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Buellia spp.
Family: Caliciaceae
Buellia image
Life habit: lichenized or rarely lichenicolous, either as obligate parasites lacking a distinctly separate thallus or as juvenile parasites establishing independent thalli with maturity Thallus: crustose, continuous to dispersed, rimose, rimose-areolate or areolate, sometimes subsquamulose or sublobate, or becoming squamulose or lobate; prothallus: frequently present, usually dark, rarely unpigmented, often forming only a thin black outline delimiting the thallus border, rarely spreading and becoming fimbriate or arachnoid, occasionally extending between the areoles as a hypothallus surface: unpigmented (i.e., pale white to dark gray) or pigmented (i.e., pale brown to deep brown, pale yellow or yellowish green), pruinose or epruinose, plane to uneven, smooth to roughened margin: determinate or indeterminate cortex: phenocorticate photobiont: primary one a Trebouxia species, secondary one absent medulla: usually white, rarely with yellow or orange pigments, often with calcium oxalate (forming clusters of colorless needle shaped crystals in H2SO4) Ascomata: apothecial, immersed to sessile, rarely stipitate, usually common, but often not formed in species that produce soredia or blastidia disc: rarely concave, usually plane, often becoming convex, black, rarely dark brown, pruinose or epruinose Ascoma anatomy: lecideine, often with necrotic material remaining attached, when emerging from the thallus (thalline veil) or pseudolecanorine (with a thalline collar of poorly differentiated thallus material), very rarely eulecanorine (i.e., with a well developed thalline exciple, but typically becoming reduced with age) thalline exciple: absent or well developed only in immature apothecia, soon reduced (mamillana-type) proper exciple: thin and poorly differentiated (aethalea-type) to conspicuously thickened and typically differentiated into an inner and outer exciple (various other exciple types, Fig. 1a-g) hymenium: hyaline, sometimes inspersed with oil droplets (Buellia s.str., i.e., "Hafellia"); paraphyses: free, rarely ±conglutinate, sparsely branched, apically swollen, with a distinct pigment cap, forming a brown, dull olive, aeruginose, or black epihymenium that can be inspersed with calcium oxalate or xanthone crystals; subhymenium: usually confluent with the hypothecium, rarely forming a distinctly separate, hyaline or faintly pigmented layer between hymenium and hypothecium; hypothecium: usually deep reddish brown, rarely pale brown to yellow, very rarely hyaline asci: clavate, Bacidia-or Biatora-type, rarely with a widened central cone and thus resembling the Lecanora-type (B. badia, cf. Dimelaena californica); (4)-8-(16-32) or even polysporous (non Sonoran: B. multispora) ascospores: hyaline when immature, becoming olive gray, soon changing to deep brown (very rarely remaining hyaline, B. oidalea), one- to pluriseptate, oblong to broadly ellipsoid, rarely citriform, usually straight, occasionally curved, obtuse (Buellia s. l.), or gradually tapering or pointed (Buellia s. str, i.e., "Hafellia"), thin- or thick-walled; spore ontogeny: septum typically formed before the development of pigmentation and/ or wall differentiation (A-ontogeny), rarely formed later (B-ontogeny; see Volume 2, Rinodina: Fig. 62); spore wall: with distinct lateral and occasionally septal thickenings (Callispora-type, Buellia s. str., i.e. "Hafellia"), or only developing septal thickenings (Physconia-type, Buellia s. l.), or not developing any wall thickenings (Buellia-type, Buellia s. l.); spore ultrastructure: wall at maturity differentiated into four to five distinct layers (1) mucilagenous sheath, (2) smooth to ornamented perispore (missing in some species), (3) intermediate layer, (4) proper spore wall, and (5) endospore; both endospore and proper wall typically form proper wall septa, some species rarely have additional endospore septa, Conidiomata: pycnidial, globose to flask-shaped, unilocular, rarely becoming divided, similar to the Umbilicaria- or, rarely, the Roccella-type; conidiophores: typically clearly differentiated from the pycnidial wall, elongate and becoming strongly branched (type V), rarely moderately long and poorly branched (type III), very rarely not clearly differentiated from the pycnidial wall (type II) conidia: simple, bacilliform or filiform, when filiform often curved Iodine reactions: thallus amyloid or not, hymenium always amyloid, inner exciple strongly amyloid in Buellia vilis (in Lugol's) Secondary metabolites: with a wide range of secondary