Molecular Plant Breeding 2024, Vol.15, No.4, 167-177 http://genbreedpublisher.com/index.php/mpb 168 2 Autotetraploidy in Rice 2.1 Historical of autotetraploids rice The phenomenon of polyploidy, characterized by the multiplication of chromosome sets (often from diploid to tetraploid), is a widespread feature of higher plants. Polyploids are traditionally recognized as autopolyploids (originating from a single parent species) and allopolyploids (arising from two hybridizing species) (Clark and Donoghue, 2018). Autotetraploids are plants that have four sets of chromosomes derived from the doubling of a diploid genome. The concept of autotetraploidy in rice dates back to the 1930s when researchers first reported spontaneously autotetraploid rice and then attempted to create polyploid rice to exploit its potential yield advantages (Nakamori, 1933; Ichijima, 1934). In rice, autotetraploids are created by colchicine treatment, induceing chromosome doubling in diploid rice lines, resulting in plants with larger cell sizes, increased biomass, and often showing enhanced heterosis compared to their diploid counterparts (Luan et al., 2007; Tu et al., 2007; Chen et al., 2022). However, despite facing challenges such as high pollen sterility and low seed fertility, which have historically limited their commercial use (Oka, 1955; Wu et al., 2015; Guo et al., 2017; Chen et al., 2020), recent advancements have led to the development of fertile autotetraploid lines to overcome early setbacks. 2.2 Development of fertile autotetraploids rice To address the bottleneck issue of low fertility in autotetraploid rice, Chinese rice researchers have made significant efforts over many years. Through the hybridization of different types of autotetraploid rice, they have bred multiple varieties of highly fertile tetraploid rice, exhibiting high fertility and strong hetrosis, thus rekindling interest in autotetraploid rice breeding. Three representative categories are noteworthy: (1) PMeS (Polyploid Meiosis Stability) Polyploid Rice: Developed by Cai et al. (2007) at Hubei University, produced through indica-japonica hybridization, followed by backcrossing and selection, and subsequent chromosome doubling and breeding. Two japonica-type tetraploid cultivars, designated PMeS-1 and PMeS-2, both display an average seed setting rate above 70%. (2) High-Fertility Tetraploid Rice: From Chengdu Institute of Biology Chinese Academy of Sciences, Tu et al. (2003) reported high-fertility tetraploid rice sterile and restorer lines, achieving a three-line system for autotetraploid rice. They have created over 100 hybrid combinations. (3) Neo-Tetraploid Rice: Derived from the crossing of autotetraploid rice Jackson-4x with tetraploid rice line 96025, followed by multiple generations of selfing and selection. Given that the genome of autotetraploid rice can be represented as AAAA, the genome of the neo-tetraploid rice should theoretically be A1A1A2A2, indicating a combination of two distinct autotetraploid genomes. Unlike allotetraploid rice, where chromosome pairing is exclusively bivalent, in autotetraploid rice, chromosome pairing is primarily a mix of bivalents and quadrivalents. The meiotic prophase I chromosome pairing in all three high-fertility tetraploid rice varieties is primarily characterized by bivalents and quadrivalents, with a low frequency of univalents, trivalents, and multivalents (pentavalents and above), as well as reduced abnormal chromosome behavior (Table 1) (Liu et al., 2023). This is likely one of the key reasons for their high fertility. 2.3 Potential advantages of autotetraploid rice Autotetraploid rice offers several potential advantages over diploid rice. The most significant benefit is the enhanced heterosis, or hybrid vigor, observed in autotetraploid hybrids. Studies have shown that autotetraploid hybrids can exhibit significantly higher yields, improved biomass, and better stress tolerance compared to diploid hybrids (Figure 1) (Tu et al., 2007; Chen et al., 2020; Ghaleb et al., 2020). Additionally, autotetraploid rice lines have been found to possess greater genetic diversity, which can be harnessed to develop new and improved rice varieties (Wu et al., 2013). The development of fertile autotetraploid lines, such as the neo-tetraploid rice, has further demonstrated the feasibility of using autotetraploids in commercial rice breeding programs (Guo et al., 2017; Wu et al., 2019). By overcoming the sterility barriers and leveraging the advantages of autotetraploidy, researchers aim to develop high-yielding, resilient rice varieties that can contribute to global food security.
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