Selection maintains genetic coupling of functional trait trade-offs despite gene flow in an oak hybrid zone.

Resource allocation trade-offs in seedlings have been widely proposed to facilitate the coexistence of tree species; however, whether selection can maintain the underlying genetic variation under gene flow remains insufficiently investigated. Using a natural hybrid zone between two East Asian oaks, we evaluated whether loci associated with allocation trade-offs showed signatures of selection despite hybridisation. Acorns collected across the zone were cultivated in a common garden, where we quantified 25 seedling growth and functional traits, and analysed whole-genome variation. We identified trade-offs between leaf function and plant architecture. Trait-associated genomic regions demonstrated strong inter-chromosomal linkage disequilibrium in the hybrids, consistent with genetic coupling, and exhibited elevated interspecific differentiation and more pronounced genetic clines. These results indicate that multilocus selection counteracts gene flow through the development of genetic coupling, thereby maintaining resource allocation trade-offs that may contribute to tree coexistence.