Walnuts (Juglans regia) have been cultivated and prized for their nutritional and economic value for thousands of years, but the evolutionary history of the species, including how it spread across Europe and Asia, has remained incompletely understood. A new genomic study analyzing walnut populations from across the species’ native and cultivated range sheds new light on the species’ origins, migration routes, and genetic diversity.

By sequencing and comparing the genomes of walnut trees collected from diverse geographic regions, the researchers reconstructed the species’ evolutionary history, identifying distinct genetic lineages that correspond to different centers of diversity. The analysis points to a complex history of population expansion, migration and, in some cases, human-mediated dispersal as walnut cultivation spread along ancient trade routes.

The team also identified genomic regions associated with important agronomic traits, including nut size, shell thickness and disease resistance, that have been shaped by both natural selection and human breeding over the centuries. These findings provide a genetic roadmap for breeders seeking to develop improved walnut varieties suited to changing climates and pest pressures.

Understanding the genetic diversity and population structure of walnut is particularly important for conservation efforts, as wild walnut populations in some regions face pressures from habitat loss and climate change. The researchers say that preserving the full range of genetic diversity captured in this study will be essential for safeguarding the species’ long-term resilience and for ensuring that valuable traits remain available for future breeding programs.

The study adds to a growing body of genomic research on tree crops, illustrating how population-level sequencing can illuminate both the deep evolutionary past of a species and practical pathways for its future improvement.

Journal: Horticulture Research