Myriospora is a genus of crustose (crust-like), rock-dwelling lichens in the family Acarosporaceae. Its small, often mosaic-cracked thalli bear minute, blackish fruiting that each release dozens of colourless spores, a feature alluded to by the genus name—which means "countless spores". Following a 2024 transfer of Acarospora molybdina into the group, around thirteen species are accepted; the type species, M. smaragdula, is a common emerald-tinged "rock scale" on sun-exposed stone across the Holarctic realm.
The genus was proposed in 1853 as a segregate of Acarospora but the original description fell short of the formal rules then in force. Wilhelm Uloth supplied a validating description in 1861, yet for more than a century lichenologists kept the group within Acarospora. Molecular studies in the early 2000s showed that the so-called A. smaragdula complex forms its own evolutionary lineage; competing names (Silobia, Trimmatothelopsis) were put forward, but in 2012 Arcadia and Knudsen demonstrated that Uloth's Myriospora antedates them and therefore has priority. The genus is distinguished by an interrupted , a very tall hymenium, and asci packed with scores of one-celled spores—characters that separate it from Acarospora in the strict sense and from the blue-green genus Caeruleum.
Species of Myriospora are ecological generalists within harsh settings: they colonise siliceous and, less commonly, calcareous rock in deserts, temperate uplands, maritime cliffs and polar outcrops, with one Antarctic endemic (M. signyensis) extending the genus to every continent except Australia. Several members tolerate heavy-metal enrichment and can bioaccumulate iron or copper, making them early pioneers on mine tailings and freshly exposed bedrock. By penetrating mineral grains and trapping airborne dust they contribute to the first stages of soil formation, paving the way for mosses, vascular plants and a broader cryptogamic community.
Taxonomy
The genus Myriospora is a lichen-forming fungal genus in the family Acarosporaceae. It was originally established (circumscribed in the mid-19th century as a segregate from the genus Acarospora. The name Myriospora ("many spores") was introduced by Johann Adam Philipp Hepp in 1853 (attributed to Nägeli) for a group of crustose lichens with numerous spores per ascus. However, Hepp's publication did not meet all formal requirements, so for a long time Myriospora was not accepted as a validly published genus and its species were included within Acarospora. In 1861, Wilhelm Uloth provided a brief validating description (in German), noting Myriospora as having "apothecia almost cup-like" with "spores one-celled, egg-shaped or elliptical, in great number in one ascus". This early validation by Uloth (1861) means the correct author citation is Myriospora . Despite this early recognition, most lichenologists treated Myriospora as a synonym of Acarospora for over a century.
In the 2000s, the genus Myriospora was resurrected and re-evaluated. Richard C. Harris (2004) informally resurrected Myriospora for a North American species (Acarospora immersa), and Harris and Knudsen (2006) further clarified the nomenclature and lectotypified the genus. They believed the name had been effectively published by Hue in 1909, but later work showed Uloth's 1861 publication had priority. During the same period, molecular phylogenetics studies provided new insights. Researchers found that the Acarospora "smaragdula group" (a subset of species around Acarospora smaragdula) formed a clade separate from Acarospora in the strict sense. For example, phylogenies by Wedin and colleagues (2005) and Crewe and colleagues (2006) showed that the A. smaragdula complex lies outside the core Acarospora lineage. This evidence supported recognizing a distinct genus for this group.
In 2011, Westberg & Wedin formalized a new genus Silobia for the A. smaragdula complex. They described several species from Sweden under Silobia (including S. smaragdula, S. dilatata, S. myochroa, etc.) based on morphology, secondary chemistry, and preliminary phylogenies. Concurrently, the lichenologists Claude Roux and Pere Navarro-Rosinés (2011) had used yet another name (Trimmatothelopsis) for essentially the same group. This led to confusion over the correct genus name for these lichens. The issue was resolved in 2012 by "Linda in Arcadia" and Kerry Knudsen, who argued that Myriospora had been validly published by Uloth and therefore had priority. They typified Myriospora on M. smaragdula (the common emerald-coloured "rock scale" lichen) and placed Silobia and Trimmatothelopsis in synonymy with Myriospora. In other words, the species that had recently been called Silobia were reassigned to Myriospora. Seven new combinations in Myriospora were published at that time (e.g. M. dilatata, M. scabrida, M. tangerina, M. myochroa, M. rhagadiza, M. rufescens, M. hassei, etc.), solidifying the genus's revival. Arcadia and Knudsen also introduced a new genus, Caeruleum, for a different lineage of blue-green Acarospora species (such as A. heppii and A. immersa) that had been erroneously included in Myriospora; those have a different ascus-type (with an amyloid apex) and are now excluded from Myriospora.
Myriospora is now accepted as a distinct genus within Acarosporaceae, comprising the former A. smaragdula species group. It is closely related to (but separate from) the genus Acarospora. The current consensus, based on morphological and molecular evidence, is that Myriospora represents a monophyletic clade defined by unique traits. In 2024, a Nordic‐Baltic phylogenetic re-assessment transferred the coastal species Acarospora molybdina into Myriospora as M. molybdina, expanding the genus to 13 currently accepted species. That same study recovered Acarospora hysgina in the core Acarospora clade, clarifying that its superficially similar thallus is an example of convergent evolution and should not be included in Myriospora. A parallel body of work (2021–2023) employed multi-locus datasets and denser taxon sampling to resurrect Trimmatothelopsis as a separate, monophyletic lineage within Acarosporaceae, sister to Myriospora rather than nested inside it; those authors recombined species such as M. benedarensis, M. versipellis, M. oreophila and M. sphaerosperma under Trimmatothelopsis.
