Aquaculture is crucial to meet the growing global demand for protein. But how can we optimize fish growth to achieve a more sustainable and productive industry? Hybridization and polyploidization, where fish combine genetic material from different species or have additional sets of chromosomes, offer exciting possibilities.
Researchers are unlocking the secrets of triploid fish, a special type of fish with faster growth rates and larger body sizes compared to their diploid parents.
A study by scientists from Hunan Normal University (China) and South China Agricultural University, published in KeAi Journal Reproduction and Breeding, explores how these techniques unlock superior growth traits in triploid fish, delving into the intriguing role of mitochondrial DNA (mtDNA) and its interaction with nuclear genes. Understanding the relationship between mtDNA and animal growth could provide valuable information for selective breeding in aquaculture.
The power of combining genomes
Hybridization, the crossing of different species, and polyploidization, the presence of additional chromosomes, are powerful tools in plant and animal breeding. In fish, interspecific hybridization has been successfully used in carp, salmon, and even cichlids to create varieties with exceptional economic characteristics, such as faster growth.
The research focuses on the red crucian carp and the common carp, which share a distant hybridization and genome duplication event millions of years ago.
The potential of triploid fish
Triploid fish, with three sets of chromosomes, offer a clear advantage. Since their gonads (reproductive organs) are underdeveloped, they channel their energy into growth, leading to faster growth rates compared to their diploid parents (two sets of chromosomes).
This research investigates two populations of allotriploid fish obtained by crossing the alopolyploid lineage with both red crucian carp and common carp.
When fish are bred for aquaculture, scientists often find phenotypic differences between parents and offspring. Allotripoids, fish with three sets of chromosomes instead of the usual two (diploids), are a good example. Studies have shown that some allotriploid populations exhibit significantly faster growth rates and higher body weight, making them ideal candidates for boosting fish production.
The study: Revealing the growth engine
The study focused on three fish species:
- Red crucian carp (Carassius auratus red var.)
- Common carp (Cyprinus carpio L.)
- Two allotriploid variations (3nR2C and 3nRC2)
These allotripoids were created by backcrossing an alotetraploid (a fish with four sets of chromosomes) with the red crucian carp and the common carp, respectively. The resulting allotripoids have been decisive in the growth of China’s aquaculture industry for decades.
The researchers conducted a meticulous comparison of the number of copies of mitochondrial DNA (mtDNA) in the muscle and liver tissues of these fish in different seasons. Mitochondria, the “powerhouses” of cells, play a crucial role in energy production. By analyzing the number of mtDNA copies, scientists were able to assess mitochondrial activity levels.
“Long-distance hybridization rapidly generates diverse genotypes and phenotypes, offering a rich resource for studying the role of genetic regulation in shaping phenotypes,” explains co-author Li Ren, researcher at the State Key Laboratory of Developmental Biology of Freshwater Fish at Hunan Normal University. “Analysis of muscle fiber diameter and mtDNA copy number revealed a positive correlation between individual size and muscle fiber diameter in all fish types. Additionally, a strong correlation emerged between muscle mtDNA copy number and body weight.”
Winter challenges and metabolic change
The study revealed a fascinating adaptation: during the winter, when the water temperature drops (around 13°C), allotriploid fish showed a decrease in the number of mtDNA copies specifically in their muscle tissues. This suggests a reduction in individual activity and energy expenditure. This metabolic change likely helps them conserve energy and maintain body weight when food availability is limited in the colder months.
“This decrease likely corresponds to a reduction in individual activity and energy expenditure aimed at maintaining body weight, which could contribute to the growth heterosis observed in allotriploid fish,” adds Ren.
In addition, the research team investigated the expression levels of specific genes involved in mitochondrial function. They observed an imbalance in the allelic expression of certain genes in allotriploid fish. This imbalance could potentially influence mitochondrial activity and contribute to their faster growth rates.
Beyond DNA: the role of genes
The study also explored the expression levels of specific genes involved in mitochondrial function. Interestingly, the researchers observed an imbalance in the expression of certain genes within triploid fish. This imbalance suggests a complex interaction between genes and ploidy levels, which ultimately influences growth traits.
The researchers analyzed the expression levels of three nuclear-encoded mitochondrial genes (tfam, tfb1m, and tfb2m). Interestingly, they found unbalanced allelic expression of tfam and tfb1m in allotriploid fish. Ren highlights the complex relationship between mtDNA: “The copy number and transcriptional efficiency of animal mtDNA are intimately linked not only to the characteristics of the mtDNA genome itself but also to the expression of specific nuclear genes. However, at present, a direct link between these three nuclear-encoded mitochondrial genes and mtDNA copy number variation remains elusive.”
A leap forward in fish farming
These findings provide valuable insights into the molecular mechanisms that regulate growth differences between fish with different ploidy levels. This knowledge can be leveraged to develop more targeted breeding programs for specific fish species, ultimately leading to a more sustainable and productive aquaculture industry.
Conclusion
“The findings enhance our understanding of the molecular regulatory mechanisms underlying differences in growth traits between fish with different ploidy levels,” the scientists conclude.
By intelligently adjusting their metabolism during resource scarcity, these fish demonstrate a remarkable adaptation for growth. This research paves the way for future advancements in fish breeding, potentially revolutionizing the aquaculture industry.
This research was supported by the National Natural Science Foundation of China, the Hunan Provincial Natural Science Foundation, the Huxiang Youth Talent Support Program, the Laboratory of Lingnan Modern Agriculture Project, the Special Funds for Construction of Innovative Provinces in Hunan Province, framed within the funds of China Agriculture Research System (CARS-45), and the 111 Project.
Contact
Shaojun Liu
State Key Laboratory of Developmental Biology of Freshwater Fish, Engineering Research Center of Polyploid Fish Reproduction and Breeding of the State Education Ministry, College of Life Sciences, Hunan Normal University, Changsha, 410081, China.
Email: lsj@hunnu.edu.cn
Li Ren
Email: renli_333@163.com
Reference (open access)
Zhang, H., Luo, M., Tai, Y., Li, M., Cui, J., Gao, X., Ren, L., & Liu, S. (2024). MtDNA copy number contributes to growth diversity in allopolyploid fish. Reproduction and Breeding, 4(2), 55-60. https://doi.org/10.1016/j.repbre.2023.12.008
