Abstract
We define a molecular connection between ribosome biogenesis and desiccation tolerance in lichens, widespread symbioses between specialized fungi (mycobionts) and unicellular phototrophs. Our experiments test whether the introns present in the nuclear ribosomal DNA of lichen mycobionts contribute to their anhydrobiosis. Self-splicing introns are found in the rDNA of several eukaryotic microorganisms, but most introns populating lichen rDNA are unable to self-splice, being either degenerate group I introns lacking the sequences needed for catalysis, or spliceosomal introns ectopically present in rDNA. Using CRISPR, we introduced a spliceosomal intron from the rDNA of the lichen fungus Cladonia grayi into all nuclear rDNA copies of the yeast Saccharomyces cerevisiae, which lacks rDNA introns. Three intron-bearing mutants were constructed with the intron inserted either in the 18S rRNA genes, the 25S rRNA genes, or in both. The mutants removed the introns correctly but had half the rDNA genes of the wildtype strain, grew 4.4 to 6 times slower, and were 40 to 1700 times more desiccation tolerant depending on intron position and number. Intracellular trehalose, a disaccharide implicated in desiccation tolerance, was detected but not at levels compatible with the observed resistance. Extrapolating from yeast to lichen mycobionts we propose that the unique requirement for a splicing machinery by lichen rDNA introns slows down intron splicing and ribosomal assembly. This effect, and the distinctive roles played by group I vs. spliceosomal rDNA introns, lead the environmental stress responses of lichen fungi to generate the twin lichen phenotypes of slow growth and desiccation tolerance.
Competing Interest Statement
The authors have declared no competing interest.
