Biatoropsis Usnearum
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''Biatoropsis usnearum'' is a species of
parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...
fungus A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
that grows exclusively on
lichen A lichen ( , ) is a hybrid colony (biology), colony of algae or cyanobacteria living symbiotically among hypha, filaments of multiple fungus species, along with yeasts and bacteria embedded in the cortex or "skin", in a mutualism (biology), m ...
species of the genus ''
Usnea ''Usnea'' is a genus of fruticose lichens in the large family Parmeliaceae. The genus, which currently contains roughly 130 species, was established by Michel Adanson in 1763. Species in the genus grow like leafless mini-shrubs or tassels anch ...
'', particularly '' U. subfloridana'', '' U. barbata'', and '' U. florida''. First described in 1934 by
Veli Räsänen Veli Johannes Paavo Bartholomeus Räsänen (24 August 1888 – 16 July 1953) was a Finnish lichenologist who made contributions to the study and documentation of Nordic and Baltic lichen funga. As a lecturer at various agricultural institution ...
, it has become a significant model organism in fungal evolution studies due to its specialised host relationships. The fungus belongs to the order
Tremellales The Tremellales are an order of fungi in the class Tremellomycetes. The order contains both teleomorphic and anamorphic species, most of the latter being yeasts. All teleomorphic species in the Tremellales are parasites of other fungi, though the ...
, though its precise
family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
classification remains uncertain. It forms distinctive swellings or
gall Galls (from the Latin , 'oak-apple') or ''cecidia'' (from the Greek , anything gushing out) are a kind of swelling growth on the external tissues of plants. Plant galls are abnormal outgrowths of plant tissues, similar to benign tumors or war ...
s on its
host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County * Host Island, in the Wilhelm Archipelago, Antarctica People * ...
lichens, ranging in colour from pale pink to dark reddish-brown, and notably suppresses the production of host defensive compounds like
usnic acid Usnic acid is a naturally occurring dibenzofuran derivative found in several lichen species with the formula C18H16O7. It was first isolated by German scientist W. Knop in 1844 and first synthesized between 1933 and 1937 by Frank H. Curd and Al ...
. While initially misclassified due to its unusual characteristics, modern microscopic and genetic studies have revealed it to be part of a
species complex In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...
, with at least three additional species now recognised. Found across Europe and North America, ''B. usnearum'' preferentially infects young, growing parts of its host lichens, particularly branch tips and small branches. The species has become particularly important in understanding how parasitic fungi adapt to new hosts, as it demonstrates evolution through switching between different host species rather than evolving alongside a single host species over time.


Taxonomy


Historical classification

The taxonomic history of ''Biatoropsis usnearum'' spans over two centuries of scientific observation.
Johann Jacob Dillenius Johann Jacob Dillen Dillenius (1684 – 2 April 1747) was a German botanist. He is known for his ''Hortus Elthamensis'' ("Eltham Garden") on the rare plants around Eltham, London, and for his ''Historia muscorum'' ("History of Mosses"), a natu ...
first documented what would later be recognised as ''B. usnearum'' in his 1742 work "Historia Muscorum" describing small fleshy nodules closely appressed to ''
Usnea ''Usnea'' is a genus of fruticose lichens in the large family Parmeliaceae. The genus, which currently contains roughly 130 species, was established by Michel Adanson in 1763. Species in the genus grow like leafless mini-shrubs or tassels anch ...
'' branches.
Erik Acharius Erik Acharius (10 October 1757 – 14 August 1819) was a Swedish botanist who pioneered the Taxonomy (biology), taxonomy of lichens and is known as the "father of lichenology". Acharius was famously the last pupil of Carl Linnaeus. Life Ac ...
, known as the "Father of Lichenology", made several observations of these structures between 1795 and 1810, referring to them variously as (seed-producing parts) and later distinguishing between normal -shaped
apothecia An ascocarp, or ascoma (: ascomata), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. As ...
(which he termed "orbilla") and what we now know to be basidiomata (which he termed "cephalodia"). Throughout the 19th and early 20th centuries, various authors applied different terms to describe these structures. Gray and Knowles used "cephalodia" in 1821 and 1929 respectively, while Smith introduced "pseudo-cephalodia" in 1918, and
Ludwig Schaerer Ludwig Emanuel Schaerer (11 June 1785 – 3 February 1853) was a Swiss pastor and lichenologist. Interested in natural history from a young age, Schaerer trained as a teacher and studied theology in Bern. During his career as a teacher, orpha ...
proposed "patellulae" in 1850.


