Genus of lichen-forming fungi
Arthothelium is a genus of lichen-forming fungi in the family Arthoniaceae.
Description.
"Arthothelium" is a genus of crustose lichens, which can either be ;dfn id="" ;immersed ;/dfn ; (embedded within the ;dfn id="" ;substrate ;/dfn ;) or superficial (growing on the surface). The thallus may spread freely or be confined to specific areas. The lichen's ;dfn id="" ;photobiont ;/dfn ; (the algae or cyanobacteria involved in its symbiosis) can vary, including types from the genus "Trentpohlia", 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 ;dfn id="" ;disc ;/dfn ;, range in colour from red-brown to black, and can sometimes have a frosted, powdery appearance ( ;dfn id="" ;pruinose ;/dfn ;). A ;dfn id="" ;thalline margin ;/dfn ;, which in some lichens surrounds the fruiting body, is absent in "Arthothelium". The ;dfn id="" ;exciple ;/dfn ;, a rim of tissue found in some lichen fruiting bodies, is also absent.
The uppermost layer of the apothecium, the ;dfn id="" ;epithecium ;/dfn ;, can be colourless, red-brown, or dark brown. Below this, the hymenium, where the spores develop, often turns blue when tested with iodine (I+). The ;dfn id="" ;hypothecium ;/dfn ;, 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 ;dfn id="" ;paraphysoids ;/dfn ;, which are sparsely to densely branched and often have swollen, red-brown tips.
The asci (spore-producing cells) are usually ;dfn id="" ;clavate ;/dfn ; (club-shaped) or ellipsoidal, containing eight spores. They are semi- ;dfn id="" ;fissitunicate ;/dfn ;, 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 ;dfn id="" ;obovoid ;/dfn ; to ellipsoidal, colourless, and ;dfn id="" ;muriform ;/dfn ;, 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 (structures producing asexual spores) resemble those found in "Arthonia". The genus generally lacks lichen products (secondary metabolites), in species found in Britain and Ireland, but elsewhere, species may contain xanthones or anthraquinones.
"Arthothelium" typically grows on smooth bark in humid, undisturbed habitats, although it is rarely found on rock. The genus includes around 50 species and has a cosmopolitan distribution, with most species occurring in tropical regions. It 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".
Species.
, Species Fungorum (in the Catalogue of Life) accept 26 species of "Arthothelium":
References.
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