Description
Thallus
Myriospora lichens have a crustose, to thallus. One recently added member, M. molybdina, departs from the typical areolate habit by forming a thin, (lobate-margined) crust with narrow, lead-grey lobes up to 0.6 mm wide. The thallus typically forms a contiguous patch of small polygonal (scale-like units) that may appear cracked like a mosaic or pavement. Areoles are usually firmly attached to the substrate (rock) by their entire lower surface, though in some species they can be slightly raised with a narrow base (squamulose). The upper surface of the thallus varies in colour from grey or brown to rust-orange or dark brown-black, sometimes with a dull olive or greenish tinge (the name smaragdula means emerald, referring to a greenish hue seen in that species). Many species develop a (fine frost-like coating of crystals) on the apothecia or thallus surface, giving a whitish or bluish bloom. For example, M. himalayensis has a white pruinose on its apothecia. The (upper surface skin) is often well-developed and can appear shiny in xeric habitats due to an epicortex polysaccharide layer. The cortex is composed of a layer of hyphae and sometimes swollen cells; it is pigmented in the upper part (giving the thallus its colour) and generally colourless below. In some cases, crystals of secondary compounds are present in the cortex and can be seen under polarized light. There is no distinct lower cortex in most species (the thallus bottom is indistinguishable and directly attached to rock), except in soil-dwelling species where a lower or rhizines may occur.
Photobiont
The is a green alga (a unicellular alga, often Trebouxia or related). A defining microscopic feature of Myriospora is the interruption of the : the algal cells, instead of forming one continuous band within the thallus, are broken into discrete clusters or patches by vertical bundles of sterile hyphae (medullary hyphae) that extend upward from the medulla towards the cortex. In cross-section, these appear as columnar bundles cutting through the algal layer. This "broken" or interrupted photobiont layer is a key diagnostic trait of Myriospora. (In Acarospora, the algal layer is typically continuous, not regularly disrupted, although a few Acarospora species can occasionally show minor interruptions.) The medullary hyphae bundles in Myriospora often reach the upper cortex, sometimes forming pustule-like bumps or faint spots on the surface where they protrude. This feature can be subtle without microscopic examination, but it reflects a distinctive internal anatomy. The continuous, unbroken photobiont layer seen in Trimmatothelopsis provides the quickest microscopic distinction between that genus and Myriospora.
Apothecia
Myriospora produces apothecia (disk-like fruiting bodies) that are usually immersed or in the areoles. Typically, each areole will develop one apothecium in its centre (occasionally 2–3 in larger areoles). The apothecium may start nearly flush with the thallus (a small pit or dot) and later become more evident as the (the rim of proper tissue around the disc) expands. In Myriospora, the apothecia often appear – meaning they have a thallus-coloured margin – but in fact the margin is formed by the expanded (sometimes called a parathecial crown) rather than the algal thallus itself. This exciple can become quite prominent and thick in some species, giving the apothecia a distinct raised rim or a nearly cup-like appearance. The (hymenial surface) of the apothecium is usually darkly pigmented – often black, brown-black, or dark red-brown – sometimes with a glossy or uneven () texture. In many specimens the disc is covered by a pruinose coating (whitish or blue-grey), which can obscure its true colour. Some species also accumulate pigments or granules on the disc surface. The apothecia are generally small (often less than 0.5 mm in diameter), but can be larger and more elevated in certain species (for example, M. signyensis in Antarctica has unusually large, prominent apothecia up to about 0.7 mm). There are no or in this genus (only apothecial ascomata).
Microscopically, Myriospora has a hymenium that is tall relative to Acarospora. The hymenium (the layer containing the asci) can reach 150–200 micrometres (μm) in height in some species, whereas many Acarospora have hymenia around 60–120 μm tall. In M. molybdina the hymenium is even taller (160–230 μm) than in the type species, whereas A. hysgina remains at 100–125 μm—one of several characters separating the two look-alikes. The paraphyses (sterile filamentous hyphae in the hymenium) are characteristically thin and unbranched in Myriospora. They are usually about 1 μm thick (occasionally up to 2 μm at mid-height) and may have slightly expanded tips with small pigmented caps, but not as markedly swollen as in some Acarospora. In contrast, Acarospora often has stouter paraphyses (roughly 2–3 μm thick) that can be branched or ; this is a useful distinction, though there is some overlap. The hymenial gel of Myriospora (and indeed most Acarosporaceae) reacts in iodine in a euamyloid or hemiamyloid manner, but the ascus apex lacks a distinct amyloid tube (i.e. the ascus tip is K/I−). This means that unlike some lichen genera, the Myriospora asci do not have a blue-staining apical ring when treated with KOH and iodine – a trait shared with Acarospora. (Exceptions in the family are those now placed in Caeruleum, which have a K/I+ blue ascus tip, hence their separation.