Text by Kristiina Mark
Lichens are symbiotic organisms formed in a mutualistic symbiosis between a terrestial green algae or cyanobacteria and (usually ascomycetous) fungus. Traditionally lichens are named after their fungal partner and thus their classification is based on the fungal counterpart. Likewise to plants, fungi also produce variety of natural products of ecological importance, also called the secondary metabolites. These substances are often used in delimiting taxa when morphological characters are scarce, however, the use of chemical characters in taxonomy is under ongoing debate.
Research investigating the utility of secondary metabolites in molecular taxonomy of lichen genus Cetrelia (Parmeliacea, Ascomycota) revealed clear correlation between lichen chemistry and phylogeny, suggesting to include information from secondary metabolites when identifying taxa in the genus Cetrelia. These organisms produce a constant set of polyphenolic compounds, specifically orcinol-type depsides and depsidones, with still unknown function. Character state analyses affirmed metabolite evolution in Cetrelia towards more complex substances, which could indicate evolutionary importance of the chemical compounds in species survival or functioning.
Citation: Mark, K., Randlane, T., Thor, G., Hur, J.-S., Obermayer, W. & Saag, A. (2019). Lichen chemistry is concordant with multilocus gene genealogy in the genus Cetrelia (Parmeliaceae, Ascomycota). Fungal Biology 123(2): 125-139, https://doi.org/10.1016/j.funbio.2018.11.013. (link to full text)
Fig. 2 from the paper. The three morphotypes in Cetrelia: sorediate (A, B), isidiate and/or lobulate (C, D), and without vegetative propagules (E, F). Scale bars 2.8 mm (A), 1.3 mm (B, C), 6.4 mm (D), 5.4 mm (E), and 6.3 mm (F). Photographed specimens: (A) Cetrelia olivetorum DNA-CKM59, Randlane and Saag, Estonia (TU); (B) C. chicitae DNA-25878, Thor, Japan (UPS, TU); (C) C. braunsiana, Kärnefelt, Russia (LD-1040527); (D) C. orientalis DNA-AT406, Skirina, Russia (LD-1062494); (E) C. alaskana DNA-CKM66, Obermayer, China (GZU); (F) C. sanguinea, Indonesia, Sumatra (TU).
The lichen genus Cetrelia represents a taxonomically interesting case where morphologically almost uniform populations differ considerably from each other chemically. Similar variation is not uncommon among lichenized fungi, but it is disputable whether such populations should be considered entities at the species level. Species boundaries in Cetrelia are traditionally delimited either as solely based on morphology or as combinations of morpho- and chemotypes. A dataset of four nuclear markers (ITS, IGS, Mcm7, RPB1) from 62 specimens, representing ten Cetrelia species, was analysed within Bayesian and maximum likelihood frameworks. Analyses recovered a well-resolved phylogeny where the traditional species generally were monophyletic, with the exception of Cetrelia chicitae and Cetrelia pseudolivetorum. Species delimitation analyses supported the distinction of 15 groups within the studied Cetrelia taxa, dividing three traditionally identified species into some species candidates. Chemotypes, distinguished according to the major medullary substance, clearly correlated with clades recovered within Cetrelia, while samples with the same reproductive mode were dispersed throughout the phylogenetic tree. Consequently, delimiting Cetrelia species based only on reproductive morphology is not supported phylogenetically. Character analyses suggest that chemical characters have been more consistent compared to reproductive mode and indicate that metabolite evolution in Cetrelia towards more complex substances is probable.