metabolites, especially depsides like atranorin and depsidones like norstictic acid (weak concentrations can usually be detected by orange-red crystals forming in K in the compound microscope), rarely also with xanthones, very rarely with usnic acid derivates, anthraquinones, or other substances; in several species no secondary metabolites have been detected Geography: world-wide; Buellia s. str. (i.e., "Hafellia") often grows in coastal regions Substrate: most species show distinct substrate preferences (e.g., only growing on bark or wood, others are restricted to siliceous, or rarely calcareous rock, very few grow on soil); however, some species typically found on one substrate occasionally found on another (e.g., B. badia usually on rock but sometimes also on wood, very rarely even on bark; B. punctata is usually on bark or wood, but rarely has been collected on rock); most species episubstratic, very few endosubstratic; a few species grow on plant debris or over bryophytes (muscicolous), or facultatively or obligately lichenicolous on other lichens. Notes: The genus Buellia was named by De Notaris after his friend Esperanzo Buelli. Buellia is characterized by black apothecia that, at least at maturity, are lecideine. Several species show tendencies to develop a lecanorine margin when emerging from the thallus, but even in species that initially have a well developed lecanorine margin, this margin eventually becomes reduced and replaced by hyphae of the proper exciple. This is an important diagnostic character in comparison with Rinodina, a genus that typically, but not always develops distinctly lecanorine apothecia. Another important character distinguishing Buellia from Rinodina, is the pigmentation of the hypothecium. Most species of Buellia have a deeply pigmented reddish brown hypothecium. In some species this pigmentation is less pronounced and their hypothecium is pale brown, pale yellow or very rarely unpigmented. In contrast, most Rinodina species have a hyaline, sometimes faintly pigmented, and only very rarely deeply pigmented hypothecium. The apical structure of the ascus is probably the most diagnostic distinguishing character between the genera. Stained with Lugol's solution, Buellia has a tholus with deeply blue flanks merging at the ascus apex. The central part does not stain. It is typically cone-shaped (i.e., narrowing towards the apex where the flanks merge). This ascus corresponds to the Bacidia-type (flanks uniformly stained) or the Biatora-type (innermost parts of the flanks more strongly stained than outer parts). In a variation of this type, the central unstained part remains cylindrical, with parallel flanks that only merge in a thin layer directly at the ascus tip. This variation is similar but not identical to the Lecanoratype of Rinodina, an ascus with a central non-amyloid part that slightly to considerably expands towards the apex, and amyloid flanks that do not merge at the tip. Both Rinodina and Buellia develop spores with distinctly pigmented walls. Immature spores are hyaline and aseptate. Premature spores soon become olive. Mature and overmature spores are deep brown. Spore septation of most species initiates well before wall differentiation or pigmentation can be observed. The premature spores are soon divided into two cells, separated by a median septum (A-ontogeny). In some species premature spores become pigmented and develop wall thickenings before septa are formed (B-ontogeny). In Rinodina these two ontogenies are typically more easily distinguished because apical thickenings can usually be observed before or after the septum formation. Ascospores of Buellia rarely show distinct apical thickenings. The terminology to describe different spore types was originally introduced in Rinodina, a genus usually characterized by ascospores with elaborate internal wall thickenings. Thickenings in Buellia are generally less conspicuous, but three different types can be distinguished, at least in species with one-septate spores: the Buellia-, the Physconia-, and the Callispora-type (Fig. 1i). The Buellia-type lacks distinct wall thickenings. Most species have uniformly thin-walled spores, others develop evenly thickened walls. In contrast, some species develop spores with distinctly localized internal thickenings, either along the septum or along the lateral wall. Spores with a distinctly swollen septum, but no lateral thickenings are referred to the Physconia-type. In contrast, spores of the Callisporatype may or may not have septal thickenings, but, more importantly, always have lateral (subapical) thickenings. Only