Assessment of genomic relationship between Oryza sativa and Oryza australinesis

The genomic relationship between Oryza sativa (2n = 24 AA) and Oryza australinesis (2n = 24 EE) has not been established. Genomic relationship between these two species was assessed by using three strategies: genomic in situ hybridization (GISH), meiotic chromosome pairing, pollen and spikelet sterility. The hybrid was produced between these two species at the International Rice Research Institute using embryo rescue technique. The chromosome pairing was examined in pollen mother cells of O. australinesis, O. sativa and the hybrid between O. sativa and O. australinesis. The hybrid was highly sterile with pollen stain ability being 0.05%. Both parents showed regular meiosis with normal chromosome pairing. The F1 hybrid exhibited limited chromosome pairing. On an average, 0 - 4 bivalents and 16 - 24 univalents were recorded at metaphase-1. The most frequent configuration was two bivalent and twenty univalent. The chromosomes of O. australiensis appeared larger and darkly stained. For genomic in situ hybridization, genomic DNA from O. australiensis was used as probe for the mitotic and meiotic chromosomes of the hybrid between O. sativa and O. australiensis. GISH revealed unequivocal discrimination of O. australiensis chromosomes that appeared yellow due to hybridization signal from O. sativa chromosomes that fluoresced red due to counterstaining with propidium iodide (PI). No cross hybridization was examined between the labeled genomic DNA of O. australiensis and the chromosomes of O. sativa. The paired chromosomes were discriminated as autosyndetic and allosyndetic pairing. Meiotic and mitotic chromosomes of the O. australiensis and O. sativa, in the hybrid were discriminated by GISH for the first time. Results showed that both genomes were highly divergent.