Megasporogenesis and Megagametogenesis in Autotetraploid Indica/Japonica Rice Hybrid

Autotetraploid indica/japonica rice hybrid combines both the advantages of polyploidy and the heterosis between indica and japonica rice. Embryo sac abortion is an important factor influencing spikelet fertility in autotetraploid rice hybrid. To clarify the cytological mechanism of embryo sac abortion, the megasporogenesis and megagametogenesis in an autotetraploid japonica/indica hybrid were examined by the whole-mount eosin B-staining confocal laser scanning microscopy (WE-CLSM) technique. Abnormalities were observed from the megasporocyte stage to the mature embryo sac stage. The degeneration of the tetrad cells and the functional megaspore was the characteristic of abnormalities during megasporogenesis. Abnormal small embryo sacs and disordered number of nuclei were frequently observed during embryo sac development. Some interesting phenomena, such as two functional megaspores, the diplospory-like megasporocyte, and five-nucleate embryo sac were found. The abnormalities that occurred during female gametophyte development resulted in more than five types of abnormal embryo sacs (i.e. embryo sac degeneration, embryo sac without female germ unit, embryo sac without egg apparatus, embryo sac with abnormal polar nuclei and abnormal small embryo sac) in autotetraploid japonica/indica hybrid. Embryo sac fertility was lower in diploid japonica/indica hybrid than in autotetraploid japonica/indica hybrid although many abnormal phenomena were observed in autotetraploid hybrid.