Modern classification

Räsänen formally described ''Biatoropsis usnearum'' in 1934, initially classifying it as an ascomycete based on what he interpreted as asci containing
hyaline A hyaline substance is one with a glassy appearance. The word is derived from , and . Histopathology Hyaline cartilage is named after its glassy appearance on fresh gross pathology. On light microscopy of H&E stained slides, the extracellula ...
, spores measuring 10–16 by 4–6.5 
μm The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a unit of length in the International System ...
. In 1939, he further specified that these supposed asci were 8-spored. However, these structures were later recognised to be young probasidia. In 1949,
Rolf Santesson Rolf Santesson (1916–2013) was a Swedish lichenologist and university lecturer. He was awarded the Acharius Medal in 1992 for his lifetime contributions to lichenology. Early life and education Santesson was born in 1916 in Trollhättan, Sw ...
was the first to challenge its classification as an ascomycete, though he incorrectly suggested the structures were galls caused by ''
Abrothallus parmeliarum ''Abrothallus parmeliarum'' is a species of lichenicolous fungus. It grows on the thallus and apothecia (fruiting bodies) of '' Parmelia'' species. The fungus was first described scientifically by the Norwegian botanist Søren Christian Sommerfe ...
''. The true nature of ''B. usnearum'' as a heterobasidiomycete was not confirmed until 1990, when detailed microscopic examination revealed a
hymenium The hymenium is the tissue layer on the hymenophore of a fungal fruiting body where the cells develop into basidia or asci, which produce spores. In some species all of the cells of the hymenium develop into basidia or asci, while in oth ...
with auricularioid basidia and no ascomycetous structures.


Current systematic position

Molecular studies have revealed that ''B. usnearum'' represents a species complex containing several distinct species. While ''B. usnearum'' sensu stricto remains widespread, recent research has shown that many specimens historically identified as this species must now be considered doubtfully identified due to morphological intermediacy between known species or lack of clear distinguishing characteristics. Three additional species were formally described from this complex in 2016: :*'' B. hafellneri'' – Distinguished by having two-celled basidia with cells that elongate laterally at maturity, and by growing on species of the ''
Usnea fragilescens ''Usnea'' is a genus of fruticose lichens in the large family Parmeliaceae. The genus, which currently contains roughly 130 species, was established by Michel Adanson in 1763. Species in the genus grow like leafless mini-shrubs or tassels anch ...
'' aggregate :*'' B. minuta'' – Characterised by small (less than 1 mm) brown to black basidiomatal galls, growing on '' Usnea barbata'' and '' U. lapponica'' :*'' B. protousneae'' – Confined to '' Protousnea dusenii'' Research has shown that host switching, rather than cospeciation, has been the primary driver of diversification within this group, particularly in host-specialised lineages. Different host-specific species have been found to occur in sympatry, suggesting that speciation occurs through adaptive specialization rather than geographic isolation. The genus ''Biatoropsis'' was initially tentatively placed in the order
Platygloeales The Platygloeales are an order of fungi in the class Pucciniomycetes. Species in the order have auricularioid basidia (tubular with lateral septa) and are typically plant parasites on mosses, ferns, and angiosperms, though '' Platygloea'' species ...
based on its transversely septate basidia, absence of
clamp connection A clamp connection is a hook-like structure formed by growing hyphal cells of certain fungi. It is a characteristic feature of basidiomycete fungi. It is created to ensure that each cell, or segment of hypha separated by septa (cross walls), rece ...
s, and strong morphological similarity to the genus '' Mycogloea''. The genus can be distinguished from related taxa by its non-deciduous basidia, which differs from ''Mycogloea'', and by the absence of distinct probasidia, unlike ''Platygloea'' which has swollen cells under the septate portion of basidia. ''Biatoropsis usnearum'' produces an
anamorph In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota: *Teleomorph: the sexual reproductive stage (morph), typically a fruiting body. *Anamorph: an ase ...
that strongly resembles '' Hormomyces aurantiacus''. The anamorph is characterised by its hyphomycetous growth form, producing long branching chains of hyaline, ellipsoid
conidia A conidium ( ; : conidia), sometimes termed an asexual chlamydospore or chlamydoconidium (: chlamydoconidia), is an asexual, non- motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also ...
measuring 3–5 by 2–3.5 Î¼m. While several species of ''Tremella'' possess transverse or oblique basidial septa similar to ''B. usnearum'', its placement outside the Tremellales is supported by its distinctive basidial morphology and the absence of clamp connections. However, definitive placement within the Platygloeales awaits ultrastructural analysis of septal pores, as suggested by Moore's 1990 work on the order. The taxonomic understanding of this species was hampered for many years because lichenologists examining infected ''Usnea'' specimens generally had limited experience with heterobasidiomycetes, leading to misinterpretation of probasidial structures as asci or spores.


