Sophie  S Steinhäuser, Ólafur S Andrésson, Arnar Pálsson, Silke Werth Fungal and cyanobacterial gene expression in a lichen symbiosis: Acclimatization and adaptation to temperature and habitat

Accepted in Fungal Biology.

The capacity of species to cope with variation in the physical environment, e.g. in temperatures and temperature fluctuations, can limit their spatial distribution. Organisms have evolved cellular mechanisms to deal with damaging effects of increased temperature and other aggravation, primarily through complex molecular mechanisms including protein refolding and DNA repair. It is of interest to see whether these responses vary with geographic location, with high vs. low elevation and on the coast vs. inland, indicating longterm acclimatization or genetic adaptation. As mutualistic symbioses, lichens offer the possibility of analyzing molecular stress responses in a particularly tight interspecific relationship. For this study, we have chosen the widespread cyanolichen Peltigera membranacea, a key player in carbon and nitrogen cycling in terrestrial ecosystems at northern latitudes. We ask whether increasing temperature is reflected in mRNA levels of selected damage control genes, and do the response patterns show geographical associations? Using real-time PCR quantification of 38 transcripts, differential expression was demonstrated for nine cyanobacterial and nine fungal stress response genes (plus the fungal symbiosis-related lec2 gene) when the temperature was increased from 5°C to15°C and 25°C. Principle component analysis (PCA) revealed two gene groups with different response patterns. Whereas a set of cyanobacterial DNA repair genes and the fungal lec2 (PC1 group) showed an expression drop at 15°C vs. 5°C, most fungal candidates (PC2 group) showed increased expression at 25°C vs. 5°C. PC1 responses also correlated with elevation. The correlated downregulation of lec2 and cyanobacterial DNA repair genes suggests a possible interplay between the symbionts warranting further studies.