The effect of hybridization on transposable element accumulation in an undomesticated fungal species

Transposable elements (TEs) are mobile genetic elements that can profoundly impact the evolution of genomes and species. A long-standing hypothesis suggests that the merging of diverged genomes within hybrids could alter the regulation of TEs and increase transposition. Higher transposition rates could potentially fuel hybrid evolution with rare adaptive TE insertions, but also cause postzygotic reproductive isolation if maladaptive insertion loads render hybrids sterile or inviable. Mixed evidence for higher TE activity in hybrids was reported in many animal and plant species.

Here, we tested for increased TE accumulation in hybrids between incipient species of the undomesticated yeast Saccharomyces paradoxus. Population genomic data revealed no increase in TE content in the natural hybrid lineages. As we could not rule out the elimination of past transposition increase signatures by natural selection, we performed a laboratory evolution experiment on a panel of artificial hybrids to measure TE accumulation in controlled conditions and in the near absence of selection.

Changes in TE copy numbers were highly dependent on the individual hybrid genotypes and were not predicted by the evolutionary divergence between the parents of a hybrid genotype. Rather, our data suggested that initial TE copy numbers in hybrids negatively impacted transposition rate, suggesting that TE self-regulation could play a predominant role on TE accumulation in yeast hybrids.