The classification of this genus in the Lecanorales was apparently based solely on the character of lichenisation. Molecular data suggest that the genus is closely related to Siphula, and that both genera are paraphyletic with the baeomycetoid lichens of the Icmadophilaceae (Platt & Spatafora 2000, Stenroos & al. 2002). The various taxa distinguished on the basis of different taxonomic concepts, do not only fail to produce ascospores but also lack any vegetative or mitotic propagules typical of other lichens (but see e.g. Lord & al. 2013). They occur on soil in arctic-alpine habitats of both Hemispheres. By analysing molecular, morphological, and chemical variation worldwide, Onut-Brännström & al. (2017) revealed the existence of three mycobiont lineages. One lineage (lineage A) is confined to the tundra region of Siberia and the Aleutian Islands, a second (lineage B) is found in the high-alpine belt of the Alps and the Carpathians Mountains, and a third (lineage C) has a worldwide distribution covering both the aforementioned ecosystems; no support was found for the recognition of taxa in Thamnolia based on either morphological or chemical characters. The three lineages were raised to species rank by Onut-Brännström & al. (2018). Jørgensen (2019), which I follow here, preferred to treat them as subspecies, also selecting a neotype and a lectotype for two subspecies, which has some important consequences with respect to the nomenclature adopted by Onut-Brännström & al. (2018): the widespread subsp. vermicularis, the geographically more restricted subsp. taurica (Eastern Alps, Tatra and Carpathians Mtns.), and subsp. tundrae (Arctic region). Molecular sequence data suggest that the two traditionally recognized chemotypes (thamnolic vs. squamaatic and baeomycesis acids), do not form well-supported, monophyletic lineages; this may be due to rare or historic recombination, repeated chemotype evolution or incomplete lineage sorting (see also Nelsen & Gargas 2009).
This short key includes the two subspecies which may occur in Italy, also distinguishing between the two chemotypes of subsp. vermicularis. The identification of samples with thamnolic acid needs molecular data.

References

Jørgensen P.M. 2019. The troublesome lichen genus Thamnolia (lichenized Ascomycetes). Lichenologist, 51, 3: 221-226.
Lord J.M., Knight A., Bannister J.M., Ludwig L.R., Malcolm W.M., Orlovich D.A. 2013. Rediscovery of pycnidia in Thamnolia vermicularis: Implications for chemotype occurrence and distribution. Lichenologist, 45, 3: 397-411.
Nelsen M.P., Gargas A. 2009. Assessing clonality and chemotype monophyly in Thamnolia (Icmadophilaceae). Bryologist, 112, 1: 42-53.
Onut-Brännström I., Tibell L., Johannesson H. 2017. A worldwide phylogeography of the whiteworm lichens Thamnolia reveals three lineages with distinct habitats and evolutionary histories. Ecology and Evolution, 7, 10: 3602-3615.
Onut-Brännström I., Johannesson H., Tibell L. 2018. Thamnolia tundrae sp. nov., a cryptic species and putative glacial relict. Lichenologist, 50: 50-75.
Platt J.L., Spatafora J.W. 2000. Evolutionary relationships of nonsexual lichenized fungi: molecular phylogenetic hypotheses for the genera Siphula and Thamnolia from SSU and LSU rDNA. Mycologia, 92: 475-48.
Stenroos S., Myllys L., Thell A., Hyvönen J. 2002b. Phylogenetic hypotheses: Cladoniaceae, Stereocaulaceae, Baeomycetaceae, and Icmadophilaceae revisited. Mycol. Prog., 1: 267-282.

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