1. Department of Biology, Faculty of Science, Shinshu University, Matsumoto 390-8621, Japan
2. Laboratory of Genetics, Wageningen Agricultural University, Dreijenlaan 2, 6703 HA Wageningen, The Netherlands
3. Department of Molecular Biology, DLO-Centre for Plant Breeding and Reproduction Research (CPRO-DLO), Postbus 16, 6700 AA Wageningen, The Netherlands