Figure legends
Figure 1. Photosymbiodemes of the lichen Peltigera britannica.
(A) green-algal tripartite lobes (apple green) emerge from a bipartite
thallus (bluish-gray) that has a cyanobacterium as a photosynthetic
partner; (B) small cyanobacterial lobes develop from the cephalodia of a
tripartite thallus.
Figure 2. Experimental setup to study differential gene
expression in Peltigera britannica photomorphs. Dark green parts
represent the cyanomorph, light green parts the tripartite morph. Lichen
thalli were exposed to 4 °C for five days (treatment “4 °C_1”), then
to 4 °C for two weeks (treatment “4 °C_2”), then to 15 °C for two
hours (treatment “15 °C”), and finally to 25 °C for two hours
(treatment “25 °C”). After each step, pieces of thallus (ca. 1 cm2) of
the cyano- and the tripartite morph were cut off from all four specimens
for RNA extraction.
Figure 3. PCA plot for the overall gene expression variance of
the mycobiont. The principal component of variance in the mycobiont was
temperature-mediated rather than photomorph-mediated (circle =
tripartite morph; triangle = cyanomorph), with clusters for low (4
°C_1; green and 4 °C_2; blue), medium (15 °C; orange) and high
temperatures (25 °C; red).
Figure 4. (A) 200 most significantly differentially expressed
fungal genes of the cyanomorph and the tripartite morph. Lighter shades
show DEGs that could not be functionally annotated. 123 DEGs were
assigned to the cyanomorph (teal color) and 77 to the tripartite morph
(green color). (B) Temperature-mediated differential gene expression of
mycobiont, chlorophytes and cyanobacteria (comparing 25 °C with 4
°C_1). Bars include the 200 most significantly differentially expressed
genes of each organism. Red bars show genes upregulated at 25°C, blue
bars genes downregulated at 25°C. Lighter shades show genes that could
not be functionally annotated. 103 of the 200 most significantly
differentially expressed fungal genes were downregulated at 25 °C, 97
were upregulated; in the photobionts, 199 genes were upregulated and one
was downregulated at 25 °C, respectively.
Figure 5. Visualization of the main biological processes (BP)
attributed to differentially expressed ascomycete genes of the
cyanomorph and the tripartite morph of Peltigera britannica. Different
BPs are shown with distinct colors; section sizes correspond to the
number of genes associated with a GOterm. In the cyanomorph, the main
BPs were oxidation-reduction process (62%), protein carbohydrate
metabolic process (18%) and protein phosphorylation (18%); a minor BP
not shown in the graph is GPI anchor biosynthetic process. In the
tripartite morph, the majority of fungal DEGs was annotated to
oxidation-reduction process (60%), followed by transmembrane transport
(32%), tricarboxylic acid cycle (1.55%) and phospholipid biosynthesis
(1.49%). Minor BPs of the tripartite morph not depicted are protein
peptidyl-prolyl isomerization, threonyl-tRNA aminoacylation, inositol
biosynthetic process and cellular potassium ion homeostasis. Analysis
based on GO annotation (Bioconductor package topGO).
Figure 6. Expression of stress proteins in the three lichen
symbiosis partners based on the counts of the corresponding genes at
each temperature. In the mycobiont (orange), upregulation of stress
proteins begins at 15 °C already and it shows high stress at 25 °C; the
same is true for the cyanobacteria (teal), although a higher number of
stress related genes are expressed at lower temperatures (4 °C_1 and 4
°C_2) than in the mycobiont. Chlorophytes (green) appear to be stressed
mainly at 25 °C.