the Callispora-type is not known from Rinodina. The Buellia-type is structurally identical with the Beltraminia-type of Rinodina. It may be assumed that these simple types represent an ancestral type, from which more elaborate spores have subsequently evolved. Nevertheless, it is problematic to use the same terminology in both genera. Recent molecular studies have demonstrated that the two genera represent two distinctly different evolutionary lineages of the Physciaceae (Helms 2003, Wedin et al. 2000, Wedin and Grube 2002). Thus, Rinodina is more closely related to Physcia and Heterodermia; whereas, Buellia is more closely related to Dimelaena and Pyxine. Structurally similar or even identical spores must therefore have evolved independently, on separate occasions. Previously, it was assumed that the crustose genera Rinodina and Buellia were closely related. Scheidegger (1993) accordingly suggested that the Physconia-type was not only found in Rinodina, but could also be observed in Buellia. However, in Buellia spores that structurally belong to the Physconia-type typically lack distinct apical thickenings during their ontogeny. In most species, the ascospores also follow a more dynamic development than in Rinodina. Septal thickenings often develop only during a brief period of the ontogeny and soon become reduced. Only a few species, like B. prospersa, have spore septa that remain conspicuously thickened. Overall, a strict spore type concept is therefore problematic. It does not reflect the evolution of ascospores; nor does it capture the dynamic processes of the spore development. A strict type concept also fails to accommodate species with pluriseptate ascospores. Species with pluriseptate spores are rarely found in Rinodina (e.g., R. intermedia, R. conradii). In Buellia the septum formation generally begins with the median septum (middle septum, central septum). During early stages of their ontogeny pluriseptate spores frequently resemble the Buellia-, Physconia-or Callispora-type, providing evidence that these species are closely related to species with one-septate ascospores. However, with the formation of additional septa this semblance is typically lost and the strict types can no longer be applied to categorize the pluriseptate ascospores. Following the formation of a median septum, two additional transverse septa divide both spore cells. These additional septa are structurally identical with the median septum, they are proper septa (i.e., formed by both endospore and proper wall). In contrast, species with one-septate spores sometimes show tendencies to develop additional "false" septa, formed only by the endospore. Endospore septa rarely occur in species with pluriseptate spores, where the majority of the septa are proper septa. Some species with pluriseptate spores only develop transverse septa. Their spores thus remain 3-septate (Fig. 1j). Species with submuriform spores typically develop only a few longitudinal septa. Species with eumuriform spores regularly form several longitudinal septa. In optical cross section submuriform spores therefore have a much smaller number of cells compared to eumuriform spores (Fig. 1j). Scheidegger (1993) distinguished several exciple types according to the development and structure of fungal plectenchyma, and their pigmentation. Fungal plectenchyma, that form the exciple, can be quite variable as they often merge into each other. Hyphae that run parallel become intertwined. As they become pigmented, the pigmentation accumulates and cell walls thus become carbonized. Cells can be short and almost isodiametric, or they become elongated, hyphae may be thin-walled with distinct cell lumina, or they become thick-walled with almost no visible cell lumen. Because of transitions, distinct texture types cannot always be distinguished. Nevertheless, general exciple types provide diagnostic categories that help to recognize species, which are otherwise very similar. The structure and development of these types can best be observed in thin microtome sections using a microscope equipped with differential interference contrast (DIC). The presence of acetone-insoluble pigments can also be significant and should be tested according to Meyer and Printzen (2000). The aethalea-type is the most common exciple-type. It is characterized by thin, ±parallel hyphae, similar in orientation and structure to the paraphyses (Fig. 1a-b). The proper exciple of an apothecium is essentially the same layer as the hypothecium (the inner envelope) and all excipular hyphae are thus connected with the hypothecium. This connection is not very