Recent loss of the Dim2 DNA methyltransferase decreases mutation rate in repeats and changes evolutionary trajectory in a fungal pathogen

DNA methylation is found throughout all domains of life, yet the extent and function differ among eukaryotes. Strains plant pathogenic fungus Zymoseptoria tritici appeared to lack cytosine (5mC) because gene amplification followed by Repeat-Induced Point mutation (RIP) resulted in inactivation dim2 methyltransferase gene. 5mC is, however, present closely related sister species. We demonstrate that occurred recently as some Z . isolates carry a functional Moreover, we show different path than previously hypothesized. mapped genome-wide distribution strains with or without alleles. Presence correlates high levels transposable elements (TEs), suggesting role genome defense. identified low carrying non-functional alleles, maintained over time, presumably an active Dnmt5 methyltransferase. Integration allele mutated restored normal levels, demonstrating de novo activity Dim2. To assess importance for evolution, performed evolution experiment, comparing genomes lacking presence alters nucleotide composition promoting C T transitions (C→T) specifically at CpA (CA) sites during mitosis, likely contributing TE inactivation. Our results density TEs polymorphic trait populations can impact evolution.