This work was supported by Grants from the State Key Program of National Natural Science Foundation of China (Grant No. 81030043), the Youth Science Fund of the National Natural Science Foundation of China (Grant No. 81000946), the National Key Sci-Tech Special Project of China (Grant No. 2008ZX10002-019), the Outstanding Youth Project of Educational Department of Hunan Province, China (Grant No. 10B090), and the Van Andel Foundation. We thank David Nadziejka, Grand Rapids, Michigan for critical reading of the manuscript.