distinct, because excipular hyphae have the same upright orientation as the paraphyses, and thus appear to form a separate layer outside the hypothecium. In species with apothecia remaining immersed, the aethalea-type exciple is more strongly reduced to a few, narrow hyphae. The inner excipular hyphae are leptodermatous to mesodermatous, narrow and parallel and thus correspond to textura oblita. The outermost cells are typically swollen and pigmented by a brown pigment cap. They are structurally similar to the inner exciple and most appropriately also referred to as textura oblita. In species with distinctly sessile apothecia, the outer part often becomes prominent and outer cells then appear distinctly swollen (Fig. 1b). Scheidegger (1993) referred to these swollen cells as textura angularis or prismatica. Buellia christophii is a good example of a species with an aethalea-type exciple and a distinctly thickened outer layer. Many, but not all species that develop an aethalea-type exciple are characterized by the pigment cinereorufa-green, an aeruginose HNO3+ violet pigment in the outer exciple and hymenium. The dispersa-type is much better developed than the aethalea-type. Its exciple can be differentiated into three distinct zones (Fig. 1c). The innermost hyphae radiate from the hypothecium towards the outer exciple. In young apothecia, the deep reddish brown pigmentation does not extend far into the central exciple, but with age the exciple becomes more evenly pigmented throughout. The inner hyphae are mesodermatous, ±parallel (textura oblita) and structurally similar to hyphae of the hypothecium. The outermost exciple cells are distinctly swollen, shorter and more isodiametric and have been referred to by Scheidegger (1993) as textura angularis, even though they are not distinctly paraplectenchymatous. This exciple corresponds to Nordin's (2000) description of an exciple, which has a distinctly paler middle part. The leptocline-type exciple is evenly pigmented throughout by a dull brown pigment (Fig. 1d). In B. halonia, the aeruginose pigment cinereorufa-green is also present, and the exciple thus appears fuscous to distinctly aeruginose. Several species assigned to this type by Scheidegger (1993) have atranorin within their exciple, which is most likely the cause for a diffuse K+ yellow reaction. This reaction is not necessarily indicative of a separate brown pigment. Nordin (2000) also recognized this evenly pigmented exciple, but does not describe the structure in detail. The hyphae of the leptocline-type are little differentiated throughout the exciple. They are mesodermatous with a distinctly pigmented wall. Initially they are parallel but with age become ±intricately interwoven (forming a dense textura intricata). The outer cells do not form a separate zone from the inner exciple. Scheidegger (1993) refers to them as textura oblita because they sometimes appear less interwoven than the inner hyphae. As previously mentioned, most species of Buellia lack a distinct lecanorine margin. The tendency to develop an additional layer of thalline material around the exciple can nevertheless be observed in many species. Some species emerge from the thallus with necrotic material forming a veil, others are surrounded by a collar or pseudothalline margin. Only a very few species emerge from the thallus with a distinctly developed thalline margin. In these species, young hyphae from the proper exciple are initially almost absent and a distinct thalline margin is first developed (Fig. 1e). This thalline margin is subsequently replaced by expanding hyphae from the proper exciple (Fig. 1f). The ontogeny was first observed in Buellia mamillana and has thus been described as mamillana-type. In Buellia mamillana, hyphae from the proper exciple remain pale during almost the entire ontogeny. Only their outermost cells become pigmented with a dull brown pigment that is also present in the paraphysal caps forming the epihymenium. This pigmentation is probably identical with the pigment cap of other Buellia species, but it is less concentrated, and the disc and margin initially often appear dark brown rather than black. This development, structure and pigmentation most closely resemble the coco's-type of Pyxine. Other species that initially form a well developed thalline exciple, later develop a proper exciple that becomes more strongly pigmented and structurally then resembles the dispersa-type. The trachyspora-type (Fig. 1g) is only known from Buellia trachyspora, a tropical to subtropical species. It is characterized by a strongly blackened (carbonized) outer