Prized worldwide for its high-quality wood and rich flavor of delicious nuts, the Persian walnut (Juglans regia) is an important economic crop. The Persian walnut is one of 22 species in the genus Juglans, which includes black and white walnuts and butternuts, grown across Europe, the Americas, and Asia. China leads world production, followed by California, Turkey, and Iran. But until now, the evolutionary history of walnuts has been unknown. Walnuts have a rich fossil record, which suggest an origin of walnuts and initial divergence into black walnuts and butternuts (white walnuts) in North America some 35-45 million years ago. With this high age, both walnut lineages would have had ample opportunity to migrate into the Old World via the Bering and North Atlantic land bridges, yet only butternuts have been detected in the fossil records of Europe and Asia, and no ancient fossils of the Persian walnut are known. Using genomic data analyzed with phylogenetic and population genetic approaches, researchers have now cracked this mystery, showing that the Persian walnut is the result of hybridization between two long-extinct species approximately 3.45 million years ago. Past analyses by the team based on some 2900 single-copy nuclear genes from 19 species of walnuts were unable to sort out the relationships among North American, Asian, and Persian walnut species. However, the researchers excluded incomplete lineage sorting as the cause of the phylogenetic uncertainty. "This led us to speculate that ancient hybridization might be involved in the origin of the Persian walnut and the American butternut," said Da-Yong Zhang, PhD, Peking University, a population geneticist who is one of the senior authors.
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