The mutational spectrum in Marasmius oreades fairy rings

Markus Hiltunen Martin Ryberg Hanna Johannesson
Organismal Biology, Uppsala University, Uppsala, Sweden

The sources of new haploid genotypes in natural populations are mutation and recombination. In fungi, having no separated germline, variation introduced during vegetative growth is likely to be carried on to the next generation. Still little is known about the genetic mechanisms that produce variation in these organisms under natural conditions. Indications exist of very low rates of spontaneous substitution in some basidiomycetes, suggesting a highly efficient mechanism working to minimize the number of mutations during vegetative growth. In addition, somatic recombination has been suggested to occur in several basidiomycetes, where haplotypes are mixed outside of meiosis, something that could carry great implications to how basidiomycetes evolve in nature. A well-suited system to study genetic variation gained during vegetative growth is so-called fairy rings; fungal mycelia that grow in circular patterns. The initial genotype that gives rise to a fairy ring is the shared throughout the ring, while different sectors have had their own evolutionary trajectories since the initial split. Here we studied genetic variation within six individual fairy rings of Marasmius oreades. By sequencing whole genomes of 5-8 fruiting bodies from different sectors of each ring, the numbers and types of new mutations were determined. The results point to a strikingly low number of mutations with substitution rates in the order of magnitude of 10-12 per cell division and base pair. This suggests either a higher-than-expected selection pressure keeping mutations to a minimum, or that the mutation rate is indeed low in these organisms. Current work also includes investigating the occurrence of somatic recombination using protoplast isolation and long-read sequencing to generate full genome sequences of the two haploid nuclei in the dikaryon.