Conclusions
This is the first study reporting differential gene expression of lichen photomorphs based on compound thalli. Our differential gene expression analyses illustrate the effects of temperature stress on all partners involved in the lichen symbiosis and show a distinct photomorph-mediated fungal gene expression pattern in P. britannica influencing mycobiont-photobiont contact and management of photobiont growth. This suggests an interplay and plasticity of fungal, cyanobacterial and algal gene expression in lichen symbioses that was previously undocumented. Moreover, our results show that the symbiosis partners possess different temperature optima. Each organism in the lichen symbiosis, be it the fungus, the alga, or the cyanobacterium, reacted to thermal stress – and each organism did so in a distinct manner. The fungi and cyanobacteria were heat stressed at 15 °C already, whereas the green algae were heat stressed only at 25 °C. Our study offers novel insights into how symbiotic partners in P. britannica photomorphs manage their interactions and responses to environmental factors such as temperature through differential gene expression. We hope that our results contribute to a better understanding of how different photosynthetic partners can influence the ecology of lichens.