1. Institute for Advanced Biosciences, Keio University, Mizukami 246-2, Kakuganji, Tsuruoka, Yamagata 997-0052, JAPAN; Graduate School of Agricultural Science, Kobe University, Rokkodai cho 1-1, Nada, Kobe, Hyogo 657-8501, JAPAN; E-mail: keizaf@gmail.co