Umbilicaria

Umbilicaria was established by Georg Franz Hoffmann in 1789 in the first fascicle of his Descriptio et adumbratio plantarum e classe cryptogamica Linnaei, quae lichenes dicuntur;^[1] because 1790 appears on the title page, some bibliographic treatments cite the work as 1789 [1790]. Hoffmann used the genus for the distinctive "umbilicate" lichens, united by their attachment to rock at a single central point. The generic name derives from the Latin umbilicus ('navel'), with the suffix –aria ('possession or connection'), referring to this navel-like holdfast.^[2] Although Umbilicaria was later found to be a later homonym of an earlier plant-genus name, it was conserved under the International Code of Nomenclature for algae, fungi, and plants, with U. hyperborea as the conserved type.^[3]

For much of its history, the genus was delimited chiefly by morphology. Traditional authors relied on features such as the foliose thallus, the central umbilicus, rhizinomorphs, the form of the black apothecia, and later the presence of thalloconidia. Classification was not entirely stable: Gyrophora was often treated as an infrageneric name within Umbilicaria, and some authors proposed dividing the genus more sharply on apothecial characters. Later workers treated such schemes with caution, however, and the growing taxonomic importance attached to thalloconidia led to a broad reassessment of species limits and name application.^[4][5]

In current classifications the genus is placed in the family Umbilicariaceae, and modern systematic work has shifted the emphasis from gross morphology alone to molecular evidence. Phylogenetic studies have supported Umbilicariaceae in the strict sense (sensu stricto) as a monophyletic group and recovered several major clades within the traditional Umbilicaria complex, including a "core Umbilicaria" clade and lineages corresponding to the historical names Agyrophora and Gyrophora. On that basis, Davydov and co-authors in 2017 proposed a broad classification in which the principal diversity of the family is treated as a single genus, Umbilicaria, divided into eight subgenera.^[5][6]

That broad single-genus treatment has not been followed uniformly, and some recent authors continue to recognize multiple genera within Umbilicariaceae. Published estimates of the number of accepted Umbilicaria species also vary: modern syntheses generally place the genus at about 70–80 species worldwide, while broader family-level summaries reflect continuing debate over circumscription. Molecular studies have also indicated that some named taxa may be non-monophyletic or more variable than earlier morphology-based classifications suggested, so both species limits and generic boundaries remain under review.^[3][4][6][7]

Species in Umbilicaria have a foliose, dorsiventral thallus — a leafy body with distinct upper and lower surfaces — that is attached to the substrate at a single central or sometimes off-centre point, producing the characteristic umbilicate (navel-like) form. When moist, the thallus is usually soft, pliable, and somewhat leathery, but it often becomes brittle on drying. The upper surface ranges from pale grey-brown to black and may be smooth, cracked, warted, folded, or marked by a net-like pattern of ridges, sometimes with a raised central area. The margins are wavy and may be entire (unbroken) or incised (deeply cut). The lower surface is smooth or warted and is sometimes pitted, rarely bearing lamellae (thin plate-like ridges); it is usually black, but in some species may be pale brownish-pink. Rhizinomorphs (root-like strands) may be present or absent on the lower surface, but they do not function in attachment to the substrate. Some species also produce isidia or soredia, structures used for vegetative reproduction. In certain taxa, the upper cortex is covered by a broad, short-lived epinecral layer — a thin surface film of dead cells. The photobiont is a chlorococcoid green alga, forming either a continuous layer or discrete colonies, while the medulla (inner tissue) may be loose or compact and is not always sharply distinct from the lower cortex.^[8]

The fruiting bodies are apothecia (open, disc-shaped reproductive structures), which may be immersed in the thallus, sessile (sitting directly on the surface), or stalked, and are often irregular in outline. Their discs are black and may be flat or convex, most commonly gyrose (with ridged, maze-like folds), ring-like, or radiating, though occasionally smooth; in some cases they bear a central sterile umbo (a small raised boss). A thick thalline margin — a rim of tissue continuous with the thallus — is usually present and may be black or dark brown, although it is lacking in some species. Internally, the hymenium (spore-producing layer) is 60–90 μm thick, with branched, septate paraphyses (sterile filaments among the spore sacs) that are hyaline or brown and often have swollen, pigmented terminal cells. The hypothecium (tissue beneath the hymenium) is thick and brown to blackish. The asci (spore-bearing sacs) are elongate-clavate (narrowly club-shaped), thick-walled, eight-spored, and of the Umbilicaria type, with an apical dome that reacts K/I+ blue (that is, staining blue when pre-treated with potassium hydroxide and then iodine). Ascospores may be colourless, ellipsoidal, and aseptate (without internal partitions), or muriform (divided by both longitudinal and transverse walls) and later becoming brown.^[8]

The pycnidia (asexual fruiting bodies) are uni- or multiloculate, and their conidiophores (spore-bearing stalks) are sometimes branched, with conidiogenous loci formed terminally or just below the septa. The conidia (asexual spores) are short, cylindrical, or slightly curved. In species that lack isidia and soredia and only rarely produce apothecia, thalloconidia may occur on the lower cortex or on rhizinomorphs; these are either single-celled or composed of several cells.^[8]

