Molecular Plant Breeding 2024, Vol.15, No.4, 167-177 http://genbreedpublisher.com/index.php/mpb 170 3 Genetic Basis of Autotetraploid Rice 3.1 Chromosomal doubling and genome stability Chromosomal doubling is a fundamental process in the development of autotetraploid rice, which involves the duplication of the entire set of chromosomes, resulting in a tetraploid organism with four sets of chromosomes. This process is typically induced using colchicine, a chemical that disrupts microtubule formation during cell division, thereby preventing chromosome segregation and leading to genome doubling (Tu et al., 2007). Understanding meiotic stability and chromosome pairing is essential for overcoming sterility barriers in autotetraploid rice. Abnormal chromosome behavior in meiocytes of autotetraploid rice during meiosis compared to diploid hybrids is one of the key reasons for reduced fertility (Wu et al., 2015). Extensive cytological studies have demonstrated that during meiosis in autotetraploid rice, pollen mother cells (PMCs) exhibit various abnormalities in chromosome pairing and segregation. These include the presence of univalents and multivalents during prophase I, chromosome dragging in metaphase I, lagging chromosomes in anaphase I and II, and the formation of micronuclei in telophase II (Luan et al., 2007). These abnormalities result in the formation of non-haploid gametes, leading to decreased pollen fertility, affecting fertilization, and ultimately reducing seed setting rates. Therefore, the stability of the genome is crucial for the successful development and fertility of autotetraploid rice hybrids (Luan et al., 2007). 3.2 Genetic and epigenetic changes The transition to autotetraploidy involves not only chromosomal doubling but also significant genetic and epigenetic changes. Transcriptome analyses have revealed that autotetraploid rice hybrids exhibit differential gene expression profiles compared to their diploid counterparts. For instance, a study identified 807, 663, and 866 differentially expressed genes (DEGs) in the anther, ovary, and leaf tissues of F1 hybrid developed by crossing neo-tetraploid with autotetraploid rice, respectively (Guo et al., 2017). These DEGs are associated with various biological processes, including photosynthesis, metabolic processes, and transport, which are crucial for the enhanced heterosis observed in autotetraploid hybrids (Guo et al., 2017). Additionally, epigenetic modifications, such as DNA methylation and histone modifications, play a significant role in regulating gene expression in autotetraploid rice (Guo et al., 2017; Li et al., 2018). Specific genes related to meiosis and epigenetic regulation, such as RAD51 and SMC2, have been identified as key players in maintaining fertility and heterosis in autotetraploid rice (Guo et al., 2017). 3.3 Polyploidy and gene expression Polyploidy significantly impacts gene expression in autotetraploid rice, leading to both additive and non-additive gene expression patterns. Comparative studies between diploid and autotetraploid rice hybrids have shown that polyploidy enhances the interaction of pollen sterility loci, resulting in increased meiosis abnormalities and pollen sterility (Wu et al., 2015; Chen et al., 2020). Moreover, polyploidy induces non-additive gene expression, where certain genes exhibit expression levels that are not simply the sum of their expression in the diploid parents. This non-additive expression is often associated with heterosis and improved agronomic traits in autotetraploid rice hybrids (Guo et al., 2017; Chen et al., 2020). Gene ontology and pathway analyses have identified key genes involved in amino acid metabolism, photosynthesis, and meiosis, which contribute to the enhanced yield and fertility observed in autotetraploid rice hybrids (Bei et al., 2019; Chen et al., 2020). In summary, the genetic basis of autotetraploid rice involves complex interactions between chromosomal doubling, genetic and epigenetic changes, and polyploidy-induced gene expression. These factors collectively contribute to the stability, fertility, and enhanced heterosis of autotetraploid rice hybrids, making them a promising resource for future rice breeding programs. 4 Challenges of Sterility in Autotetraploid Rice Autotetraploid rice hybrids, while promising for their potential to enhance heterosis and increase yield, face significant challenges related to sterility. These challenges are primarily due to complex genetic and cytological factors that affect pollen and embryo sac development, leading to reduced fertility and seed set.
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