Phenotypic Characterization And Genetic Mapping Of A New Mutant Gene For Male Sterility In Rice

Male sterility is an inheritance phenomenon found widely in plants. The male sterile plant can't produce normal male gametophyte for reproduction, while the female counterpart is normal.So far, there are several dozens of male sterile mutants or genes reported in rice.In rice anther development process, the microspore mother cell firstly undergoes meiosis to produce the microspore in the anther locule,and the microspore further developed into pollens through mitosis, then the anther split and released maturity pollens.for fertilization.Any factors participated abnormally in anther development,premeiosis cell division, meiosis, microspore mitosis, or pollen vacuolation would resulted in anther abnormality , which finally caused male sterility.In recent years, with the development of the rice genome sequencing and annotation, the molecular mechanisms of pollen development in rice are gradually clear.we reported herein the phenotypic characterization, genetic analysis and molecular mapping of a sterile rice material derived from a spontaneous mutation, with the aim of making foundations for gene cloning and further understanding of the molecular mechanism in the controlling of rice male fertility.XS1 is a rice male sterile mutant which derived from a spontaneous mutation. The floret of the mutant, consisting of six stamens and one pistil, looks the same as that of the wild type in the male-female organs, except that the filaments are long and thin, and the anthers are withered in white transparece. It is verified that in XS1 no pollen grains can be stained with 1% I2-KI solution and the anther locules are always hollow. Paraffin slices indicate that the mutant microspores are abnormally condensed and agglomerated to form a deep-stained mass at the late microspore stage ,which results in the ceasing of the vacuolation process of microspores, and, therefore, the mutant forms no functional pollens for production. There was no detectable difference between the wild type and the mutant anther at the stage from microspore mother cell to meiosis.During this period, the formation process of microspore and the transfomation of the outer-4-layer in the muant are morphologically same as those in the wild type. Normal epidermis, endothecium, middle layer, tapetum, and microsporocytes were found in both wild-type and the xml anthers .Subsequently, the xml mutant anther had detectable morphological abnormalities after micorospores were released from the tetrads. During the later micorospore stage, wild-type micorospores increased the volume to undergo vacuolation and the uninucleate pollen developed to trinucleate pollen through two mitotic divisions. However, the mutant anther was failing in vacuolation. Observations indicated that all the micorospores were abnormally condensed and agglomerated to form a deep-stained mass, and the latter then extremely expanded the volume and occupied the majority of the locule. Finally, it was entirely collapsed and therefore no mature pollen grains were formed except a hollow locule . These observations suggest that the xml mutation resulted in the defects in pollen vacuolation, not affecting the outer-layer development. Thus, we concluded that the mutant's male sterility characteristic was a consequence of lacking of mature pollen grains caused by the arrest of the pollen vacuolation.During the heading stage, the individual plants in the F1 and F2 progenies from the crosses between XS1 and other normal rice lines were investigated. In the four F1 progenies, all plants exhibited wild-type phenotype, suggesting that the mutant trait is recessive. In the four F2 populations, all the segregation rates of feitility and sterility plants fit the ratio of 3:1 ; and, additionally, in the 3 BC1F1 populations, all the segregation rates of feitility and sterility plants fit the ratio of 1:1. Therefore, the mutant trait is controlled by one recessive gene, given its phenotypic character, termed VR1 (Vacuolation retardation 1).The polymorphisms between XM1 and other rice lines G630, M63, 9311 were examined with 512 SSR markers and the most polymorphism richment population XS1/G630 was selected for mapping. Firstly, total 103 SSR polymorphic markers were selected and used to survey in a small populations which was composed of the two parents, four of wild type F2 plants , and six of F2 mutants plants. The result showed that 3 SSR markers RM470, RM303 and RM317 located on chromosome 4 were obviously associated with the XS1 phenotype. Then, the three markers were used to survey all the mutant plants in the same F2 population, and were all verified to be linked to VR1 with the genetic distance of 3.4 cM, 2.6 cM and 2.4 cM, respectively. VR1 is a new gene located on chromosome 4 which controlled male fertility in rice .