Repeat-induced point mutation and gene conversion coinciding with heterochromatin shape the genome of a plant pathogenic fungus

Abstract

Meiosis is associated with genetic changes in the genome - via recombination, gene conversion, and mutations. The occurrence of gene conversion and mutations during meiosis may further be influenced by the chromatin conformation, in analogy to what is known for mutations during mitosis. To date, however, the exact distribution and type of meiosis-associated changes and the role of the chromatin conformation in this context is largely unexplored. Here, we determine recombination, gene conversion, and de novo mutations using whole-genome sequencing of all meiotic products of 23 individual meioses in Zymoseptoria tritici, an important pathogen of wheat. We could confirm a high genome-wide recombination rate of 65 cM/Mb and see higher recombination rates on the accessory compared to core chromosomes. A substantial fraction of 0.16% of all polymorphic markers was affected by gene conversions, showing a weak GC-bias, and occurring at higher frequency in regions of constitutive heterochromatin, indicated by the histone modification H3K9me3. The de novo mutation rate associated with meiosis was approx. three orders of magnitude higher than the corresponding mitotic mutation rate. Importantly, repeat-induced point mutation (RIP), a fungal defense mechanism against duplicated sequences, is active in Z. tritici and responsible for the majority of these de novo meiotic mutations. Our results indicate that the genetic changes associated with meiosis are a major source of variability in the genome of an important plant pathogen and shape its evolutionary trajectory. ImportanceThe impact of meiosis on the genome composition via gene conversion and mutations is mostly poorly understood, in particular for non-model species. Here, we sequenced all four meiotic products for 23 individual meioses and determined the genetic changes caused by meiosis for the important fungal wheat pathogen Zymoseptoria tritici. We found a high rate of gene conversions and an effect of the chromatin conformation on gene conversion rates. Higher conversion rates were found in regions enriched with the H3K9me3 - a mark for constitutive heterochromatin. Most importantly, meiosis was associated with a much higher frequency of de novo mutations than mitosis. 78% of the meiotic mutations were caused by repeat-induced point mutations - a fungal defense mechanism against duplicated sequences. In conclusion, the genetic changes associated with meiosis are therefore a major factor shaping the genome of this fungal pathogen.