Population-level transposable element expression dynamics influence trait evolution in a fungal crop pathogen

Rapid adaptive evolution is driven by strong selection pressure acting on standing genetic variation within populations. How adaptive genetic variation is generated within species and how such variation influences phenotypic trait expression is often not well understood though. Here, we focused on recent activity of transposable elements (TEs) using deep population genomics and transcriptomics analyses of a fungal plant pathogen with a highly active content of TEs in the genome. Zymoseptoria tritici causes one of the most damaging diseases on wheat, with recent adaptation to the host and environment being facilitated by TE-associated mutations. We obtained genomic and RNA-sequencing data from 146 isolates collected from a single wheat field. We established a genome-wide map of TE insertion polymorphisms in the population by analyzing recent TE insertions among individuals. We quantified the locus-specific transcription of individual TEs within the population and revealed considerable variation in transcription across individual TE loci. About 20% of all TE copies show activity in the genome implying that genomic defenses such as repressive epigenetic marks and repeat-induced polymorphisms are ineffective at preventing the proliferation of TEs in the genome. A quarter of recent TE insertions are associated with expression variation of neighboring genes providing broad potential to influence trait expression. We indeed found that TE insertions are likely responsible for variation in virulence on the host and secondary metabolite production. Our study emphasizes how TE-derived polymorphisms segregate even in individual populations and broadly underpin transcription and adaptive trait variation in a species.