Diversity of abnormal embryo sacs in indica/japonica hybrids in rice demonstrated by confocal microscopy of ovaries

Embryo sac abortion occurs frequently in the spikelet of indica/japonica hybrids in rice. To provide a comprehensive understanding of embryo sac sterility in typical indica/japonica hybrids, a simple eosin B staining procedure for use with confocal microscopy was developed to observe the mature embryo sacs of 35 indica/japonica F(1)s making up an incomplete diallel cross design ( 5 x 7). A diversity of abnormal embryo sacs existed in the hybrids. The main types were classified into five categories: (i) embryo sac degeneration (ESD), (ii) embryo sac without egg apparatus, (iii) embryo sac without female germ unit, (iv) embryo sac with abnormal polar nuclei and ( v) abnormal small embryo sac (ASES). The frequencies of abnormal embryo sacs in the 35 hybrids ranged from 26.0% to 76.3%, with an average of 48.6%. The highest frequency of abnormality was 31.3% ESD. The frequencies of various abnormalities varied with parental genotype and environment.