1. Institute of Agrobiological Sciences National Agriculture and Food Research Organization 1‐2 Owashi Tsukuba305‐8634Japan
2. Japan Science and Technology Agency (JST) Precursory Research for Embryonic Science and Technology (PRESTO) 4‐1‐8 Honcho Kawaguchi Saitama332‐0012Japan
3. Department of Biological Sciences Purdue University West Lafayette IN47907‐1392USA
4. Kihara Institute for Biological Research Yokohama City University 641‐12, Maioka‐cho Yokohama244‐0813Japan