community involving gelatinous lichens and free-living Nostoc colonies. Annales Botanici Fennici 39: 93–99.[Web of Science]
Parmasto, E. 1978. The genus Dictyonema (‘Thelephorolichenes’). Nova Hedwigia 29: 99–144.[Web of Science]
Paulsrud, P., J. Rikkinen, AND P. Lindblad. 1998. Cyanobiont specificity in some Nostoc-containing lichens and in a Peltigera aphthosa photosymbiodeme. New Phytologist 139: 517–524.[CrossRef][Web of Science]
Paulsrud, P., J. Rikkinen, AND P. Lindblad. 2000. Spatial patterns of photobiont diversity in some Nostoc-containing lichens. New Phytologist 146: 291–299.[CrossRef][Web of Science]
Piercey-Normore, M. D. 2004. Selection of algal genotypes by three species of lichen fungi in the genus Cladonia. Canadian Journal of Botany 82: 947–961.[CrossRef]
Piercey-Normore, M. D., AND P. T. DePriest. 2001. Algal switching among lichen symbioses. American Journal of Botany 88: 1490–1498.
Posada, D., AND K. A. Crandall. 1998. MODELTEST: Testing the model of DNA substitution. Bioinformatics 14: 817–818.
Purvis, O. W., B. J. Coppins, D. L. Hawksworth, P. W. James, AND D. M. Moore. 1992. The lichen flora of Great Britain and Ireland. Natural History Museum Publications and British Lichen Society, London, UK.
Purvis, W. 2000. Lichens. Natural History Museum, London, UK; Smithsonian Institution, Washington D.C. USA.
Rambold, G., T. Friedl, AND A. Beck. 1998. Photobionts in lichens: Possible indicators of phylogenetic relationships? Bryologist 101: 392–397.
Richardson, D. H. S. 1999. War in the world of lichens: Parasitism and symbiosis as exemplified by lichens and lichenicolous fungi. Mycological Research 103: 641–650.[CrossRef][Web of Science]
Rikkinen, J. 2003. Ecological and evolutionary role of photobiont-mediated guilds in lichens. Symbiosis 34: 99–110.[Web of Science]
Rikkinen, J., I. Oksanen, AND K. Lohtander. 2002. Lichen guilds share related cyanobacterial symbionts. Science 297: 357.
Rippka, R., J. Deruelles, J. B. Waterbury, M. Herdman, AND R. Y. Stanier. 1979. Generic assignments, strain histories and properties of pure cultures of cyanobacteria. Journal of General Microbiology 111: 1–61.
Ronquist, F., AND J. P. Huelsenbeck. 2003. MrBayes 3: Bayesian phylogenetic inference under mixed models. Bioinformatics 19: 1572–1574.
Roskin, P. A. 1970. Ultrastructure of the host–parasite interaction in the basidiolichen Cora pavonia (Web.) E. Fries. Archiv fur Mikrobiologie 70: 176–182.[CrossRef]
Rudi, K., O. M. Skulberg, AND K. S. Jakobsen. 1998. Evolution of cyanobacteria by exchange of genetic material among phyletically related strains. Journal of Bacteriology 180: 3453–3461.
Sanders, W. B. 2001. Lichens: The interface between mycology and plant morphology. Bioscience 51: 1025–1035.[CrossRef][Web of Science]
Santesson, R. 1952. Foliicolous lichens. I. A revision of the taxonomy of the obligately foliicolous, lichenized fungi. Symbolae Botanicae Upsalienses 12: 1–590.
Schultz, M., W.-R. Arendholz, AND B. Büdel. 2001. Origin and evolution of the lichenized ascomycete order Lichinales: Monophyly and systematic relationships inferred from ascus, fruiting body and SSU rDNA evolution. Plant Biology 3: 116–123.[CrossRef]
Schultz, M., AND B. Büdel. 2002. Key to the genera of the Lichinaceae. Lichenologist 34: 39–62.[CrossRef][Web of Science]
Sipman, H. J. M. 1999. Lichens. In J. L. Luteyn [ed.], Páramos: a checklist of plant diversity, geographical distribution, and botanical literature, 41–53. Memoirs of The New York Botanical Garden, New York Botanical Garden, Bronx, New York, USA.
Smith, J. E., AND J. Sowerby. 1849. English botany. Coloured figures of British plants, vol. 4, supplement. [Publisher unknown], London, UK.
Stanier, R. Y., AND G. Cohen-Bazire 1977. Phototrophic prokaryotes: The cyanobacteria. Annual Review of Microbiology 31: 225–274.[CrossRef][Web of Science][Medline]
Thompson, J. N. 1994. The coevolutionary process. University of Chicago Press, Chicago, Illinois, USA.
Trembley, M. L., C. Ringli, AND R. Honegger. 2002. Differential expression of hydrophobins DGH1, DGH2 and DGH3 and immunolocalization of DGH1 in strata of the lichenized basidiocarp of Dictyonema glabratum. New Phytologist 154: 185–195.[CrossRef][Web of Science]
Tschermak-Woess, E. 1988. The algal partner. In M. Galun [ed], CRC handbook of lichenology, vol. I, 39–92. CRC Press, Boca Raton, Florida, USA.
Turner, S., T. C. Huang, AND S. M. Chaw. 2001. Molecular phylogeny of nitrogen-fixing unicellular cyanobacteria. Botanical Bulletin Academia Sinica 42: 181–186.
Wetmore, C. M. 1970. The lichen family Heppiaceae in North America. Annals of the Missouri Botanical Garden 57: 158–209.[CrossRef][Web of Science]
Wirtz, N., H. T. Lumbsch, T. G. A. Green, R. Türk, A. Pintado, L. Sancho, AND B. Schroeter. 2003. Lichen fungi have low cyanobiont selectivity in maritime Antarctica. New Phytologist 160: 177–183.[CrossRef][Web of Science]
Yeager, C. M., J. L. Kornosky, R. E. Morgan, E. C. Cain, F. Garcia-Pichel, D. C. Housman, J. Belnap, AND C. R. Kuske. 2007. Three distinct clades of cultured heterocystous cyanobacteria constitute the dominant N2-fixing members of biological soil crusts of the Colorado Plateau, USA. FEMS Microbiology Ecology 60: 85–97.[CrossRef][Medline]
Zwickl, D. J. 2006. Genetic algorithm approaches for the phylogenetic analysis of large biological sequence data sets under the maximum likelihood criterion. Ph.D. dissertation, University of Texas, Austin, Texas, USA.
CiteULike Complore Connotea Del.icio.us Digg Facebook Reddit Technorati Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  S. Werth and V. L. Sork Identity and genetic structure of the photobiont of the epiphytic lichen Ramalina menziesii on three oak species in southern California Am. J. Botany, May 1, 2010; 97(5): 821 - 830. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