Description

''Biatoropsis usnearum'' forms distinctive growths, known as basidiomata, on its host lichens. These structures show considerable variation in their appearance, but typically appear as rounded, convex swellings with a narrowed base. The basidiomata can range in colour from pale pink to reddish brown or black, and measure between 0.2 and 2.5 mm in diameter. Their surface is usually smooth, though occasionally it may become warty. The texture is cartilaginous, similar to firm jelly. The species in its strict sense (''
sensu stricto ''Sensu'' is a Latin word meaning "in the sense of". It is used in a number of fields including biology, geology, linguistics, semiotics, and law. Commonly it refers to how strictly or loosely an expression is used in describing any particular c ...
'') is characterised by large, pale pinkish brown basidiomata that may sometimes become darker. These typical forms can be distinguished from other members of the species complex by this consistent colouration and size, though some specimens may darken due to parasitic fungi. The development of these structures follows a characteristic pattern. The infection begins in the outer protective layer () of the host lichen, where it triggers changes in the host's own thread-like structures (
hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...
e). The first visible signs are pale to reddish spots on the lichen's surface, where the number of algal cells is already reduced. As the fungus grows upward through the host's cortex, it begins producing reproductive structures, eventually forming a mature gall that lacks algal cells entirely. The internal structure of these growths consists of microscopic threads called hyphae, which measure 2-3 Î¼m in width. These hyphae have uniform walls and lack specialised connecting structures known as clamp connections. The fungus connects to its host through specialised feeding structures called
haustoria In botany and mycology, a haustorium (plural haustoria) is a rootlike structure that grows into or around another structure to absorb water or nutrients. For example, in mistletoe or members of the broomrape family, the structure penetrates th ...
l branches. These consist of a rounded "mother cell" measuring 2.5–4.5 Î¼m in diameter, from which extends a very fine filament 0.5–1 Î¼m thick and 3–7 Î¼m long. The reproductive layer, called the hymenium, appears clear or colourless under the microscope, though it may sometimes show a reddish-brown colouration in its upper portion. This layer contains numerous probasidia, which are early stages of the reproductive structures. The mature reproductive structures, called basidia, are club-shaped to nearly cylindrical and divided by 1–3 cross-walls. They measure 20–44 Î¼m long by 3–6.5 Î¼m wide. From these basidia emerge long, thin extensions called epibasidia, which can reach up to 85 Î¼m in length while maintaining a width of 2–3 Î¼m. The spores produced by ''B. usnearum'' are nearly spherical to oval-shaped, with a distinctive projection point called an apiculum. They measure 4.5–8 Î¼m long by 4–7.5 Î¼m wide. The fungus also produces an asexual reproductive form (anamorph) that creates long, branching chains of clear, oval-shaped cells measuring 3–5 by 2–3.5 Î¼m. When ''B. usnearum'' infects a host lichen, it often causes a characteristic bending or curving of the lichen's branches at the point of infection. The fungus preferentially infects young and growing parts of the host, particularly branch tips, fibrils (small branches), and areas beneath developing reproductive structures. Notably, when the fungus infects a part of the lichen, it suppresses the production of usnic acid, a characteristic lichen compound, in the infected area. While the fungus is most commonly found growing on various species of ''Usnea'' lichens worldwide, it has also been documented on the related genus ''Protousnea''. Interestingly, it has never been found on Usnea species from the subgenus ''Neuropogon'', suggesting some degree of host specificity. Field observations have shown that when multiple ''Usnea'' species grow together, not all species are equally susceptible to infection by ''B. usnearum''. Advanced microscopic imaging techniques reveal that in mature galls of ''B. usnearum'', the basidia are situated at the surface of the gall, while infective hyphae with haustoria are abundant in the central zone and extend to a lesser degree into the basal layers. When stained with special dyes, the fungal cells show a distinctive orangish colour that distinguishes them from the greenish host cells. This structured organization differs from some other gall-forming lichenicolous fungi, like '' Tremella cetrariicola'', where the parasitic hyphae are distributed more evenly throughout the gall.


Habitat and distribution

''Biatoropsis usnearum'' sensu stricto is found growing specifically on six ''Usnea'' species: '' U. subfloridana'', '' U. barbata'', '' U. cavernosa'', '' U. florida'', '' U. glabrescens'', and '' U. intermedia''. The species has been confirmed from several locations in Europe (Austria, Finland, Poland, Sweden, and Scotland) and North America (Alberta and British Columbia in Canada, Minnesota in USA). However, its true distribution is likely more extensive than currently documented. Many historical records and reported locations for this species need to be reevaluated, as specimens previously identified as ''B. usnearum'' may represent other species in the complex. Research suggests there may be patterns in how these fungi have evolved to interact with their hosts – lichenicolous fungi that are phylogenetically related to lichens themselves tend to target the algal symbionts in lichen associations, while those from primarily non-lichenised fungal groups (like ''B. usnearum'') appear better adapted to parasitising the fungal component of the lichen. The fungus preferentially infects young and growing parts of the host, particularly branch tips, fibrils (small branches), and areas beneath developing reproductive structures. Notably, when the fungus infects a part of the lichen, it suppresses the production of usnic acid, a characteristic lichen compound, in the infected area.


References

{{Taxonbar, from=Q10429310 Tremellomycetes Fungus species Fungi described in 1934 Lichenicolous fungi Taxa named by Veli Räsänen