Biatoropsis usnearum
From Wikipedia, the free encyclopedia
| Biatoropsis usnearum | |
|---|---|
 | |
Scientific classification  | |
| Kingdom: | Fungi |
| Division: | Basidiomycota |
| Class: | Tremellomycetes |
| Order: | Tremellales |
| Genus: | Biatoropsis |
| Species: | B. usnearum |
| Binomial name | |
| Biatoropsis usnearum Räsänen (1934) | |
Biatoropsis usnearum is a species of parasitic fungus that grows exclusively on lichen species of the genus Usnea, particularly U. subfloridana, U. barbata, and U. florida. First described in 1934 by Veli Räsänen, it has become a significant model organism in fungal evolution studies due to its specialised host relationships. The fungus belongs to the order Tremellales, though its precise family classification remains uncertain.[1][2] It forms distinctive swellings or galls on its host lichens, ranging in colour from pale pink to dark reddish-brown, and notably suppresses the production of host defensive compounds like usnic acid. While initially misclassified due to its unusual characteristics, modern microscopic and genetic studies have revealed it to be part of a species complex, 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.
Historical classification
The taxonomic history of Biatoropsis usnearum spans over two centuries of scientific observation. Johann Jacob Dillenius first documented what would later be recognised as B. usnearum in his 1742 work "Historia Muscorum" describing small fleshy nodules closely appressed to Usnea branches.[3] Erik Acharius, known as the "Father of Lichenology", made several observations of these structures between 1795 and 1810,[4][5] referring to them variously as froredningsdelar (seed-producing parts) and later distinguishing between normal disc-shaped apothecia (which he termed "orbilla") and what we now know to be basidiomata (which he termed "cephalodia").[6]
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 proposed "patellulae" in 1850.[6]
Modern classification
Räsänen formally described Biatoropsis usnearum in 1934, initially classifying it as an ascomycete based on what he interpreted as asci containing hyaline, simple spores measuring 10–16 by 4–6.5 μm.[7] In 1939, he further specified that these supposed asci were 8-spored.[8] However, these structures were later recognised to be young probasidia.[6]
In 1949,[9] Rolf Santesson was the first to challenge its classification as an ascomycete, though he incorrectly suggested the structures were galls caused by Abrothallus parmeliarum. The true nature of B. usnearum as a heterobasidiomycete was not confirmed until 1990, when detailed microscopic examination revealed a hymenium with auricularioid basidia and no ascomycetous structures.[6]
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.[10] Three additional species were formally described from this complex in 2016:[11]
- B. hafellneri – Distinguished by having two-celled basidia with cells that elongate laterally at maturity, and by growing on species of the Usnea fragilescens aggregate[11]
- B. minuta – Characterised by small (less than 1 mm) brown to black basidiomatal galls, growing on Usnea barbata and U. lapponica[11]
- B. protousneae – Confined to Protousnea dusenii[11]
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.[12]
The genus Biatoropsis was initially tentatively placed in the order Platygloeales based on its transversely septate basidia, absence of clamp connections, 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.[6]
Biatoropsis usnearum produces an anamorph that strongly resembles Hormomyces aurantiacus. The anamorph is characterised by its hyphomycetous growth form, producing long branching chains of hyaline, ellipsoid conidia measuring 3–5 by 2–3.5 μm.[6]
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.[6]
