Mobile introns shape the genetic diversity of their host genes

Self-splicing introns populate several highly conserved protein-coding genes in fungal and plant mitochondria. In fungi, many of these introns have retained their ability to spread to intron-free target sites, often assisted by intron-encoded endonucleases that initiate the homing process. Here, leveraging population genomic data from Saccharomyces cerevisiae, Schizosaccharomyces pombe, and Lachancea kluyveri, we expose non-random patterns of genetic diversity in exons that border self-splicing introns. In particular, we show that, in all three species, the density of single nucleotide polymorphisms increases as one approaches a mobile intron. Through multiple lines of evidence we rule out relaxed purifying selection as the cause of uneven nucleotide diversity. Instead, our findings implicate intron mobility as a direct driver of host gene diversity. We discuss two mechanistic scenarios that are consistent with the data: either endonuclease activity and subsequent error-prone repair have left a mutational footprint on the insertion environment of mobile introns or non-random patterns of genetic diversity are caused by exonic co-conversion, which occurs when introns spread to empty target sites via homologous recombination. Importantly, however, we show that exonic co-conversion can only explain diversity gradients near intron-exon boundaries if the conversion templates comes from outside the population. In other words, there must be pervasive and ongoing horizontal gene transfer of self-splicing introns into extant fungal populations.