exciple of small, globular cells (textura globularis) and a reddish brown, large-celled, and distinctly paraplectenchymatous inner exciple (textura angularis). The inner exciple pigment is ±identical to leptoclinoides-brown. The outer hyphae are blackened as a result of an extremely strong pigment concentration throughout the small globular cells. The pigment shows the same color reactions as pigment B sensu Scheidegger (1993). It is probably identical to elachista-brown sensu Meyer and Printzen (2000) The vilis-type is unique. It is only known from Buellia vilis (Fig. 1h). This species has a hyaline hypothecium of thin, strongly interwoven hyphae, which extend throughout the inner exciple and react very strongly iodine-blue. It is the only species with a distinctly amyloid exciple. The outermost exciple cells are very deeply pigmented. They are strongly carbonized, and can barely be distinguished from one another. The blackish red pigment is called atrared sensu Meyer and Printzen (2000). It reacts deep purple with HNO3, a very diagnostic reaction (distinctly different from the HNO3+ reddish violet reaction of cinereorufa green). In the Sonoran Region species on bark and rocks have been collected extensively; species from these substrates are therefore fairly well known. In contrast, species on soil have been very rarely collected. Several reasons may account for the few available collections. Calciferous soils preferred by most terricolous species discussed here are a rare substrate throughout the Sonoran Region. Soil crusts in general are strongly affected by disturbance and terricolous lichen communities throughout the region today are fragmented, often highly endangered and in poor condition. Finally, terricolous herbarium specimens are often poorly preserved, their soil substrates easily disintegrate, and specimens are thus often in bad condition for the study of their morphology and anatomy. The range of material available for this study was therefore often not sufficient to assess variability of the terricolous species discussed here. Some of the material that was available does not fit into the four terricolous taxa distinguished here and we decided not to describe new species for these specimens. The relatively large variation of characters so far discussed, indicates that the genus concept employed here is heterogeneous and, to some extent, artificial. More than 16 different genera have therefore been proposed to establish more narrowly defined, presumably natural groups. A very wide genus concept is, nevertheless, applied here for several reasons: (1) no molecular studies of a wide range of specimens representing all different groups have so far been conducted, (2) very few of the newly proposed genera are well circumscribed, and these may eventually be confirmed as monophyletic groups by molecular studies, (3) the majority of newly proposed genera is based on characters, which are not exclusive, but, to the contrary, frequently occur within other genera or different species groups; character overlap is thus quite significant, (4) in some instances, genera are based on a single character alone, but otherwise appear heterogeneous, and (5) even if a few groups are fairly well characterized, accepting these as well delimited genera, will leave a fairly large "residue" of heterogeneous species that cannot easily be accommodated. If a narrow genus concept were applied, almost all of these "residual" species would have to be placed into several newly described, monotypic genera. This approach by no means clarifies how these species are related. On the contrary, it would obscure the fact that the phylogeny of these species still remains unresolved. Genera that could be accepted with some hesitation are briefly discussed here. Even these genera are not without challenges. Diplotomma was one of the first genera distinguished from Buellia s. l. Even though not originally described to generally accommodate species with pluriseptate spores, the genus soon became a conglomerate of species with pluriseptate spores. Nordin (2000) clarified that many species with pluriseptate spores were not necessarily closely related and suggested that Diplotomma s. str. could only be accepted for a relatively small group of species with pluriseptate spores. These species are also characterized by pruinose apothecia, a thallus rich in calcium oxalates, and a thick perispore. Bungartz and Nash (2004b) discussed that B. amabilis may belong to this group, even though it has one-septate spores. According to Molina et al. (2002), Diplotomma represents a distinctly