Symbiont-specific responses to environmental cues in a threesome lichen
symbiosis
Abstract
Photosymbiodemes are a special case of lichen symbiosis where one
lichenized fungus engages in symbiosis with two different photosynthetic
partners, a cyanobacterium and a green alga, to develop two distinctly
looking photomorphs. We investigated differential gene expression in
photosymbiodemes of the lichen Peltigera britannica at different
temperatures representing mild and putatively stressful conditions and
compared gene expression of thallus sectors containing cyanobacterial
photobionts with thallus sectors with both green algal and
cyanobacterial photobionts. Firstly, because of known ecological
differences between photomorphs, we investigated symbiont-specific
responses in gene expression to temperature increases. Secondly, we
quantified photobiont-mediated differences in fungal gene expression.
High temperatures expectedly led to an upregulation of genes involved in
heat shock responses in all organisms in whole transcriptome data. As
expected, the expression of genes involved in photosynthesis was
increased in both photobiont types at 15 and 25 °C. The green algae
exhibited thermal stress responses mainly at 25 °C, the fungus and the
cyanobacteria already at 15 °C, demonstrating symbiont-specific
responses to environmental cues and symbiont-specific ecological optima.
Furthermore, photobiont-mediated differences in fungal gene expression
could be identified, with upregulation of distinct biological processes
in the different morphs, showing that interaction with specific
symbiosis partners profoundly impacts fungal gene expression.