**6. Genetic mechanism of rice heterosis**

Heterosis, also known as hybrid vigor, is the phenomenon in which progeny of diverse inbred varieties outperform both parents in terms of yield, panicle size, and number of spikelets per panicle, number of productive tillers, stress tolerance and other factors. This phenomenon has been extensively exploited in crop production as a powerful force in plant evolution. After the successful development of hybrid maize in 1930, other crop breeders, including rice breeders, were inspired to use the concept of hybrid production by exploiting heterosis. In fact, the exploitation of heterosis has been the most practical achievement of genetics and plant breeding research [26]. The impact of this phenomenon can be judged by the fact that the number of grains per square meter in rice varies significantly between (1) wild ancestors with just a few hundred (2) improved inbred varieties with about 40,000, and (3) rice hybrids with about 52,000. Rice heterosis was first reported by Jones (1926) who observed that some F1 hybrids had more culms and greater yield than their parents. Between 1962 and 1967, a variety of proposal came from around the world for commercial exploitation of heterosis to become a major component of national and international rice improvement programs. Rice breeders from Japan, China, United States, India, the former Soviet Union and Philippines, for example, began working on projects to use rice heterosis. However, progress had been hampered by rice's inability to be strictly self-pollinated crop, as opposed to corn which is needed for hybrid seed development, extremely difficult [27–29].
