In the lichen symbiosis, the fungal and algal partners constitute a closely integrated system. The combination of fungal and algal partners changes along climate gradients in many species, and is expected to be adaptive. However, the functional mechanisms behind this symbiosis-mediated environmental adaptation are unknown. We investigate which transcriptional profiles are associated with specific fungal-algal symbiont pairings found in lichens from high-elevation (Lower Supratemperate) and low-elevation (Lower Mesomediterranean) sites at two extremes of a climatic gradient on Mt. Limbara, Sardinia. Using laboratory-acclimatized thalli, we find that lichen fungal and algal symbionts show variable expression profiles between high- and low-elevation individuals: circadian- and temperature-associated genes for fungi and light-responsive genes for algae show climate-specific patterns. High- and low-elevation individuals differentially express sugar transporters in both symbionts, pointing to symmetrical and climate-dependent sugar transport mechanisms between them. A light pulse treatment identified asymmetries between fungal and algal light response, with high- and low-elevation fungal symbionts but only low-elevation algal symbionts showing a response. Together, these results tie previously-observed genomic variation along climatic gradients in a lichen species to functional differences in transcription for the fungal and algal symbionts, contributing to our understanding of environmental specialization and niche-specific partner combinations in lichens.

Lichen thalli were sampled in the field (Sardinia, Italy) and dried before transport to Frankfurt, Germany. Dried individuals were split into two pieces (LL and DD) and then rehydrated within 3 days of field collection and allowed to acclimatize over 3 days in a plant growth chamber (12h light/12h dark cycles, 60 μmol photons m^-2 s^-1 at 16° C).

After 3 days, samples were exposed to 24 h of total darkness before LL group was additionally exposed to 20 minutes of light (60 μmol photons m^-2 s^-1). All samples were then harvested into liquid nitrogen.

After tissue harvesting from lichen thalli (150 mg), RNA was extracted with TRI Reagent (Zymo Research Europe GmbH, Freiburg, DE) according to the manufacturer’s instructions. The extracted RNA was then sent to Novogene (Cambridge, UK) for library construction, quality control and 150 bp paired-end sequencing with NovaSeq. All sequence information can be found on the European Nucleotide Archive (ENA) under project accession number PRJEB72275.

Data from: Fungal and algal lichen symbionts show different transcriptional expression patterns in two climate zones

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