1. Department of Life Sciences, Graduate School of Arts and Science, The University of Tokyo, Tokyo 153-8902, Japan
2. Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Tokyo 102-0076, Japan
3. Present address: Department of Bioinformatic Engineering, Graduate School of Information Science and Technology, Osaka University, 1-5 Yamadaoka, Suita, Osaka 565-0871, Japan.