A New Strategy of Rice Breeding in the 21st Century Ⅱ. Searching a New Pathway of Rice Breeding by Utilization of Double Heterosis of Wide Cross and Polyploidization

CAI De-Tian, YUAN Long-Ping, LU Xing-Gui (College of Life Science, Hubei University, Wuhan 430062) (National Hybrid Rice Research and Development Center in China, Changsha, 410125 ) (Hubei Agricultural Academy, Wuhan 430070)

Breeding strategy; Allopolyploid; Apomixis ;Wide compatibility ; Ph gene

This is the second one of a series of papers about the new strategies of rice breeding in the 21st century. It is suggested that the tactics of new pathway of rice breeding by utilization of wide cross and polyploidization, i. e using heterosis of intergenomes and polyploid and the role of some special genes. Making a general survey of the history of rice breeding, their research strategies were based on sexual reproduction and diploid, in general, all used the combination of superior genes of cultivated and wild rice in the same genome (AA). Reviewing on the tendance of crop evolution, rice is a diploid with smaller genome, DNA content and chromosome size. Increasing the number of its genomes, getting up its ploidy level, using heterosis of allopolyploid will be a new pathway of rice breeding. To counter the key problem of low percentage of filled seeds in autotetraploid , such measures can be used to raise seed fertility by genetic principle, as enlarging their relative distance of parentages, reducing the formation of polyvalent chromosome and utilizing apomixis, wide compatibility and Ph genes. There are three steps to realize the strategy. First, selecting terminal type of Orysa sativa ssp indica, japonica and javanica involved American cultivars cross each other to induce intersubspecific polyploid. Second, inducing O. sativa, O. glaberrima and O. barthii to form interspecific polyploid. Third, inducing cultivated rice with AA genome and wild rice with different genomes in other 8 genomes from B to J to form interspecific allopolyploid. Some germplasms with special genes such as wide compatibility, apomixis and Ph genes will be taken part of important role in the cross. Wide compatibility will be useful to overcome male sterility and female abortion of F1 of intersubspecies. The characteristic of apomixis without meiosis and fertilization will be helpful to avoid segregation of progeny because of chromosome pairing. And Ph gene as in wheat with impression of the pairing o f partial homologeous chromosomes, can be used to prevent the pairing of partial homologeous chromosomes in intergenomes of allopolyploid and formation of multivalent, chromosomal bridge, lag chromosome. These will ensure for their progenies with high rate of seed set and good stability in utilization of selecting allopolyploid rice and apomictic rice. Recent progress in this experiment have provided a good results. Using apomictic lines as a parent to cross with O. satva ssp indica and japonica and cross between O. sativa ssp Asian and American cultivars had obtained some superier polyploid plant lines with powerful heterosis and high up to 85 % seed set. It has provided a very good background to realize the new breeding strategy.