Lichenicolous fungus
While the study of lichenicolous fungi dates back to the mid-18th century, recent decades have seen significant advancements through modern research methods, including molecular techniques, metagenomics, and sophisticated imaging.
[3] Lichenicolous fungi represent a highly diverse group, with over 2,000 known species across 280 genera, reflecting a wide range of ecological strategies and relationships with their lichen hosts.
[1] Lichen thalli provide a complex and varied habitat for lichenicolous fungi, characterised by biological gradients that range from actively growing to decaying parts.
The presence of these biological gradients within lichen thalli may act as a catalyst for the evolution and adaptation of lichenicolous fungi, potentially explaining the wide range of ecological relationships observed, from commensalism to parasitism.
[8] These can be categorised into three main groups:[8] Advancements in molecular techniques and increased exploration have significantly expanded the known diversity of lichenicolous basidiomycetes, nearly doubling the species count since 2018.
[3] The description of the new genus Crittendenia in the Pucciniomycotina represents a significant taxonomic development, as it redefines the classification of several host-specific parasitic species previously placed under Chionosphaera.
[5] Lichenicolous fungi exhibit diverse ecological relationships with their lichen hosts, ranging from harmful parasitism to neutral commensalism and potentially beneficial (mutualistic) interactions.
While many lichenicolous fungi are often considered parasitic or commensalistic, recent research suggests that some species may play important ecological roles within the lichen thallus.
These findings suggest that the ecological impact of lichenicolous fungi may be more nuanced and potentially beneficial than previously understood, contributing to the health and longevity of lichen communities in various environments.
For example, species such as Stigmidium arthrorhaphidis, Cercidospora trypetheliza, and C. soror have been observed infecting Arthrorhaphis citrinella, which itself grows parasitically on Baeomyces, Cladonia squamules, or decaying lichens.
For instance, the Koralpe mountain range in Austria, with its stable conditions and variety of microhabitats, supports a high diversity of both lichens and lichenicolous fungi.
The presence of exposed boulders and cliffs provides numerous microniches for these fungi to exploit, ranging from the outer cortex to the inner medulla of lichen thalli.
[16] In alpine and polar regions, the stability and longevity of lichen thalli provide consistent microhabitats, allowing lichenicolous fungi to establish long-term populations.
Research in the Koralpe Mountain area revealed high beta diversity due to numerous microenvironmental conditions supporting various fungal species.
A striking example is Xanthoriicola physciae on Xanthoria parietina, which can give the host a soot-spattered appearance and potentially reduce its photosynthetic area to the point of local death.
Some homobasidiomycetes, such as Athelia arachnoidea, show seasonal peaks in their development and can survive as small sclerotia or bulbils on bark or mosses after killing their lichen hosts, appearing to have a facultatively lichenicolous lifestyle.
For instance, Chaenothecopsis consociata (Mycocaliciales) typically invades thalli of Chaenotheca chrysocephala (Caliciaceae, Lecanorales), but can also associate with Dictyochloropsis symbiontica to form its own crustose thallus.
Similarly, Athelia arachnoidea is necrotrophic on various lichen taxa, free-living algae, and bryophytes, but has also been identified as the sexual state of Rhizoctonia carotae, a postharvest disease of carrots.
Africa and much of Asia remain poorly explored for lichenicolous basidiomycetes, and the current low numbers of known species likely underrepresent the true diversity in these regions.
A study conducted in this region identified 63 lichen and 41 lichenicolous fungal species within a relatively small area, illustrating the rich biodiversity of these communities.
In regions such as Hungary, India, and parts of the Holarctic, including North America, Russia, and Sweden, national checklists have documented numerous species of lichenicolous fungi.
This method has demonstrated that lichen communities can serve as reliable surrogates for predicting lichenicolous fungal diversity, aiding in the conservation and study of these specialised fungi.
[24] In 1810, Acharius published detailed colour illustrations distinguishing between normal disc-shaped apothecia and the nodule-like, bulging structures he termed cephalodia in Usnea.
[27] Several researchers made significant contributions during this period, preparing detailed illustrated critical accounts of both the taxonomy and biology of selected species.
Henri Olivier provided a detailed account of lichenicolous fungi from France (1905–1907), while Léon Vouaux published a worldwide flora with keys and descriptions of all known species (1912–1914).
[3][4] Henri Olivier gave a detailed account of lichenicolous fungi from France (1905–1907), while Léon Vouaux published a worldwide flora with keys and descriptions of all known species (1912–1914).
[24] In the early 20th century, Werner and his co-workers carried out developmental and biological investigations on a few species, contributing to the growing body of knowledge about lichenicolous fungi.
[37] Conidiomata in lichens can take various forms, including roughly spherical ("globose") or flask-shaped ("pycnidia"), cupuliform ("acervular"), cushion-like ("sporodochia"), hooded or peltate ("campylidia"), or erect ("synnemata", "hyphophores").
However, distinguishing foreign hyphae within lichen thalli from the mycobiont proper remains a significant challenge, highlighting the need for integrating both traditional and modern research approaches.
[16] This approach uses statistical techniques like co-correspondence analysis to create predictive models, which have shown that certain lichen species, especially those that are abundant and widely distributed, can reliably indicate the diversity of lichenicolous fungi.
