Improvement for yield level will still be the target of first priority in the coming century. According to the report of IRC of FAO in 1994, the yield of modern varieties has become stagnant. Hybrid rice is the only technology presently available to overstep these yield barriers. 4 Having been in a position to review the hybrid rice technologies, I would like to discuss the potential of this technology.
Doubtlessly, hybrid rice using cytoplas-mic male sterility(CMS) is one of the most significant achievements after the introduction of semi-dwarf high yielding varieties. The yield increase is estimated to be 15-20 % over the ordinary varieties. It is reported that this hybrid rice covered nearly half of the total rice areas in China. But the initial gains by hybrid rice have not been improved in China, perhaps due to the difficulty in breeding stable CMS lines and shifted emphasis on grain quality. Since the 1990s hybrid rice breeding has been one of the first priority programs in India, where hybrid rice is being increased from the initial adoption of 50,000 ha in the mid-1990s.
What is interesting in hybrid rice breeding is a series of unique innovations in the technology. After the success of the CMS system, hybrid seed production along the idea of two line hybrids has been a fascinating target. With the use of environment-dependent genic male sterility (EGMS), fertile plants can be propagated by self-pollination under one set of conditions, while the same genotype can be male sterile and be hybridized with any other variety. When the new type of hybrid was first proposed in China, few scientists were confident in it. But it is now a reality showing further yield increase. Hybrid rice technology will be improved further by the further study of EGMS.
Another idea is to overcome hybrid sterility between different groups of rice varieties (Table 5.1). Partial sterility is commonly found in the panicles of F1 hybrids between Indica and Japonica groups in rice. It is known as a barrier in the use of pronounced heterosis of Indica-Japonica hybrids. I started a genetic study in the early 1980s, and found that the panicle sterility in Indica-Japonica hybrids is caused by an allelic interaction at locus S-5 on chromosome 6, where Indica and Japonica varieties have S-5i and S-5j, respectively (Fig. 5.1.) The heterozygote S-5i/S-5j pro duces semisterile panicles because of the partial abortion of the female gametes carrying S-5j. Some varieties such as Ketan Nangka (KN) and Dular, have a neutral allele S-5n, and the genotype S-5j/S-5n and S-5j/S-5n produce fertile panicles. S-5n is called the wide compatibility gene (WCG), and has been incorporated into Indica or Japonica varieties to overcome the sterility in Indica-Japonica hybrids. 5 In the past decade, several Indica-Japonica hybrids which have the neutral allele S-5n have been developed to determine the yield potential in China. Such hybrids showed strong heterosis, but their seed set were unstable under some environments. One way to solve the problem is to use Javanica varieties instead of Japonica. This idea has been utilized for the new generation of hybrids, inter-subspecific two line hybrids, which showed increased yield and were planted to 0.7 million ha in 1996 in China.
An ultimate technology in hybrid rice breeding will be the use of apomixis, which functions in some plant species to produce genetically the same progeny via seed. In this way hybrid plants may produce hybrid seed without any artificial crossing. So far there is no basis on which further progress can be seen in rice breeding.
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