Acknowledgements
The present research was funded by National Key Technology R&D Program (grants 2008BADB0B04). The authors would like to thank Ming Zhou and Guo Cai for their help in sample collection. They also thank Prof. Zhaobin Song, Weicai Chen, Zhenxin Fan and Liwei He at Sichuan University for their valuable help and revisions of the manuscript.
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.