Inheritance of the Male Sterility in the Oryza sativa Taichung 65 Like Cytoplasmic Substitution Lines Derived from O. rufipogon K61 Strain

Abstract

RT61A line and RT61B line were isolated by recurrent backcrossing, in which Oryza rufipogon K61 line was used as the initial female parent and O. sativa c.v. Taichung 65 as the recurrent pollen parent. The phenotypes of the RT61A line and the RT61B line were identical to that of Taichung 65 except that RT61A line have one-half sterile pollen (semisterile) and RT61B line represent complete pollen and seed fertilities. In order to elucidate the genetic mechanism of this sterility and its restoration an investigation was conducted by following crossing experiment between Taichung 65 and RT61A line, and RT61B line.
When RT61B line was crossed with Taichung 65, all of the offspring showed complete pollen and seed sterilities, indicating that this line have a male sterile cytoplasm and no fertility restoration gene and Taichung 65 also does not have this gene. When RT61A line was crossed with paternal Taichung 65, the resulting F₁ was segregated into pollen semisterility and normal seed fertility individuals and complete pollen sterility and seed sterility individuals at a ratio of approximately one to one. This phenomenon coincided with a segregation pattern from monoallelic genes and can be explained as follows. RT61A line had male sterile cytoplasm and a pair of fertility restoration genes in heterozygous status. In the next generation when this line is crossed with Taichung 65 having recessive genes for restoration fertility, therefore, heterozygous individuals will have pollen semisterility and normal seed fertility and homozygous individuals for recessive genes will have complete pollen and seed sterility. When the heterozygous individuals were self-pollinated, they produced all normal seed fertility plants in F₂ progeny while their pollen fertility was normally in one-half individuals but semisterile in the other. There were no complete male sterile plants. Such mode of inheritance indicated gametophytic one, that is, pollen grains having dominant restoration gene were normally developed, while recessive pollen grains were degenerated in a certain stage of pollen development. On the other hand, all F₁ and F₂ individuals obtained by crossing Taichung 65 as a maternal parent with paternal RT61A line showed normal seed and pollen fertilities, thus indicating a possibility that cytoplasm of Taichung 65 will offer normal developed pollen grains of both dominant and recessive genes. Such a possibility was confirmed because, when 18 F₂ individuals were selected at random and crossed with the sterile line, these individuals were found to consist of three groups as follows: 7 F₂ individuals which yielded all progeny of normal seed fertility and pollen semisterility without segregation, 6 F₂ individuals which yielded seed and pollen sterile plants and normal seed fertile and pollen semi-sterile plants at a ratio of one to one, and 5 F₂ individuals which yielded progeny of seed and pollen sterilities only.
From above results, it was concluded that the male sterility and its restoration found in these lines was induced by interaction with cytoplasm and a single locus for fertility restoration and the mode of restoration was gametophytic.