Rice Genomics and Genetics 2024, Vol.15, No.6, 277-286 http://cropscipublisher.com/index.php/rgg 278 development of parental lines with enhanced compatibility and yield potential. Advanced genomic and transcriptomic analyses have further elucidated the complex interactions between different genetic components, paving the way for more targeted and efficient breeding strategies 2 History and Development of Hybrid Rice 2.1 Development history of hybrid rice The development of hybrid rice has been a significant milestone in agricultural science, particularly in the quest to meet the increasing global demand for rice. The introduction of semidwarf varieties and hybrid rice in the last century marked two major breakthroughs that significantly boosted rice productivity (Qian et al., 2016). The first successful hybrid rice variety, developed in China, demonstrated the potential of hybrid vigor or heterosis to enhance yield. This success led to the widespread adoption of hybrid rice, which has since become a dominant form of rice cultivation in China and has extended worldwide (Li et al., 2016). Despite initial challenges, such as seed purity issues and sensitivity to environmental conditions, subsequent improvements in hybrid rice breeding have led to the development of high-yielding varieties like ‘Liangyoupeijiu’ and ‘Liangyou E32’, which have been successfully commercialized and cultivated over large areas. 2.2 Basics and concept of heterosis Heterosis, or hybrid vigor, refers to the phenomenon where hybrid offspring exhibit superior qualities compared to their parents. This concept has been extensively utilized in rice breeding to enhance yield and quality traits. Heterosis in rice is primarily achieved through the exploitation of genetic diversity between different subspecies, such as indica and japonica (Wu, 2009). The genetic basis of heterosis involves the interaction of multiple quantitative trait loci (QTLs) that cumulatively drive yield heterosis by regulating key yield components like spikelet number per panicle and effective panicle number (Li et al., 2016). Studies have identified specific heterosis-associated genes, such as qSS7 and qHD8, which contribute to the dominance effects observed in high-yielding hybrid varieties (Lin et al., 2020). The application of molecular biotechnology, including genome sequencing and marker-assisted selection, has further enhanced the understanding and utilization of heterosis in rice breeding. 2.3 Genetic basis of intersubspecific hybridization Intersubspecific hybridization, particularly between indica and japonica rice varieties, has been a key strategy in developing high-yielding hybrid rice. This approach leverages the genetic diversity between the two subspecies to create hybrids with superior traits. The development of indica-japonica hybrids, such as ‘Yayou 2’, demonstrated the potential of intersubspecific heterosis to achieve high yields, although initial attempts faced challenges related to seed purity and environmental sensitivity (Li et al., 2005). Recent advancements have focused on developing parental lines with favorable genes from both subspecies, utilizing wide compatibility and thermosensitive genic male sterility (TGMS) genes to facilitate hybridization. The integration of molecular marker-assisted selection has enabled the identification and pyramiding of heterosis genes from different rice ecotypes, further enhancing the yield potential of hybrid rice. The success of China’s ‘super’ hybrid rice breeding project, which combines the ideotype approach with intersubspecific heterosis, underscores the effectiveness of this strategy in breaking the yield ceiling of irrigated rice crops (Peng et al., 2008). 3 Biological Basis of Intersubspecific Heterosis 3.1 Definition and types of intersubspecific heterosis Intersubspecific heterosis, also known as hybrid vigor, refers to the phenomenon where hybrid progeny resulting from the cross between two different subspecies exhibit superior traits compared to their parents. This can manifest in various forms such as increased yield, enhanced growth rates, and improved resistance to diseases. The types of heterosis can be broadly categorized into three main genetic mechanisms: dominance, overdominance, and epistasis. Dominance refers to the masking of deleterious recessive alleles by dominant alleles, overdominance involves the superior performance of heterozygous genotypes over both homozygous parents, and epistasis is the interaction between different gene loci that results in enhanced performance (Zhang et al., 2008; Goff and Zhang, 2013; Fujimoto et al., 2018; Paril et al., 2023).
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