Rice Hybrid Weakness is Caused mainly by the Enhanced Stress-Response

Hybrid weakness is a type of reproductive barrier found in many plant species and is important to plant evolution. Compared with heterosis, hybrid weakness has received less attention in evolutionary genetics studies. In rice, the hybrid weakness of the F1 progenies between the Jamaica- and temperate Japonica-types has been intensively genetically surveyed, and it has been found to be controlled by two complementary genes, Hwc1 and Hwc2. The defective development of the hybrid F1 seedlings was found to be mainly due to abnormal root growth, resulting in non-continuous growth and the eventual lethality of the plants. Detailed genome-wide analyses using the hybrid F1 plant and parents showed that, in contrast to heterosis, in which photosynthesis- and starch metabolism-related genes are preferentially expressed, the abscisic acid (ABA)-response and abiotic-/biotic- and defense-related genes were significantly up-regulated in the roots of the hybrid F1, resulting in suppressed growth of the whole plant. This indicates that the mechanisms of heterosis and hybrid weakness differ and provides informative clues to facilitate the understanding of the mechanisms controlling the reproductive isolation and hybrid weakness.

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