Genus of lichens
Austrotrema is a genus of lichen-forming fungi in the family Graphidaceae. It is primarily found in the Australian-Southeast Asian region. The genus is characterized by small, pore-like ;dfn id="" ;apothecia ;/dfn ; with a double margin, faintly amyloid ascospores, and stictic acid chemistry. "Austrotrema" species grow on tree bark and have a continuous thallus with a ;dfn id="" ;cortex ;/dfn ; that is ;dfn id="" ;prosoplectenchymatous ;/dfn ;–featuring densely packed, filamentous fungal hyphae that run parallel to the surface of the lichen, creating a compact and firm texture. The genus is closely related to "Thelotrema" and "Leucodecton", but can be distinguished from them based on molecular phylogenetic data, specific morphological traits, and its secondary chemistry. Currently, "Austrotrema" comprises three species.
Taxonomy.
"Austrotrema" was circumscribed by lichenologists Ian Medeiros, Robert Lücking, and H. Thorsten Lumbsch in 2017. The genus was created to accommodate the three species of the "Thelotrema bicinctulum" species complex, which molecular phylogenetic analysis showed to be sufficiently genetically distinct to warrant placement in a new genus. "Austrotrema" is characterized by its small, pore-like ;dfn id="" ;apothecia ;/dfn ; with a double margin, faintly amyloid ascospores, and stictic acid chemistry. The type species for this genus is "Austrotrema bicinctulum", originally described as "Thelotrema bicinctulum" by William Nylander in 1861. The genus name "Austrotrema" is derived from the distribution of the type species in the Australian-Southeast Asian region.
Description.
"Austrotrema" lichens grow on the bark of trees and have a continuous thallus (body) with no prothallus. The ;dfn id="" ;cortex ;/dfn ; of these lichens is ;dfn id="" ;prosoplectenchymatous ;/dfn ;, featuring an irregular to thick layer of photobiont "Trenepohlia" green algal cells, interspersed with calcium oxalate crystal clusters. The medulla is indistinct and located between the upper and lower layers of the ;dfn id="" ;periderm ;/dfn ;. The minute ;dfn id="" ;ascomata ;/dfn ; are round and can be either immersed or barely ;dfn id="" ;erumpent ;/dfn ;. The ;dfn id="" ;disc ;/dfn ; is almost entirely covered by a narrow pore, with a light to dark grey colour and a white- ;dfn id="" ;pruinose ;/dfn ; appearance. The ;dfn id="" ;proper margin ;/dfn ; is distinct and may be slightly to deeply fissured, surrounded by a pale rim formed by the double margin. The ;dfn id="" ;thalline margin ;/dfn ; is thin, smooth, and ranges from yellow to olive-brown, with entire to slightly eroded or fissured edges.
The ;dfn id="" ;excipulum ;/dfn ; is ;dfn id="" ;paraplectenchymatous ;/dfn ; and separated from the covering thalline layer by a split. A ;dfn id="" ;columella ;/dfn ; is absent, while the ;dfn id="" ;hypothecium ;/dfn ; is prosoplectenchymatous and hyaline. The hymenium is hyaline and clear, while the ;dfn id="" ;epithecium ;/dfn ; is gray and granular. The unbranched ;dfn id="" ;paraphyses ;/dfn ; have smooth tips, and ;dfn id="" ;periphysoids ;/dfn ; are present to indistinct. The asci are fusiform, with eight ;dfn id="" ;ascospores ;/dfn ; per ascus. The ascospores are hyaline, ellipsoid, tapering at one end, and transversely septate to somewhat ;dfn id="" ;muriform ;/dfn ; with lens-shaped ;dfn id="" ;lumina ;/dfn ;. ;dfn id="" ;Conidiomata ;/dfn ; have not been observed to occur in this genus.
The secondary chemistry of "Austrotrema" lichens includes stictic acid as the major component, along with various related substances in minor or trace amounts.
Comparison with related genera.
"Austrotrema" bears resemblance to certain species of "Thelotrema", and its distinction from "Thelotrema" at the genus level can only be made with molecular phylogenetic data. However, "Austrotrema" differs from similar "Thelotrema" species with transversely septate or small muriform ascospores and stictic acid due to its dense, prosoplectenchymatous cortex. "Austrotrema" can also be confused with "Leucodecton" species, but its close relationship with "Nadvornikia hawaiiensis" places it in the same tribe as "Austrotrema". Within the Wirthiotremateae tribe, "Austrotrema" can be distinguished from "Asteristion" by its small ascomata with entire, pore-like thalline margin, and from "Wirthiotrema" by its less glossy thallus that does not split internally. The thalli of "Wirthiotrema" species tend to be more greenish, while those of "Austrotrema" are brownish.
References.
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