Phylogenetic Differentiation of Cultivated Rice, XX Analysis of the Genetic Basis of Hybrid Breakdown in Rice

Abstract

The gametic-development gene hypothesis, formerly put forward by the senior writer, could convincingly explain the sterility of F1 hybrids and related phenomena in cultivated rice, but could not account for the occurrence of sterility in F2 and later hybrid derivatives. It was found that among many intervarietal crosses, F1 fertility and mean F2 fertility were not correlated. Experiments were then conducted to throw a light on the genetic basis of sterility of the latter type, using cross combinations which showed a high fertility of F1 hybrids. Their fertility in F2 segregated in a wide range, and from a few partially sterile F2 plants, partially sterile lines which bred true for sterility have been established. Embryological observations showed that both male and female gametes partly degenerate after normal meiosis.The partially sterile lines were then back-crossed to the parental strains, and the F1, F2 and F3 generations were investigated regarding fertility variation. The results led to the conclusion that duplicate factors might be involved sustaining the development of gametes on the condition that at least two dominants are present in the plant, and that this diplontic sterility might be due to certain recessive combinations of those factors. The parental strains were considered to have different combinations, A1A1a2a2 and a1a1A2A2. A linkage relation was found between such a fertility factor and the apiculus pigmentation gene C. In the sense that those factors behave as genes, they were called Duplicate-fertility genes. The genetic basis of this sterility thus postulated is similar to that formerly shown for the segregation of weak plants in F2. Both phenomena, which spell a breakdown of hybrids, seems to be the isolating barriers frequently found in rice.