Arthothelium
Arthothelium is a genus of lichen-forming fungi in the family Arthoniaceae. Species in the genus typically form crusts on smooth bark in humid, undisturbed habitats. They are distinguished from the related genus Arthonia by their spores, which are divided by both transverse and longitudinal walls into a brick-like pattern. The genus has a cosmopolitan distribution, with most species occurring in tropical regions.
Taxonomy
Arthothelium was introduced as a new genus by the Italian lichenologist Abramo Bartolommeo Massalongo in 1852, for species that earlier authors had treated among genera such as Arthonia, Graphis, and Opegrapha. In the protologue, Massalongo defined the genus by its gelatinous apothecia, which are concealed by the thallus when young but later become swollen, misshapen, and rough-wrinkled, and by ovoid to pear-shaped asci bearing 6–8 spores with only barely visible paraphyses. He described the spores as ovoid-ellipsoid and initially colourless, then increasingly divided by 8–10 transverse septa and 1–2 longitudinal septa, and finally darkening to a sooty brown at maturity; the thallus itself was described as a uniform, closely attached crust.Arthothelium is considered polyphyletic, meaning the genus likely includes species that do not share a common evolutionary ancestor, and further study is needed on its type species, Arthothelium tremellosum.
Description
Arthothelium is a genus of crustose lichens, which can either be or superficial. The thallus may spread freely or be confined to specific areas. The lichen's can vary, including types from the genus Trentepohlia, the family Chlorococcaceae, or may be absent altogether.The reproductive structures resemble apothecia, a common form of lichen fruiting body, and come in various shapes, from flat to convex, and may be elongated or star-like. These structures, known as the, range in colour from red-brown to black, and can sometimes have a frosted, powdery appearance. A, which in some lichens surrounds the fruiting body, is absent in Arthothelium. The, a rim of tissue found in some lichen fruiting bodies, is also absent.
The uppermost layer of the apothecium, the, can be colourless, red-brown, or dark brown. Below this, the hymenium, where the spores develop, often turns blue when tested with iodine. The, the supportive tissue beneath the hymenium, is variable in thickness and ranges from dark red-brown to dark brown in colour. The hymenium contains filaments called, which are sparsely to densely branched and often have swollen, red-brown tips.
The asci are usually or ellipsoidal, containing eight spores. They are semi-, meaning they have two wall layers that split during spore release, and feature a large apical dome with a distinct ocular chamber. The ascospores are to ellipsoidal, colourless, and, meaning they are divided by multiple internal walls, creating a brick-like pattern. This is a key distinguishing feature from similar genera like Arthonia, where the spores are only divided by transverse walls. Conidiomata resemble those found in Arthonia. The genus generally lacks lichen products, in species found in Britain and Ireland, but elsewhere, species may contain xanthones or anthraquinones.
Habitat and distribution
Arthothelium typically grows on smooth bark in humid, undisturbed habitats, although it is rarely found on rock. The genus has a cosmopolitan distribution, with most species occurring in tropical regions.Species
, Species Fungorum accept 26 species of Arthothelium:Arthothelium ampliatum Arthothelium atrorubrum Arthothelium aurantiacopruinosum Arthothelium bacidinum Arthothelium cinereoargenteum Arthothelium desertorum Arthothelium dictyosporum Arthothelium diffluens Arthothelium evanescens Arthothelium feuereri Arthothelium frischianum Arthothelium huegelii Arthothelium hymeniicola Arthothelium infuscatum Arthothelium insolitum Arthothelium interveniens Arthothelium isidiatum Arthothelium japonicum Arthothelium lirellans Arthothelium macounii Arthothelium macounioides Arthothelium macrothecum Arthothelium magenteum Arthothelium microsporum Arthothelium miesii Arthothelium norvegicum Arthothelium orbilliferum Arthothelium polycarpum Arthothelium pulverulentum Arthothelium punctatum Arthothelium ruanum Arthothelium scandinavicum Arthothelium spectabile Arthothelium subspectabile- ''Arthothelium velatius''