Polyploidy, the condition of having more than two sets of chromosomes, is common in crop domestication and improvement, driving traits such as higher biomass, stress tolerance, and novel fruit qualities. Traditionally, chromosome doubling in watermelon has relied on tissue culture techniques, which are inefficient and often unpredictable. In Arabidopsis thaliana, disruption of the OSD1 gene leads to the formation of unreduced gametes, laying the foundation for engineering apomixis systems. However, cucurbit species contain only a single homolog of OSD1/UVI4, raising questions about whether their functions diverge from model plants. Because of these challenges, it is necessary to conduct in-depth research on the role of ClOSD1 in watermelon.
A research team from Northwest A&F University and Kaifeng Academy of Agriculture and Forestry Sciences reports new insights into chromosome regulation in watermelon. Their study, published (DOI: 10.1093/hr/uhae288) online in Horticulture Research on January 1, 2025, reveals that disruption of the ClOSD1 gene induces both somatic and gametic ploidy doubling. Using CRISPR/Cas9 editing, the team created mutant plants that displayed tetraploidization and abnormal reproductive behaviors, offering the first evidence that OSD1 in watermelon simultaneously regulates mitosis and meiosis, a dual role distinct from findings in model species.
Through sequence alignment, researchers identified ClOSD1 as the sole homolog of OSD1/UVI4 in watermelon. Expression analysis showed the gene is active in vegetative tissues and reproductive organs, with peaks during meiotic prophase I. To determine its function, CRISPR/Cas9 mutants were created, including homozygous, heterozygous, and biallelic lines. All mutants exhibited enhanced growth traits, such as deeper green leaves, larger flowers, and thicker stems. Flow cytometry and chromosome counts confirmed nearly doubled ploidy compared with wild-type plants, indicating that endomitosis, not tissue culture artifacts, caused the observed chromosome doubling.
Mutants also showed altered gamete development. Homozygous mutants produced 100% dyads instead of tetrads, yielding tetraploid pollen, while heterozygotes generated a mix of diploid and tetraploid gametes. Reciprocal crosses revealed reduced seed set, with most seeds empty and only a few viable triploids produced. These findings indicate a dosage-dependent role of ClOSD1: somatic cells are highly sensitive to reduced protein levels, while reproductive cells retain partial tolerance. Overall, the study demonstrates that ClOSD1 disruption drives both somatic and gametic ploidy doubling, establishing a unique regulatory mechanism in watermelon distinct from Arabidopsis.
“Our results reveal an unexpected divergence in how OSD1 functions across species,” said Dr. Li Yuan, corresponding author of the study. “While in Arabidopsis the disruption mainly affects gamete formation, in watermelon it simultaneously triggers somatic chromosome doubling. This dual effect opens new opportunities for polyploid crop breeding but also cautions against directly applying apomixis strategies developed in model plants to cucurbits. Understanding these species-specific mechanisms is critical for designing targeted genetic approaches to harness polyploidy while preserving fertility.”
This research provides a genetic mechanism for inducing polyploidy in watermelon, a globally important horticultural crop. By manipulating ClOSD1, breeders could develop stable polyploid lines with enhanced vigor and potentially improved fruit traits. However, the concurrent induction of unreduced gametes and fertility issues highlights challenges for using this approach in hybrid seed production or clonal reproduction. Future studies will explore alternative genes, such as PS1 or TAM, to establish apomixis systems in cucurbits. Ultimately, this work lays the groundwork for precision ploidy engineering in watermelon and related horticultural crops.
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References
DOI
10.1093/hr/uhae288
Original Source URL
https://doi.org/10.1093/hr/uhae288
Funding information
This work was supported by the National Natural Science Foundation of China (32372734), the National Youth Talent Program (A279021801), the National Key Research and Development Program of China (2023YFE0206900), the Key Program of the National Natural Science Foundation of China (U23A20208), Luoyang Major Science and Technology Tackling Key Issues Project (Public Announcement and Leadership, 2301024A), Earmarked Fund for China Agriculture Research System (CARS-25), Key-Area R&D Program of Guangdong Province (2022B0202060001), Key R&D Program of Shaanxi province (2023-YBNY-008,2024NC-YBXM-032), and the Central Guidance on Local Science and Technology Development Fund of Henan Province Z202318111037.
About Horticulture Research
Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2023. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.