Megasporogenesis and Megagametogenesis in Autotetraploid Indica/Japonica Rice Hybrid

Autotetraploid indic, a/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 dce hybrid. To clarify the cytological mechanism of embryo sac abortion, the megasporogenesis and megagametogenesis in an autotetraploid japonicaAndica 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/ndica hybdd. Embryo sac fertility was lower in diploid japonica/ndica hybdd than in autotetraploid japonicaAndica hybrid although many abnormal phenomena were observed in autotetraploid hybrid.

Abnormalities Occurring during Female Gametophyte Development Result in the Diversity of Abnormal Embryo Sacs and Leads to Abnormal Fertilization in indica/japonica Hybrids in Rice

Embryo sac abortion is one of the major reasons for sterility in indicaljaponica hybrids in rice. To clarify the causal mechanism of embryo sac abortion, we studied the female gametophyte development in two indicaljaponica hybrids via an eosin B staining procedure for embryo sac scanning using confocal laser scanning microscope. Different types of abnormalities occurred during megasporogenesis and megagametogenesis were demonstrated. The earliest abnormality was observed in the megasporocyte. A lot of the chalazal-most megaspores were degenerated before the mono-nucleate embryo sac stage. Disordered positioning of nucleus and abnormal nucellus tissue were characteristics of the abnormal female gametes from the mono-nucleate to four-nucleate embryo sac stages. The abnormalities that occurred from the early stage of the eight-nucleate embryo sac development to the mature embryo sac stage were characterized by smaller sizes and wrinkled antipodals. Asynchronous nuclear migration, abnormal positioning of nucleus, and degeneration of egg apparatus were also found at the eight-nucleate embryo sac stage. The abnormalities that occurred during female gametophyte development resulted in five major types of abnormal embryo sacs. These abnormal embryo sacs led to abnormal fertilization. Hand pollination using normal pollens on the spikelets during anthesis showed that normal pollens could not exclude the effect of abnormal embryo sac on seed setting.