separate, monophyletic group and should therefore be accepted as a genus. However, they also included the placodioid Diploicia in Diplotomma. Scheidegger and Mayrhofer (in Scheidegger 1993, p. 341) provided the first valid description for Amandinea, a genus originally introduced by Choisy (1950). In their description in Scheidegger (1993), they emphasized the filiform conidia and different conidiophores, but also mention that the genus often lacks secondary metabolites. As the type they selected Buellia coniops (Wahlenb.) Th. Fr., an arctic-alpine, saxicolous species.. Subsequently almost any species with filiform conidia was transferred from Buellia or Rinodina into Amandinea. Other characters, like the formation of a lecanorine margin, the presence of secondary metabolites, the spore type, the hypothecium color, the exciple structure or pigmentation were all ignored. Instead the length of the conidia became the only, dominant character that now defined the genus. As a consequence Amandinea "grew" into a poorly defined amalgam of species held together only by the length of their conidia. This situation is almost certainly as artificial as accepting a sensu lato concept for Diplotomma, lumping any species with pluriseptate spores into that genus. Analogous to this situation, a core group of arctic-alpine species closely related to Buellia coniops may eventually be confirmed as Amandinea s. str. Subtropical to tropical, predominantly epiphytic species with a range of secondary metabolites hardly belong to this core. Species with bacilliform conidia (Buellia badia, Buellia schaereri) must certainly not be included in the genus. Marbach (2000), Nordin (2004), Nordin and Tibell (2005), and Kalb (2004) accepted Tetramelas, a small, apparently well defined group of arctic-alpine, terricolous species, characterized by the secondary metabolite 6-Omethylarthothelin or related xanthones. The species in Tetramelas have spores that are similar to the Callisporatype, but usually have less pronounced lateral (subapical) wall thickenings. Their hymenium is not inspersed, but occasionally contains a few oil droplets from a scarcely inspersed hypothecium. Some species are not easily placed into Tetramelas based on these classical characters alone. Buellia chloroleuca contains 6-O-methylarthothelin or traces of other xanthones. It has spores that resemble the Callispora-type, and the hymenium typically contains a few oil droplets. Although it does not grow terricolously, Nordin (2004) included it in Tetramelas. Nordin and Tibell (2005) described a lichenicolous species that apparently does not contain xanthones. Reports of whether the terricolous Buellia papillata does contain 6-O-methylarthothelin are contradictory. Further molecular studies will help to refine the genus concept of this group (Nordin and Tibell 2005). "Hafellia" is one of the few, reasonably well delimited groups in Buellia s. l. It is characterized by a strongly oilinspersed hymenium, Callispora-type ascospores, bacilliform conidia, frequently a thick and well developed thallus that often contains norstictic acid as well as some unusual secondary metabolites, such as diploicin or placodiolic acid. For nomenclatural reasons, the genus name Hafellia must be regarded as a synonym of Buellia s. str. Buellia disciformis, a species that shares all typical characters of "Hafellia", is the listed type of Buellia. Obviously, this type species of Buellia cannot belong to a different genus. A proposal by Moberg et al. (1999) therefore suggested changing the listed type of Buellia from B. disciformis to B. aethalea. Types are usually not listed in the Botanical Code unless a name is conserved. Buellia disciformis was chosen as the type of Buellia, when the genus name was conserved against Gassicurtia. This listing would have had to be changed, if the proposal by Moberg et al. (1999) had been accepted. This would have been the first case in the history of the Botanical Code when a conserved type would have been replaced by another type. The procedure was not recommended by the Committee of Fungi, who voted against it (Gams 2004). Their decision to reject the proposal was accepted by general vote at the XVII Botanical Congress in Vienna, 2005. Therefore, the group of species formerly included in "Hafellia" must now be regarded as Buellia s. str. If this core group of species is regarded as a separate genus, all species that are not closely related, will have to be excluded from Buellia. This decision has considerable consequences for the taxonomy of the genus Buellia. If a narrow species concept is adopted, only a few