Athallia pyracea
Athallia pyracea is a species of crustose lichen in the family Teloschistaceae. It forms a very thin, often inconspicuous greyish crust on bark, with scattered to crowded orange fruiting bodies typically 0.3–1.0 mm across. The species grows mainly on the nutrient-enriched bark of deciduous trees such as aspen, poplar, and ash, and is considered a nitrophilous lichen. It is widespread in temperate regions of the Northern Hemisphere, occurring across Europe, North America, and parts of Asia. The yellow and orange parts of the thallus and apothecia turn purple when treated with potassium hydroxide. The species was transferred to Athallia in 2013 as part of a molecular reclassification of the Teloschistaceae.
Taxonomy
Athallia pyracea was originally described by Erik Acharius in his 1803 work Methodus qua omnes detectos lichenes, as Parmelia cerina var. pyracea. It was characterized as a greyish crust on tree bark with minute orange apothecia that become slightly convex and have a grey to yellowish margin. Acharius also remarked that it is only weakly distinct from var. cerina and that the crust tends to darken with age.After Acharius's introduction, the taxon was repeatedly recombined as lichen classification shifted through the 19th century, moving among several genera used for crustose lichens with orange apothecia. It appeared early on in Lecidea and Lecanora, and was also treated in segregate concepts such as Biatora, Placodium, and Callopisma, reflecting changing ideas about which mattered most for generic placement. From the later 1800s onward it was largely treated under the name Caloplaca pyracea and its close variants, a reclassification that had been suggested by the German mycologist Philipp Franz Wilhelm von Zwackh-Holzhausen in 1862. A parallel "pyrithroma" line was later linked back to pyracea and handled at various ranks under Lecanora, Placodium, Callopisma, and Caloplaca. In mid-20th-century British treatments, the name usage around this complex was still unsettled. Arthur Wade, treating the species under Caloplaca, regarded C. pyracea as a synonym of C. holocarpa and argued that holocarpa had nomenclatural priority because it traces back to Lichen holocarpus, predating the earliest species-rank use of "pyracea". He also regarded the "pyrithroma" element as a largely upland expression within the same continuum, remarking that intermediates occur and that there was little basis for keeping it distinct.
In a later reclassification of the xanthorioid lichens, Ulf Arup, Patrik Frödén and Ulrik Søchting split several reduced-thallus species out of the broad traditional concept of Caloplaca and proposed the new genus Athallia for a well-supported monophyletic lineage within the subfamily Xanthorioideae. In their circumscription, Athallia includes species that typically have a poorly developed thallus, a weak or amorphous when present, mostly apothecia, and spores with a medium to long septum; chemically, the species share the "chemosyndrome A" anthraquinone pattern. On this basis they made the new combination Athallia pyracea for Acharius's taxon, treating it as the appropriate name for the species that had long been known as Caloplaca pyracea and explicitly linking the combination back to the original Acharius basionym and type.
Description
Athallia pyracea forms a very thin, crustose thallus on bark that is sometimes partly immersed in the substrate. The surface is flat to slightly warty and often cracked into small areoles about 50–200 μm in diameter. Its colour ranges from pale greyish or greyish yellow to orange-yellow or orange; many thalli show a greyish background with scattered yellow-to-orange spots. A thin, grey may be visible at the margins. The is usually weakly developed, consisting of an amorphous layer of indistinct, roughly isodiametric cells.Apothecia are usually abundant, scattered to crowded, and sit to on the thallus. They are round to somewhat irregular and typically 0.3–1.0 mm across, with an orange that is flat to slightly convex. A is generally present, though it can become excluded in some apothecia. The is usually 35–50 μm thick and may be slightly raised or level with the disc; the is orange and granular. Microscopically, the hymenium is colourless and about 70–90 μm thick, with a hyaline that is sometimes oil-inspersed. Paraphyses are unbranched or branched near the tips, with the upper cells swollen to about 4–7 μm in diameter; the asci contain eight spores. The ascospores are ellipsoid to broadly ellipsoid, typically 10–15.5 × 5.5–8 μm, and, with the septum usually 3.8–5.5 μm thick. The yellow parts of the thallus and the apothecia give a K+ reaction.