SUMMARYA post-zygotic hybridization barrier is often observed in the endosperm of seeds produced by interspecific or interploidy crosses. In Arabidopsis thaliana, for example, hybrid endosperm from both types of cross shows altered timing of cellularization and an altered rate of nuclear divisions. Therefore, it has been proposed that interspecific and interploidy crosses share common molecular mechanisms for establishment of an effective species barrier. However, these two types of hybridization barrier may be initiated by different intrinsic cues: the interspecific cross barrier arises after hybridization of genomes with differences in DNA sequences, while the interploidy cross barrier arises after hybridization of genomes with the same DNA sequences but differences in ploidy levels. In this study, we performed interploidy crosses to identify components of the post-hybridization barrier in the endosperm of rice. We performed an intra-cultivar cross of autotetraploid (4n) 3 diploid (2n) rice, and found precocious cellularization and a decreased rate of nuclear division in the syncytial endosperm. By contrast, seeds from the reciprocal cross showed delayed cellularization and an increased rate of nuclear division. This differential effect on nuclear division rates contrasts with the outcome of rice interspecific crosses, which were previously shown to have altered timing of cellularization without any change in nuclear division rates. Thus, we propose that the post-zygotic hybridization barrier in rice endosperm has two separable components, namely control of the timing of cellularization and control of the nuclear division rates in the syncytial stage of endosperm development.

In most eudicot and monocot species, interspecific and interploidy crosses generally display abnormalities in the endosperm that are the major cause of a post-zygotic hybridization barrier. In some eudicot species, however, this type of hybridization barrier can be overcome by the manipulation of ploidy levels of one parental species, suggesting that the molecular mechanisms underlying the species hybridization barrier can be circumvented by genome dosage. We previously demonstrated that endosperm barriers in interspecific and interploidy crosses in the genus Oryza involve overlapping but different mechanisms. This result contrasts with those in the genus Arabidopsis, which shows similar outcomes in both interploidy and interspecific crosses. Therefore, we postulated that an exploration of pathways for overcoming the species hybridization barrier in Oryza endosperm, by manipulating the ploidy levels in one parental species, might provide novel insights into molecular mechanisms. We showed that fertile hybrid seeds could be produced by an interspecific cross of female tetraploid Oryza sativa and male diploid Oryza longistaminata. Although the rate of nuclear divisions did not return to normal levels in the hybrid endosperm, the timing of cellularization, nucellus degeneration and the accumulation of storage products were close to normal levels. In addition, the expression patterns of the imprinted gene MADS87 and YUCCA11 were changed when the species barrier was overcome. These results suggest that the regulatory machinery for developmental transitions and imprinted gene expression are likely to play a central role in overcoming species hybridization barriers by genome dosage in the genus Oryza.

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