species that do not belong to Buellia s. str. ("Hafellia) can still be accommodated within previously described genera. Even though "Hafellia" appears to be fairly well delimited, several species in Buellia s. l. share some but not all characters with this group. Thus, Marbach (2000) described Baculifera to accommodate species with a similar spore type, but lacking oil inspersion within their hymenium. He emphasized that Baculifera was characterized by elongate bacilliform rather than short bacilliform conidia. Nevertheless, he included one species with short bacilliform conidia, arguing that this could be justified by the presence of a K+ yellow-green pigment in the exciple and epihymenium. This pigment is described here as mi-cromera-green. Not all of the species that Marbach (2000) placed into Baculifera contain the pigment. Nevertheless, "outside" Baculifera the pigment is only known from a single "Hafellia" species, Buellia conspirans. Marbach (2000) also emphasized the presence of norstictic acid as an important character of Baculifera. This secondary metabolite is, however, also present in many "Hafellia" species. Norstictic acid is generally common and occurs in many species that appear not closely related to either "Hafellia" or Baculifera. Marbach (2000) re-established the genus Gassicurtia to accommodate species with distinctly broadened, fusiform conidia and rusty red pigments, thereby rendering the conservation of Buellia against Gassicurtia obsolete. Three Sonoran species contain pigments in their medulla, B. endoferruginea, B. halonia, and B. capitis-regum. Contrary to all species included by Marbach (2000) in Gassicurtia, Buellia endoferruginea contains norstictic acid and has bacilliform conidia. The species therefore appears to be an "intermediate" between Gassicurtia (rusty red pigments, broadly fusiform conidia, no norstictic acid) and Baculifera (no rusty pigments, elongate bacilliform conidia, norstictic acid). Its distinctly thickened, areolate thallus resembles B. capitis-regum, a species with a deep yellow medulla, but muriform spores and a hymenium inspersed with oil droplets. The only other Sonoran species with a similar pigment in the medulla is B. halonia. In B. halonia, the rusty red, K+ purple pigment is only occasionally present in parts of the lower medulla. The pigmentation is fairly inconspicuous and has therefore previously been overlooked. Grube et al. (2004) described two species from Australia, which are also characterized by rusty red pigments and emphasized that both species appeared to be not closely related to Gassicurtia. Presently, the genus Gassicurtia thus appears not well delimited. If Buellia is accepted in a strict sense, "residual" groups of species that were formally included in Buellia s.l. would have to be segregated and placed into new genera. For some of these "residual" groups, genus names are available. Thus, species in the Buellia aethalea-group (Bungartz and Nash2004a) could be placed into Melanaspicilia. Buellia tesserata could be united with Dimelaena radiata. Buellia badia and Dimelaena californica could be joined within Monerolechia. Most other species, however, cannot easily be transferred into existing genera even though they form groups of apparently closely related species. These groups typically share some characters, but they are often not clearly distinguished from one another. For example, species in the Buellia excelsa-group are characterized by an oil inspersed hymenium. Because of their Physconiatype ascospores and the distinctly bullate to subsquamulose thallus, they, nevertheless, appear not closely related to "Hafellia" or Baculifera. Instead they show affinities with the Buellia dispersa-group, species that develop a similar thallus, have Physconia-type ascospores, and a dispersatype exciple. Finally, a relatively large group of species remains for which no taxonomic affinity is currently known. These species could only be placed into newly described, separate and monotypic genera. These uncertainties are by no means different from the situation in other large and heterogeneous genera like Rinodina, Caloplaca, or Lecanora. These genera are just as heterogeneous and artificial as Buellia s. l. Their phylogeny also remains largely unresolved. Nevertheless, these genera have not yet been split. All species are retained here within Buellia s. l. All descriptions include brief references to taxonomic affinities for each species. In many cases this "affinity" refers to a genus name, or a taxonomic group. For quite a large number any taxonomic affinity remains, however, unknown.