Author contributions: Y-H. Gao designed the project. D. Yan, X-Y. Liu and B-T. Dan carried out the data collection. Y-H. Gao, Y-N. Zhu and L. Huang analysed the data and prepared the figures. Y-H. Gao, D. Yan, Y-N. Zhu and L. Huang drafted the manuscript. All the authors have revised the manuscript critically, approved the version submitted for publication and have agreed to be accountable for all aspects of the work.