The genus Cetrelia was traditionally regarded as belonging to the Cetrarioid group of Parmeliaceae, based on the presence of marginal apothecia, but is now considered to belong to the Parmelioid clade based on inferences from molecular data (see e.g. Crespo & al. 2010). The taxonomy at species level is quite peculiar, because of the treatment of chemical characters. Combining morphotypes and chemotypes, 18 species are distinguished (Mark & al. 2016). The morphotypes are distinguished first of all by the presence/absence of soredia, isidia and lobuli. The main chemical species, which were not accepted by several authors, did not form a monophyletic group in the ITS-analysis carried out by Thell & al. (2002), which supports the use of chemotaxonomy within the genus. A very useful attempt for a morphological characterisation of the four species occurring in Central Europe was published by Obermayer & Mayrhofer (2007). These authors, from the analysis of several hundred samples in southeastern Europe, and especially in the eastern Alps, found the following percent occurrencies: 2% C. chicitae, 13% C. olivetorum, 30% C. cetrarioides, and 55 % C. monachorum. Most of the old Italian records are unreliable, as the four species were not distinguished in the past.

References

Crespo A., Kauff F., Divakar P.K., del Prado R., Pérez-Ortega S., Amo de Paz G., Ferencova Z., Blanco O., Roca-Valiente B., Núñez-Zapata J., Cubas P., Argüello A., Elix J.A., Esslinger T.L., Hawksworth D.L., Millanes A., Molina M.C., Wedin M., Ahti T., Aptroot 2010. Phylogenetic generic classification of parmelioid lichens (Parmeliaceae, Ascomycota) based on molecular, morphological and chemical evidence. Taxon, 59, 6: 1735-1753.
Mark K., Randlane T., Saag A. 2016b. Phylogeny and species delimitation in the lichen genus Cetrelia. IAL 8, Book of Abstracts, Helsinki: 45.
Obermayer W., Mayrhofer H. 2007. Hunting for Cetrelia chicitae (Lichenized Ascomycetes) in the Eastern European Alps (Including an attempt for a morphological characterization of all taxa of the genus Cetrelia in Central Europe). Phyton, 47, 1-2: 231-290.
Thell A., Stenroos S., Feuerer T., Kärnefelt I., Myllis L., Hyvönen J. 2002. Phylogeny of cetrarioid lichens (Parmeliaceae) inferred from ITS and β-tubulin sequences, morphology, anatomy and secondary chemistry. Mycol. Prog., 1, 4: 335-354.

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