History and Dynamics of an Extensive Plant Hybrid Zone on the Great Plains of North America

Hybridisation affects the spatial and temporal patterns of morphological and genetic variation, shaping species evolution. Asclepias speciosa and A. syriaca provide an excellent system for uncovering how these patterns are generated. The ranges of A. speciosa and A. syriaca overlap on the North American Great Plains, and multiple intermediates are observed in this contact zone, consistent with hybridisation. However, other processes, such as preservation of ancestral polymorphism or selective pressure imposed by environmental clines, could explain the presence of morphological intermediates. In this study, we characterised patterns of variation within and between A. speciosa and A. syriaca using morphological and genetic data to validate the hybrid origin of intermediates, evaluate the impact of hybridisation on the parental species and reconstruct the demographic history of hybridisation events. In addition, we explored species-specific associations of genetic variation with climatic variables. We demonstrated that hybridisation best explains the occurrence of large numbers of morphologically intermediate individuals in the contact zone and documented bidirectional and asymmetric genetic introgression. We found a strong relationship between precipitation patterns and genetic variation in A. speciosa and A. syriaca, which suggests that the hybrid zone is maintained by differences in annual precipitation. We discovered that the timing of secondary contact is relatively recent, coinciding with rapid range shifts during the Pleistocene. Our findings provide new insights into the dynamics of hybridisation on the North American Great Plains, the setting for numerous contact zones between taxa affiliated with eastern and western biotas.