Abstract:

The two rice indica/japonica combinations, Asominori/IR24 and 02428/IR24, have strong heterosis, but low seed-setting rates. To understand the genetic bases of the sterility of the two hybrids，a study for identification of gene loci causing hybrid sterility were carried out by using a set of CSSL population with genetic background of japonica variety Asominori and chromosome substitution segments of indica IR24. The results showed that the sterility of Asominori/IR24 is mainly controlled by three gene loci, namely S-5 on chromosome 6, S-24(t) and S-31(t) on chromosome 5, and that of 02428/IR24 is influenced by S-24(t), a locus conferring pollen fertility. S-31(t) was first identified and tentatively nominated, and furthermore, a neutral gene on S-31(t) was also identified in a japonica variety 02428, which had been well documented with a neutral allele S-5n on S-5. The genetic background has a great effect on expression of fertile genes. In the japonica background, S-24(t) at Sⁱ/S^j heterozygous genotype could decrease spikelet fertility by about 70%, but S-31(t) and S-5 were two loci of hybrid semi-fertility. In the heterozygous genetic background of indica/japonica hybrid, the F₁ seed setting rate could be averagely increased by 22.5% when S-5ⁱ/S-5^j genotype replaced with S-5ⁱ/S-5ⁱ, which improved the F₁ fertility near to normal level, but S-24(t) and S-31(t) did not have the significant substitution effects when S-5 was at S-5ⁱ/S-5^j genotype. Therefore, S-5 was proved to be a key locus conferring spikelet sterility of rice indica/japonica hybrids. Finally, an alternative strategy named “alleles substitution method” was put forward to overcome low spikelet fertility in exploiting rice inter-subspecific heterosis.

Key words: Rice, Intersubspecific hybrid, Fertility, Chromosome segment substitution lines (CSSLs)