Parmelia sulcata, commonly known as the hammered shield lichen or cracked-shield lichen, is a foliose lichen in the family Parmeliaceae. First described by Thomas Taylor in 1836, it is one of the most prevalent lichen species globally, known for its resilience to pollution and cosmopolitan distribution across temperate and cold regions of both hemispheres. P. sulcata forms a circular thallus up to 10 cm (4 in) in diameter, with a glaucous white to grey upper surface and a black lower surface, featuring broadly lobed structures with both marginal and laminal soralia and a distinctive reticulate pattern of pseudocyphellae. Taxonomically, Parmelia sulcata has a complex history. Molecular phylogenetics studies have revealed significant genetic variability within what was traditionally considered a single species, identifying a complex of cryptic species, including Parmelia encryptata and P. barrenoae, and demonstrate the necessity of molecular data for accurate species identification. P. sulcata maintains a highly specific symbiotic relationship with green algae of the genus Trebouxia, particularly species within Trebouxia clade I, as its photobiont partner. Ecologically, Parmelia sulcata grows on various substrates, including bark, wood, and rocks. It accumulates pollutants such as heavy metals and radionuclides, reflecting air quality and contamination levels, making it a useful bioindicator in pollution studies across Europe, North America, and other regions. Beyond its ecological importance, P. sulcata has been used in traditional medicine and as a source of natural dyes.
Systematics
Historical taxonomy Parmelia sulcata was first described by Thomas Taylor in 1836 under its current name. Taylor characterised its thallus as orbicular and stellate, with a glaucous (dull grayish-green) white colour when dry and a glaucous green colour when wet, and a dark brown underside with black fibres. He described the lobes as incised, somewhat concave, and reticulated with elevated ridges. The thallus features oblong or linear eruptions consisting of a fine greyish-brown powder. Apothecia are central and substipitate, with a brown disk and a smooth or powdery exterior. Taylor noted that Parmelia sulcata was distinct from Parmelia saxatilis, being larger, whiter, less imbricated, more concave, and more green when wet. He also highlighted that the species is more common on rocks in County Kerry than on trees and could attain a diameter of a foot or more. Taylor mentioned that it had been previously mistaken for Parmelia conspersa (now in Xanthoparmelia) in some plant lists. Parmelia sulcata has a large number of named varieties and forms, but no subspecies despite its extensive range. Most taxonomists have left the species in the genus to which Taylor originally assigned it, though the mycologist Maurice Choisy assigned it to the genus Parmotrema in 1952. In 1962, Mason Hale and Syo Kurokawa designated a specimen from Lough Bray, County Kerry, as the lectotype for Parmelia sulcata. This specimen, collected by Taylor in 1812 and housed in the Farlow Herbarium of Cryptogamic Botany, was supported by Taylor's own notes, confirming its identification. Despite attempts to extract DNA from the original lectotype, no viable DNA was obtained, leading to the selection of an epitype from a specimen collected near the original type locality in County Kerry in 2006. This epitype helps stabilize the application of the name Parmelia sulcata in both molecular and morphological studies.
Molecular studies Molecular phylogenetics studies revealed significant genetic variability within Parmelia sulcata, indicating that it is a complex of cryptic species. Molina and colleagues (2011) used three molecular markers: nuclear internal transcribed spacer, abbreviated as nuITS; nuclear intergenic spacer rDNA, abbreviated as nuIGS; and partial β-tubulin gene to study the biodiversity of P. sulcata across four continents. They identified two monophyletic groups: Parmelia sulcata sensu stricto ("in the strict sense") and a new cryptic species named Parmelia encryptata. The study highlighted that P. sulcata as traditionally circumscribed did not form a monophyletic group. The researchers found that specimens initially identified as P. sulcata based on morphological characters did not cluster together in phylogenetic analyses. Instead, they formed distinct, well-supported clades, indicating the presence of multiple cryptic species within what was previously thought to be a single species. P. encryptata was described as morphologically similar to P. sulcata but genetically distinct, with a unique insertion in the internal transcribed spacer region and specific nucleotide differences. Another member of this complex, the widely distributed Parmelia barrenoae, was recognised as a distinct species in 2005.
Naming The genus name Parmelia is a compound of two Greek words: parme, meaning 'fruit bowl' and -eileo, meaning 'enclosed'. This likely refers to the lecanorine apothecia of species belonging to the genus. The specific epithet sulcata is derived from sulcatus, meaning "grooved" or "furrowed". The species is known by a number of colloquial names, including hammered shield lichen, cracked-shield lichen, powdered crottle, furrowed shield lichen, powdered shield, and waxpaper lichen.
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