Evidence of an additional wild contributor, Malus orientalis Uglitzk., to the genome of cultivated apple varieties in the Caucasus and Iran

Anthropogenic and natural divergence processes remain poorly studied in crop-wild fruit tree complexes, especially in the Caucasus, a pivotal region for plant domestication. We investigated anthropogenic and natural divergence processes in apples in the Caucasus using 26 microsatellite markers amplified in 550 wild and cultivated samples. We found two genetically distinct cultivated populations in Iran that are differentiated from Malus domestica, the standard cultivated apple worldwide. Coalescent-based inferences showed that these two cultivated populations originated from specific domestication events of M. orientalis in Iran. One of the Iranian clusters comprised both cultivated and forest trees, suggesting that either farmers use local wild apple for cultivation or that some forest trees are feral cultivars. We found evidence of substantial wild-crop and crop-crop gene flow in the Caucasus, as has been described in apple in Europe. In the Caucasus, we identified seven genetically differentiated populations of wild apple (Malus orientalis). Niche modeling combined with genetic diversity estimates indicated that these populations likely resulted from range changes during past glaciations. This study identifies Iran as a key region in the domestication of apple and M. orientalis as an additional contributor to the cultivated apple gene pool. Domestication of the apple tree therefore involved multiple origins of domestication in different geographic locations and substantial crop-wild hybridization, as found in other fruit trees. This study also highlights the impact of climate change on the natural divergence of a wild fruit tree and provides a starting point for apple conservation and breeding programs in the Caucasus.