, We acknowledge Rasmus Nielsen for helpful discussions and comments on an early version of this work. This study makes use of data generated by the NextGen Consortium. The European Union's Seventh Framework Programme (FP7/2010-2014) provided funding for the project under grant agreement no 244356-NextGen, and 91731309) and the Taishan Scholars Program of Shandong Province (No. ts201511085)

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, Associate editor: Ya-Ping Zhang