A complex interplay of genetic introgression and local adaptation during the evolutionary history of three closely related spruce species.

As climate change triggers unprecedented ecological shifts, it becomes imperative to understand the genetic underpinnings of species' adaptability. Adaptive introgression significantly contributes to organismal adaptation to new environments by introducing genetic variation across species boundaries. However, despite growing recognition of its importance, the extent to which adaptive introgression has shaped the evolutionary history of closely related species remains poorly understood. Here we employed population genetic analyses of high-throughput sequencing data to investigate the interplay between genetic introgression and local adaptation in three species of spruce trees in the genus (, , and ). We find distinct genetic differentiation among these species, despite a substantial gene flow. Crucially, we find bidirectional adaptive introgression between allopatrically distributed species pairs and unearthed dozens of genes linked to stress resilience and flowering time. These candidate genes most likely have promoted adaptability of these spruces to historical environmental changes and may enhance their survival and resilience to future climate changes. Our findings highlight that adaptive introgression could be prevalent and bidirectional in a topographically complex area, and this could have contributed to rich genetic variation and diverse habitat usage by tree species.