Crop-to-wild gene flow and spatial genetic structure in the closest wild relatives of the cultivated apple

Crop-to-wild gene flow have important evolutionary and ecological consequences and require careful consideration in conservation programs for wild genetic resources of potential use in breeding programs and in assessments of the risk of transgene escape into natural ecosystems. Using 26 microsatellites and a set of 1181 trees, we investigated the extent of introgression from the cultivated apple, Malus domestica, to its three closest wild relatives, M.sylvestris in Europe, M.orientalis in the Caucasus, and M.sieversii in Central Asia. We found footprints of introgression from M.domestica to M.orientalis (3.2% of hybrids), M.sieversii (14.8%), and M.sylvestris (36.7%). Malus sieversii and M.orientalis presented weak, but significant genetic structures across their geographic range. Malus orientalis displayed genetic differentiation with three differentiated populations in Turkey, Armenia, and Russia. Malus sieversii consisted of a main population spread over Central Asia and a smaller population in the Tian Shan Mountains. The low Sp values suggest high dispersal capacities for the wild apple relatives. High potential for crop-to-wild gene flow in apples needs to be considered in the implementation of in situ and ex situ actions for the conservation of wild apple genetic resources potentially useful to plant breeding.