Parental divergence as a predictor of hybridization outcomes

Noora Poikela , Kalle Tunström, Rike Stelkens, Anna Runemark, Jonna Kulmuni

Hybridization can generate both fitness benefits and genetic incompatibilities, yet predicting how these opposing effects shape hybrid fitness and genome evolution remains a major challenge in evolutionary genomics. Parental genetic divergence is expected to play a central role, as increasing divergence can promote heterosis and adaptive novelty while simultaneously increasing incompatibilities and barriers to gene flow. Here, we review the theoretical and empirical evidence linking parental divergence to homoploid hybridization outcomes and use meta-analyses to identify general patterns across taxa. We find that heterosis tends to decline with increasing parental divergence, consistent with accumulation of incompatibilities with divergence. Furthermore, we show that greater parental divergence is associated with lower minor parent ancestry in hybrid genomes, potentially explained by stronger constraints on hybrid genome formation set by accumulating incompatibilities. We also find a significant association between mitochondrial and major parent ancestry, consistent with mitonuclear incompatibilities constraining hybrid genome evolution. In contrast, our results concerning minor parent ancestry on sex chromosomes are inconclusive. Together, these findings suggest that parental divergence is an important predictor of both hybrid fitness and the retention of introgressed ancestry, while highlighting substantial variation in hybridization outcomes across taxa. Our analyses also reveal major limitations arising from inconsistent reporting and gaps in available data, underscoring the need for more standardized approaches to understanding and predicting the evolutionary consequences of hybridization.