1. Graduate School of Agriculture, Iwate University, Iwate 020-8550, Japan
2. Department of Bioscience, Tokyo University of Agriculture, Tokyo 156-8502, Japan
3. Laboratory of Electron Microscope, Division of Technical Support, Iwate University, Iwate 020-8550, Japan
4. Institute of Livestock and Grassland Science, NARO, Ibaraki 305-0901, Japan