Micarea


Micarea is a genus of lichen-forming fungi in the family Pilocarpaceae. The widely distributed genus contains 156 species and new species are described actively. Species in the genus are crustose lichens and their photobiont is a single-celled green alga. The genus was established by the Swedish mycologist Elias Fries in 1825. Micarea lichens are typically small and often overlooked, producing minute fruiting bodies that are usually whitish, grey, or bluish in colour. Many species reproduce both sexually through spores and asexually through specialised structures, and they are found on substrates ranging from tree bark and dead wood to rocks and even moss.

Taxonomy

Micarea was circumscribed by Elias Magnus Fries in his 1825 work Systema Orbis Vegetabilis. In his, Fries characterised the genus by an effuse, crust-like thallus made up of aggregated gelatinous, together with free, almost spherical apothecia that are "always open" and lack a distinct margin. He recorded it from rocks and especially damp wood, and remarked that sterile material of Micarea prasina could be difficult to tell apart from algae-like crusts recognised at the time.
There have been some taxonomic disputes about the genus. Nowadays, the genus is accepted, although it is paraphyletic and in need of further research. Micarea prasina is the type species of the genus.

Description

Species of Micarea are small crustose lichens whose thallus forms a thin, spreading crust that may sit on the surface or be partly immersed in the substrate. The thallus is commonly built from tiny, almost spherical granules called . These granules may remain distinct, merge into convex areoles, or form a cracked or scurfy crust; in some species the areoles break down into soredia for asexual dispersal, while isidia are only rarely present. A distinct marginal is usually absent or inconspicuous, and the thallus typically lacks a true outer, though some species develop a thin, colourless outer layer. The photobiont is usually a single-celled green alga with thin-walled cells about 4–7 μm in diameter, often occurring in pairs. A few species also form small cephalodia that contain cyanobacteria such as Nostoc or Stigonema.
Sexual fruiting bodies are usually present but are often minute. They are commonly whitish to grey or bluish, but may be brown to black, and they are not. The apothecia are typically immarginate and sit close to the thallus. Stalked forms occur only rarely, and a thallus-derived margin is absent in almost all cases. Internally, the apothecial margin ranges from poorly developed to well developed and is formed from radiating, branched hyphae. The pigments in these tissues can give characteristic reactions in standard chemical tests, and the spore-bearing layer is gelatinised and may turn blue in iodine. The hymenium contains mostly branched paraphyses, though some species also have sturdier, unbranched paraphyses; the tips are not, or only slightly, swollen and lack a dark apical cap. The asci bear eight spores and are club-shaped, with apical structures that show characteristic staining patterns. Ascospores are colourless and smooth, lack a, and may be or transversely septate. Their shapes range from ellipsoid and ovoid to spindle-shaped or needle-like. Asexual reproductive structures are usually pycnidia, which may be immersed, sessile, or raised on short stalks. Three conidium types are produced: macroconidia, mesoconidia, and microconidia. Chemically, many species have no detectable secondary metabolites by thin-layer chromatography, while others produce a range of substances. Gyrophoric acid occurs in several species.

Reproduction

Lichen-forming fungi have developed diverse reproduction strategies. Genus Micarea is used as a model for studying how reproductive traits and environmental factors relate to speciation, because species differ in both substrate use and reproductive mode. Some Micarea species are generalists that can grow on several substrates, while others are restricted to narrow microhabitats. In the Micarea prasina group, some species are predominantly sexual, while others often lack sexual structures but bear numerous pycnidia that produce asexual conidia. The roles of the different conidium types are still not fully understood, but mesoconidia are thought to act as asexual propagules, based in part on specimens that regularly have only mesopycnidia and no apothecia. Recent phylogenetic analyses together with ancestral state reconstruction indicate that shifts in reproduction mode have evolved independently several times in the group, and that facultative and obligate lignicoles can be sister species. This pattern is consistent with a facultative lignicole ancestor, and suggests that shifts from bark to dead wood can coincide with a move towards asexual reproduction. One proposed explanation is that dead wood is a short-lived habitat, so species confined to it may need to colonise new suitable substrata quickly; faster dispersal via asexual propagules could then promote reproductive divergence and contribute to speciation.

Species

, Species Fungorum accepts 156 species of Micarea.