Fine Mapping Of S9, A Gene Responsible For Intersubspecific Hybrid Sterility In Rice (Oryza Sativa L.)

Currently, rapid growth of the world population is accompanied by the rapid growth of the fast demand for food supply. Especially in developing countries of Asia and Africa, with the fast development of population, the demands of food supply are increasing quickly. Rice is the staple food in developing countries of Asia and Africa and one of the world’s three major food crops. So the raising in yield of rice is important for the food security and economic security of the world. However, no big breakthrough has been made in the increase of rice yield since the 1980s. Some of the major factors, including the narrow genetic resources, led to the current status of rice yield. Currently, the use of rice heterosis is between varieties inside the subspecies, and the increasing in rice yield has not improved greatly. But, there are strong heterosis between subspecies and good genetic potential or yield potential.We can bring a new leap for rice breeding by using heterosis between subspecies. However, there is a major factor that restricts the using of heterosis between subspecies. That is serious reproductive isolation between subspecies, which resulting in intersubspecific hybrid sterility. It will affect spikelet fertility and restrict the raising in rice yield. Therefore, it’s necessary to study the genetic mechanism of intersubspecific hybrid sterility and the genes responsible for it. That is a prerequisite to overcome hybrid sterility between subspecies and make use of strong heterosis between subspecies.We used two parent materials in our study. Baimifen is a landrace in Yunnan province and Ketan Nangka is a japonica in Indonesia. There was a reported gene, named S9 responsible for intersubspecific hybrid sterility between Baimifen and Ketan Nangka. The fine mapping and genetic analysis of it were finished. We explained the mechanism of hybrid sterility between subspecies, combined with the cytological observation. The main results of intersubspecific hybrid sterility in this study were as follows:1. The cytological observation of the hybrid sterility F1 plant.This paper surveyed related cytological indexes affecting spikelet fertility to analyze the causes of hybrid sterility between subspecies.The seed setting rate of hybrid F1 plant was 54.4±10.8(%), and that of parent materials Ketan Nangka and Baimifen were both about 90%. The seed setting rate of hybrid F1 plant exhibited typical semi-sterility. The I2-KI(1%) staining showed that the pollen fertility of hybrid F1 plant, Ketan Nangka and Baimifen was normal and closed to 90%. There were no significant differences between hybrid F1 plant and two parent materials in the germination of pollens and elongation of pollen tubes on the stigmas. These results would presume that the fertility of the male gametes was normal and not the cause of spikelet sterility. But the fertility of the female gametes may be the reason for spikelet sterility.Laser confocal microscopy assay showed that partial mature embryo sacs of hybrid F1 plants were sterility:undifferentiated embryo sac, embryo sacs without female germ-unit, embryo sacs with unknown cells, degenerative embryo sacs or empty embryo sacs. Statistics show that the number and ratio of sterility embryo sacs were correlated to the spikelet sterility. This finding preliminary proved that the spikelet sterility was due to partial sterility mature embryo sacs of hybrid F1 plants.Parafin section assay indicated that the spikelet sterility of hybrid F1 plant was caused by sterility embryo sacs. There were some kinds of sterility embryo sacs:degenerative embryo sacs, empty embryo sacs, embryo sacs with abnormal numbers of polar nuclei, embryo sacs with unknown cells, abnormal small embryo sacs, polar nucleis and antipodal cells in abnormal position.2. Fine-mapping of the intersubspecific hybrid sterility gene S9.Using of 88 individual plants in BC1F1 group for linkage analysis, the female gamete sterility was found to be controlled by S9. It was on the long arm of the chromosome 4 and located to the region between markers P15 and N4-13.We selected 874 extreme individual plants from BC2F3 population, and screened out 18 recombinants. S9 was finally fine-mapped to the region between molecular marker jx2 and N4-13, with an interval of approximately 249kb.