Rice Genomics and Genetics 2024, Vol.15, No.6, 277-286 http://cropscipublisher.com/index.php/rgg 279 3.2 Genetic diversity and heterosis Genetic diversity plays a crucial role in the manifestation of heterosis. The greater the genetic distance between the parental lines, the higher the potential for heterosis. This is because diverse genetic backgrounds can bring together a wide array of beneficial alleles and gene interactions. Studies have shown that the genetic basis of heterosis in rice involves multiple quantitative trait loci (QTLs) that cumulatively contribute to yield heterosis. For instance, the RH8 gene has been identified as a major QTL for yield heterosis in rice, highlighting the importance of specific genetic loci in driving hybrid vigor (Huang et al., 2016; Li et al., 2016; Liu et al., 2020). Additionally, the presence of polymorphic promoter cis-regulatory elements and differential gene expression in hybrids further underscores the complex genetic interplay underlying heterosis. 3.3 Common intersubspecific hybrid combinations and their advantages and disadvantages Several intersubspecific hybrid combinations have been developed to exploit heterosis in rice. One notable example is the hybrid combination Liang-you-pei 9 (LYP9), which has shown significant yield advantages due to better parent heterosis (BPH) of spikelet number per panicle (SPP) and paternal parent heterosis (PPH) of effective panicle number (EPN). Another example is the use of multiplex CRISPR-Cas9 genome editing to produce clonal diploid gametes and tetraploid seeds, enabling clonal propagation of F1 hybrids and maintaining their heterozygosity (Wang et al., 2019). However, these hybrid combinations also come with certain disadvantages. The cost of hybrid seed production can be high, and the beneficial traits of hybrids may be lost in subsequent generations due to genetic segregation (Wang et al., 2019; Paril et al., 2023). Additionally, the complexity of genetic interactions and the need for precise genetic engineering pose significant challenges in the development and maintenance of high-yielding hybrid varieties (Fujimoto et al., 2018). 4 Genetic Basis and Breeding Strategies for High-Yield Traits 4.1 Key indicators and genetic basis of high-yield traits High-yield traits in hybrid rice are primarily influenced by both additive and non-additive gene actions. Studies have shown that the additive variance is a significant component of the total genotypic variance, which is crucial for the selection of superior parental lines and hybrids (Gaballah et al., 2022). The principal component analysis (PCA) has identified key yield component traits, such as grain yield, spikelet number per panicle, and plant height, which are essential indicators of high yield (Duan et al., 2013). The genetic basis of these traits often involves complex interactions between multiple quantitative trait loci (QTLs) and specific alleles that contribute to heterosis, or hybrid vigor, resulting in superior performance of hybrids compared to their parents (Huang et al., 2015; Li et al., 2016; Liu et al., 2020). 4.2 Important yield-related genes and their functions Several key genes have been identified as crucial for enhancing yield traits in hybrid rice. The Gn1a gene, which influences spikelet number per panicle, is a major determinant of grain yield. The DEP1 gene is associated with dense and erect panicle architecture, contributing to increased grain number and yield (Duan et al., 2013). Another important gene, Ghd7, plays a significant role in regulating heading date and plant height, which are critical for optimizing the growth period and maximizing yield potential (Li et al., 2016). These genes, along with others like Sd1 for plant height and IPA1 for ideal plant architecture, collectively contribute to the high-yield traits observed in hybrid rice varieties. 4.3 Application of molecular marker-assisted selection and gene editing in high-yield breeding The integration of molecular marker-assisted selection (MAS) and gene editing technologies has revolutionized high-yield breeding in hybrid rice. MAS allows for the precise selection of desirable traits by identifying specific genetic markers linked to high-yield QTLs, thereby accelerating the breeding process (Qian et al., 2016). Gene editing tools, such as CRISPR/Cas9, enable targeted modifications of key yield-related genes, facilitating the development of rice varieties with enhanced yield potential. These advanced breeding strategies have unlocked the potential for rational design in rice breeding, combining wide-cross compatibility and intersubspecific heterosis to
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