Abstract
The application of heterosis (hybrid vigor) has brought great success to plant breeding, particularly of hybrid rice, achieving significant yield increases. Attempts to explore the heterosis of inter-subspecific hybrids between indica and japonica rice, which result in even greater yield increases, have greatly increased in the past decades. However, because of the reduced seed setting rate in F1 hybrids as a result of increased reproductive isolation, the application of inter-subspecific hybrids in rice has slowed. Understanding the balance between heterosis and the reproductive isolation of inter-subspecific hybrids will facilitate the strategic design of inter-subspecific hybrid breeding. In this study, five indica and seven japonica rice varieties were chosen as the parental lines of a complete diallel mating design. Data from six group traits from all of the hybrids and inbred lines were collected. We found that the grain weight per plant, grain number per panicle, tiller per plant, thousand grain weight and plant height, which reflected increased heterosis, were associated with the genetic divergence index (GDI) of the parents. Meanwhile, owing to the reduced seed setting rate, which was also associated with the parents' GDI, the grain production of the hybrids was negatively affected. After analyzing the relationships between the GDI of indica-japonica parents and the grain weight per plant of the F1 hybrids, an ideal GDI value (0.37) for the two indica-japonica parents that could provide an optimal balance between the inter-subspecific heterosis and reproductive isolation was proposed. Our findings will help in the strategic design of an inter-subspecific hybrid rice breeding program by identifying the ideal indica and japonica parents for a hybrid combination to achieve hybrid rice with an optimal yield. This strategic design of an inter-subspecific hybrid rice breeding program will be time saving and cost effective.
Highlights
Rice is the main staple food for almost half of the world’s population
Regarding the remaining three yield-related traits, the highest average value of the relative mid-parent heterosis was the tiller number per plant, which was as high as 15.38%
The magnitudes of heterosis in these three traits were all smaller than the grain weight per plant
Summary
Rice is the main staple food for almost half of the world’s population. In China, rice constitutes 40% of the total calorie intake [1]. Asian-cultivated rice (Oryza sativa L.) consists of two subspecies, indica and japonica. Regarding morphology and living conditions, indica varieties exhibit greater plant heights, longer leaves, and heat and moisture tolerance but are sensitive to low temperatures. These varieties are cultivated at low latitudes and in humid regions. Japonica varieties have lower plant heights and shorter leaves than do those of indica but are tolerant to low temperatures These varieties are more suitable for high latitudes and for the lower latitudes of high altitude cultivation. Reproductive isolation emerges in the hybridization between these two subspecies
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