Extra chromosomes of 3 trisomic types, M, N and O, discovered recently in rice were clarified. Adding these 3 types to 9 types previously found, a complete set of 12 primary trisomics, having the homogeneous background of the cultivar Nipponbare, was established in japonica rice. The relationships among trisomics, chromosomes and linkage groups made clear up to the present time are shown.
