Low-temperature-sensitive steriUty has become one of the major obstacles in indica-japonica hybrid rice breeding. The objectives of this paper were to evaluate the extent of the fertility reduction and to determine the genetic basis of low-temperature-sensitive sterility. Seventeen varieties were crossed in various ways to produce 21 F|S including 16 indica-japonica hybrids. Fertility of the F]S and their parents was examined under both high and low temperature conditions. Considerable reduction in spikelet fertility was observed under low-temperature conditions in the majority of the indica-japonica hybrids having at least one wide compatibility parent. However, the extent of fertility reduction varied greatly, depending on the parental genotypes. Data from five pairs of reciprocal crosses indicated that the cytoplasm had no effect on fertility reduction. The more-or-less bimodal distribution of the fertihty segregation of one BCiF, and two F^ populations under low-temperature conditions suggested that the lowtemperature-sensitive sterility was controlled by only one or a few genes. It was also shown that the low-temperature-sensitivity is not related to wide compatibility. We conclude that it is possible to develop indicajaponica hybrids with wide compatibility and also insensitivity to the low-temperature conditions.
To improve the width of compatibility for overcoming various sterilities in inter‐subspecific hybrid rice, some elite lines combining several sterility‐neutral genes were developed and the effects on mitigating various hybrid sterilities were tested. From Akihikari// IR36/Dular, neutral genes at ga11 and six sterility loci, S5, S7, S8, S9, S15 and S16, were combined and elite lines were obtained in their successive progeny. Four of the lines tested were confirmed to combine the neutral alleles S5‐n, S7‐n, S8‐n, S9‐n, S15‐n and S16‐n at the sterility loci and, among them, two harboured an additional gamete abortion‐neutral allele, ga11‐n. F1s, which used the lines and various testers as parents, mitigated the spikelet sterilities by six sterility loci and gamete abortion by a gametophyte gene, ga11. These lines could be selectively used as parents or donors to increase the width of compatibility of rice varieties for improving fertility in inter‐subspecific hybrid rice breeding.
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