Genus of lichens
Asteristion is a genus of lichen-forming fungi in the family Graphidaceae. It has seven species. Previously considered a synonym of either "Phaeotrema" or "Thelotrema", molecular evidence led to its resurrection as a distinct genus. "Asteristion" lichens are corticolous (bark-dwelling), featuring a continuous thallus with a loose to hardened ;dfn id="" ;cortex ;/dfn ; and a ;dfn id="" ;photobiont layer ;/dfn ; containing calcium oxalate crystals. The ;dfn id="" ;ascomata ;/dfn ;, or fruiting bodies, are characterized by their large, often ;dfn id="" ;chroodiscoid ;/dfn ; appearance and the presence of distinct ;dfn id="" ;periphysoids ;/dfn ;. The secondary chemistry of these lichens includes major concentrations of stictic acid and minor to trace amounts of associated substances.
Taxonomy.
"Asteristion" belongs to the tribe Wirthiotremateae in the family Graphidaceae. This tribe, revised in 2017 to account for newly clarified phylogenetic relationships, contains the genera "Wirthiotrema", "Austrotrema", "Asteristion", and "Nadvornikia". These genera are characterized by having the substance stictic acid, in addition to minor or trace amounts of related substances. Originally considered a synonym of either "Phaeotrema" or "Thelotrema", "Asteristion" was later recognized as a section of "Thelotrema" by Tatsuo Matsumoto in 2000. Molecular evidence and extended phylogenetic analysis reported in the 2017 study have since revealed that the type species, "Asteristion erumpens", does not belong to "Thelotrema" or any other recognized genus. Consequently, "Asteristion" has been resurrected as a distinct genus, which currently comprises seven species that were formerly members of the "Chapsa platycarpa" species complex.
The genus name "Asteristion" derives from the star-like appearance of the recurved ;dfn id="" ;thalline margin ;/dfn ; commonly found in this genus, particularly evident in the type species.
Description.
"Asteristion" lichens are corticolous, featuring a continuous, epiperidermal thallus with a smooth to uneven surface and no prothallus. The thallus comprises a loose to prosoplectenchymatous cortex, a ;dfn id="" ;photobiont layer ;/dfn ; with calcium oxalate crystals, and a thin epi- to endoperidermal medulla. The ;dfn id="" ;photobiont ;/dfn ; in "Asteristion lichens" is "Trentepohlia", a green algal genus.
The ;dfn id="" ;ascomata ;/dfn ; in these lichens are rounded to angular and immersed to ;dfn id="" ;erumpent ;/dfn ;. The ;dfn id="" ;disc ;/dfn ;, which ranges in colour from flesh to brown and is white- ;dfn id="" ;pruinose ;/dfn ;, can be covered or exposed. The ;dfn id="" ;proper margin ;/dfn ; is distinct, fissured to recurved, and separated from the ;dfn id="" ;thalline margin ;/dfn ; by a narrow split, forming a prominent rim around the disc. The thalline margin is white to pale brown and entire to recurved.
"Asteristion" lichens have an entire, hyaline to pale yellow, ;dfn id="" ;paraplectenchymatous ;/dfn ; excipulum, separated from the covering thalline layer by a split. A ;dfn id="" ;columella ;/dfn ; is absent, and the ;dfn id="" ;hypothecium ;/dfn ; is ;dfn id="" ;prosoplectenchymatous ;/dfn ; and hyaline. The hymenium is hyaline and clear, while the ;dfn id="" ;epithecium ;/dfn ; is grey and granular. The paraphyses are unbranched with smooth apices, and ;dfn id="" ;periphysoids ;/dfn ; are present. The asci are fusiform, with fusiform to ellipsoid ascospores that are transversely septate to ;dfn id="" ;muriform ;/dfn ;, somewhat ;dfn id="" ;distoseptate ;/dfn ; with lens-shaped ;dfn id="" ;lumina ;/dfn ;, and hyaline to brown. Ascospores may be non-amyloid or amyloid. ;dfn id="" ;Conidiomata ;/dfn ; have not been observed to occur in "Asteristion" lichens.
The secondary chemistry of these lichens includes major concentrations of stictic acid and minor to trace amounts of related substances.
Comparison with related genera.
"Asteristion" species can be distinguished from other genera in the Wirthiotremateae tribe by their large, often ;dfn id="" ;chroodiscoid ;/dfn ; ascomata and the presence of distinct periphysoids (lateral paraphyses). They may resemble "Chapsa" and its allies, such as "Astrochapsa", but can be differentiated by the presence of a distinctly free excipulum forming a double margin, as opposed to a fused excipulum in "Chapsa".
Species.
As of April 2023[ [update]], Species Fungorum (in the Catalogue of Life) accept seven species of "Asteristion".
The taxon "Asteristion erumpens" Leight. (1870) is now named "Phaeotrema erumpens".
References.
 ;templatestyles src="Reflist/styles.css" / ;