Rice Genomics and Genetics 2024, Vol.15, No.4, 190-202 http://cropscipublisher.com/index.php/rgg 192 interactions between alleles at a single locus or the presence of advantageous alleles that function optimally when heterozygous. In rice, evidence for overdominance has been found in studies identifying specific loci where heterozygous combinations outperform both homozygous parents. For example, QTL mapping has revealed loci associated with yield and other agronomic traits that exhibit overdominant effects, contributing to the overall hybrid vigor (Luo et al., 2013). Epistasis refers to the interaction between genes at different loci, where the effect of one gene depends on the presence of one or more modifier genes. Gene interactions can significantly influence heterosis by creating complex networks that enhance the overall performance of the hybrid. Epistatic interactions can result in non-additive genetic effects, contributing to the superiority of hybrids over their parents. Studies in rice have demonstrated that epistatic interactions play a crucial role in heterosis. By using advanced genomic tools, researchers have identified numerous epistatic QTLs that contribute to traits such as yield, stress tolerance, and disease resistance (Yu et al., 1997). These findings underscore the importance of considering gene interactions in hybrid breeding programs. 2.3 Genetic basis of heterosis Quantitative Trait Loci (QTL) mapping is a powerful tool used to identify genomic regions associated with complex traits, including those contributing to heterosis. By crossing genetically diverse parents and analyzing the resulting progeny, researchers can pinpoint specific loci that influence hybrid vigor. QTL mapping has been instrumental in uncovering the genetic basis of heterosis in rice. Numerous QTLs associated with yield, biomass, and other agronomic traits have been identified in hybrid rice. For example, studies have identified QTLs on chromosomes 1, 2, and 6 that significantly contribute to yield heterosis (Huang et al., 2016). These QTLs often contain multiple genes, each playing a role in the observed heterotic effects. The identification and characterization of these QTLs provide valuable targets for marker-assisted selection (MAS) and genomic selection (GS) in rice breeding. Advances in genomic technologies have revolutionized the study of heterosis, enabling researchers to delve deeper into the molecular mechanisms underlying hybrid vigor. High-throughput sequencing, transcriptomics, and proteomics have provided insights into the gene expression patterns and regulatory networks involved in heterosis. Genomic studies have revealed that hybrids often exhibit unique gene expression profiles compared to their parents. These differences in gene expression can lead to enhanced metabolic activities, improved stress responses, and optimized growth processes in hybrids. For instance, transcriptomic analyses have shown that hybrid rice plants have upregulated genes associated with photosynthesis, nutrient uptake, and stress tolerance (Zhang et al., 2018). Moreover, epigenetic modifications, such as DNA methylation and histone modifications, have been implicated in the regulation of heterosis. These epigenetic changes can influence gene expression and contribute to the phenotypic superiority of hybrids. Understanding the role of epigenetics in heterosis offers new avenues for manipulating and enhancing hybrid performance through targeted breeding strategies. The genetic basis of heterosis in rice is multifaceted, involving complex interactions between genes, regulatory networks, and environmental factors. QTL mapping and genomic studies have provided valuable insights into the loci and molecular mechanisms that drive hybrid vigor. By integrating these findings into breeding programs, it is possible to develop superior hybrid rice varieties that can meet the growing demands for food security and agricultural sustainability. 3 Breeding Strategies for Exploiting Heterosis 3.1 Traditional hybrid breeding Traditional hybrid breeding has been the backbone of hybrid rice development for decades. This approach involves several key steps, including inbreeding and line development, followed by crossbreeding techniques to produce hybrid varieties.
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