Lepraria incana is a species of dust lichen in the family Stereocaulaceae. First described scientifically by Johann Jacob Dillenius in 1741, and then formally by Carl Linnaeus in 1753, it is the type species of the genus Lepraria. The thallus of this species is green to greyish-green, and powdery – as if made of tiny granules. These granules are soredia, which are asexual reproductive structures. Like most members of genus Lepraria, the lichen has few distinguishing features, lacking both a medulla and sexual reproductive structures (apothecia). Chemically, the lichen is characterised by the presence of the secondary chemicals known as divaricatic acid and zeorin. The lichen prefers to grow on bark on the base of trees in moist and partly shaded locations, but will also grow on dead wood, silica-rich rocks, or soil. It is not particularly selective in terms of its bark substrate, and has been recorded growing on a wide variety of both deciduous and coniferous trees. Lepraria incana is common and widespread in Europe and Asia. Although it was previously thought to be even more widely distributed, with records from North America and South America, research has shown that at least some of the Lepraria incana lookalikes on those continents – although morphologically indistinguishable – are genetically distinct, and they have been described as unique species. Lepraria incana is relatively tolerant to air pollution and some studies have investigated its use as a biomonitor. The lichen is known to harbour several species of lichenicolous fungi as well as a mycovirus.
Taxonomy The species was one of the 80 lichens formally described by Carl Linnaeus in his 1753 work Species Plantarum. The taxon was originally named Byssus incana, as Linnaeus considered it to be a species of alga. He cited a polynomial originally published by the German botanist Johann Jacob Dillenius in his 1742 work Historia muscorum, and referred to Dillenius' illustration of the species. The polynomial (Byssus pulverulenta incana) given by Linnaeus included a diagnosis that was unaltered from the one given by Dillenius. In some cases, because these illustrations represent the only remaining original material available, nomenclatural rules allow for these images to serve as lectotypes. The type specimen was collected in the British Isles. Linnaeus also included the species in his 1743 work Flora Svecica, an account of the plants and cryptogams growing in Sweden. The Swedish lichenologist Erik Acharius transferred it to the genus Lepraria in 1803. It is the type species of Lepraria, having been designated so by Jack Laundon in 1992. Because there was no associated specimen in the Linnaean Herbarium, he selected Dillenius' 1741 illustration to serve as the lectotype of this taxon. Laundon had previously attempted to typify this species in 1963, but did so incorrectly. An epitype for Lepraria incana was designated by Per Magnus Jørgensen and colleagues in 1994. Lepraria incana has been transferred to several other genera in its taxonomic history. Chronologically, these include Lepra (Friedrich Heinrich Wiggers, 1780), Verrucaria (Philipp Gottfried Gaertner, Georg Friedrich Wilhelm Meyer and Johannes Scherbius, 1801), Pulveraria (Heinrich Gustav Flörke, 1807), Lecidea (Erik Acharius, 1814), Patellaria (Kurt Polycarp Joachim Sprengel, 1827), and Crocynia (Auguste-Marie Hue, 1924). Acharius proposed the variety Lepraria incana var. latebrarum in 1810, based on the species he had originally published in 1799 as Lichen latebrarum; this lichen is now known as Dendrographa latebrarum.
Description
The thallus of Lepraria incana is leprose (powdery), and greenish grey in colour, often with blue tinge. It has a diffuse margin with neither distinct lobes, nor a distinct cortex, forming continuous patches with diameters up to about 8 cm (3.1 in), or forming smaller colonies of poorly developed patches or areoles. The thallus consists of abundant powdery soredia (little balls of algae wrapped in fungus hyphae) up to 50 μm in diameter. A medulla is sometimes present, but if so, it is usually poorly developed. Consoredia (diaspores made of aggregated soredia) are sometimes present, and they measure up to 110 μm. Overall, the appearance of the lichen can be quite variable, as sometimes it forms more or less loose cotton-like cushions, while other times it consists only of soredia that can be sparse or dense. The photobiont partner of the lichen is a spherical (coccoid) member of the green algal phylum Chlorophyta, with a diameter of up to 18 μm. In terms of secondary chemicals, Lepraria incana contains divaricatic acid, zeorin, and nordivaricatic acid (trace amounts). Some specimens have trace amounts of atranorin, although this may be a contaminant. It also contains anthraquinones such as parietin, fallacinal, parietinic acid, and citreorosein. A chemical analysis on specimens of the lichen from Japan revealed several terpenes, including pavoninin-2, terpecurcumin Q, ergosterol acetate, taxuspine C, and lantadene A methyl ester. The thallus has a bluish tinge when viewed under ultraviolet light. The expected results of standard lichen spot tests on this species are K− or + (producing a faint yellow colour), C−, KC−, and Pd−.
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