Progress of <italic>indica-japonica</italic> hybrid sterility and wide-compatibility in rice

Abstract

As a main staple food for world population, utilization of heterosis in rice has increased crop productivity tremendously. The Asian cultivated rice ( Oryza sativa L.) consists of two subspecies, indica and japonica . Genetic differentiation between indica and japonica subspecies appears to be a major source of genetic diversity in the cultivated rice gene pool. A strategic plan for utilization of inter-subspecific heterosis was proposed half a century ago, indicating that strong heterosis in indica-japonica hybrids provides an effective approach to increase the yield potential for rice breeding. Hybrid sterility that caused by postzygotic reproductive isolation has been a major barrier for utilization of such heterosis between indica and japonica . Genetic analyses have identified a large number of loci conditioning hybrid sterility in rice, which regulate fertility in heterozygous genetic background by complex allelic and epistasis interactions. Six locus-pairs regulating hybrid embryo-sac or pollen sterility have been cloned recently in rice, providing fresh understandings in mechanism of hybrid sterility. The molecular and evolutionary mechanisms underlying reproductive isolation are summarized with respect to the rice hybrid sterility genes, which reveal three evolutionary genetic models including the parallel divergence model, the sequential divergence model, and the parallel-sequential divergence model. There also exists a special group of rice germplasm, referred to as wide-compatibility varieties that produce fertile hybrids when crossed to both indica and japonica varieties. The wide-compatibility genes in these varieties enable hybridization and provide valuable genetic resources for utilization of inter-subspecific heterosis, which take diverse forms owing to the divergent mechanisms in reproductive barriers. Therefore, understanding in hybrid sterility and reproductive isolation in rice would greatly facilitate the utilization of heterosis in breeding programs. This paper summarized the progress in genetic and molecular mechanisms regulating rice hybrid sterility, and discussed the recent understanding in wide-compatibility genes and their utilizations in breeding of wide-compatibility varieties. These advances may greatly facilitate the exploration of inter-subspecific heterosis in breeding programs.