Many species of Umbilicaria produce secondary metabolites belonging mainly to the orcinol-derived depside, tridepside, and β-orcinol depsidone groups.^[6][9] In a broad high-performance liquid chromatography survey of 33 species, gyrophoric acid was detected in 31 and was described as the characteristic lichen substance of the genus, while lecanoric acid and umbilicaric acid were also frequent.^[9] Chemistry can help separate some species, but its taxonomic value is uneven and is usually strongest when used alongside morphology rather than on its own.^[9][4] Intraspecific variation in lichen substances has also been documented, so chemically defined forms do not always correspond neatly to species boundaries.^[9][10]

Umbilicaria reproduces both sexually and asexually.^[3][11] Sexual reproduction is by apothecia producing ascospores, and the genus also forms pycnidia that produce conidia.^[3] Asexual reproduction often involves thalloconidia, which are fungal propagules produced from the lower cortex or on rhizinomorphs, while some species instead produce lichenized propagules such as isidia or soredia.^[3][12][11]

Thalloconidia have long been important in the classification of Umbilicaria and can be informative at the species level.^[12][6] Across Umbilicariaceae, species with thalloconidia tend to produce lichenized propagules less often, and many also form apothecia only rarely, suggesting partial substitution among dispersal strategies.^[6][11] Because thalloconidia do not contain the photobiont, establishment from them, like establishment from ascospores, still requires acquisition of a compatible algal partner.^[7]

Studies of a few species suggest that Umbilicaria species that reproduce mainly by ascospores may associate with a broader range of Trebouxia partners than mainly vegetative species, although this pattern has not yet been demonstrated across Umbilicaria as a whole.^[13][14] Field work on glacier forelands has also found that sexually reproducing species can be especially effective colonizers of newly exposed rock, whereas detached thallus fragments appear to play a smaller role in establishment in at least some alpine species.^[15]

Species of Umbilicaria are mostly saxicolous (rock-dwelling) lichens that grow chiefly on exposed rock surfaces in cold climates, and they are especially characteristic of cool-temperate, boreal, alpine, and polar environments.^[4][5] Most occur on siliceous, usually acidic rock rather than calcareous substrates.^[4][16] Although the genus is overwhelmingly rock-dwelling, rare departures from this pattern have been reported.^[17] Regional floras record species on boulders, scree, rock walls, cliffs, and other long-exposed outcrops.^[18] In tropical regions, Umbilicaria is largely confined to open high-mountain habitats above the tree line, where suitable sunlit rock is available.^[19]

The genus has a worldwide distribution and is a major component of saxicolous lichen floras in boreal and alpine regions.^[5] Modern syntheses indicate that diversity is greatest in the Northern Hemisphere, while occurrences in the mid-latitude Southern Hemisphere are more geographically restricted.^[4] Umbilicaria is nevertheless well represented in some Southern Hemisphere mountain systems and in Antarctica.^[4][16] In maritime Antarctica it can be widespread and locally abundant on stable acidic rock, but is less frequent on sites exposed to strong wind and sea spray.^[16]

Biogeographic patterns within the genus combine broad-ranging cold-climate species with narrower regional endemics.^[5][7] Molecular work in the central Andes has identified an endemic Umbilicaria element that dominates part of the alpine to subalpine elevational range there.^[5] Several species are circumpolar or bipolar in distribution, and long-distance dispersal has been proposed as an explanation for at least some of these disjunct ranges.^[7]

Umbilicaria species form lichens with green algal photobionts, most commonly members of the genus Trebouxia.^[14][13] Studies of co-occurring species show that photobiont specificity can be relatively low in some systems.^[14] Comparative work on U. esculenta and U. muehlenbergii further suggests that reproductive strategy helps shape how narrowly or broadly species associate with algal partners.^[13] Hestmark and co-workers reported that about one third of Umbilicaria species reproduce mainly by thalloconidia, which are strictly fungal propagules produced from the lower cortex.^[14] Differences in reproductive mode therefore seem to influence dispersal and the establishment of new fungal-algal partnerships within the genus.^[13][14]

Many species occupy physically severe environments and have been studied for their ability to survive freezing and resume activity after rehydration.^[20][21] In one experiment, five Umbilicaria species resumed gas exchange after ten years of storage at −20 °C, although recovery differed among species and hydration states.^[20] In U. aprina, net photosynthesis has been recorded at subzero temperatures after rehydration from snow, showing that at least some Antarctic species can remain metabolically active below 0 °C.^[21] Water-storage capacity also varies with thallus anatomy, especially medullary structure, which helps explain differences in hydration behaviour among species.^[22]

Members of the genus are also important pioneers on newly exposed rock surfaces.^[15][7] A 240-year glacier foreland study in Norway found that four alpine species grew slowly—roughly 0.9 to 2.4 mm per year in radial growth—and typically required about 50–80 years from establishment to first reproduction.^[15] Young volcanic lava can likewise be colonized by Umbilicaria, as shown for U. krascheninnikovii on twentieth-century lava flows in Kamchatka.^[7] In maritime Antarctica, propagules appear to be dispersed mainly by wind, although fragments of Umbilicaria have also been found in skua and seabird nests.^[16][5]