Inter-specific hybrids are a useful source for increasing genetic diversity. Some reproductive barriers before and/or after fertilization prevent production of hybrid plants by inter-specific crossing. Therefore, techniques to overcome the reproductive barrier have been developed, and have contributed to hybridization breeding. In recent studies, identification of molecules involved in plant reproduction has been studied to understand the mechanisms of reproductive barriers. Revealing the molecular mechanisms of reproductive barriers may allow us to overcome reproductive barriers in inter-specific crossing, and to efficiently produce inter-specific hybrids in cross-combinations that cannot be produced through artificial techniques. Inter-specific hybrid plants can potentially serve as an elite material for plant breeding, produced through the merging of genomes of parental species by allopolyploidization. Allopolyploidization provides some benefits, such as heterosis, increased genetic diversity and phenotypic variability, which are caused by dynamic changes of the genome and epigenome. Understanding of allopolyploidization mechanisms is important for practical utilization of inter-specific hybrids as a breeding material. This review discusses the importance of reproductive barriers and the effect of allopolyploidization in crop breeding programs.
Keywords: endosperm barrier; genomic rearrangements; heterosis; hybrid necrosis; inter-specific hybrid; non-additive expression; pre-zygotic barrier.