From divergence to contact: demographic history and genomic context shape introgression across independent damselfly hybrid zones

Abstract Hybridization outcomes often vary across space and time, yet the relative roles of demographic history and chromosomal differences in shaping introgression remain unclear. Here, we investigate three replicated hybrid zones between the damselflies Ischnura elegans and Ischnura graellsii across Spain to test whether genomic introgression patterns are repeatable across independently formed zones. Using genome-wide data, we combined demographic modeling, genomic cline, and functional annotation of introgressed loci. Demographic inference supported three independent secondary contact events of different ages: The South-east zone forming first (~207 years ago), followed by the North-west (~73.5 years) and North-central (~33 years) hybrid zones. Despite similar cline steepness across autosomes, asymmetric gene flow from I. graellsii into I. elegans was observed, with a low overlap of introgressed loci between zones. These loci were mainly associated with broad physiological and regulatory functions. In contrast, the X chromosome showed steeper clines, suggesting strong intrinsic genomic constraints. Together, demographic history explains geographic heterogeneity in introgression, whereas chromosome architecture imposes consistent constraints. These findings highlight how replicated hybrid zones can disentangle contingent versus repeatable genomic responses during early stages of speciation with gene flow.