mtDNA copy number contributes to growth diversity in allopolyploid fish

Phenotypic differences between diploid parents and their allotriploid offspring are common in aquaculture breeding. Some allotriploid populations exhibit rapid growth rates and increased body weight, which are significant for supporting fisheries development. Understanding the genetic mechanisms underlying these traits is crucial for implementing diverse breeding strategies to achieve high production in fish farming. Here, we collected the following fish species for our study: red crucian carp (Carassius auratus red var., 2nRR), common carp (Cyprinus carpio L., 2nCC), and two allotriploids (3nR2C and 3 nRC2). These allotriploids were obtained through backcrossing an allotetraploid of C. auratus red var. × C. carpio L. (4nR2C2, ♂) with C. auratus red var. and C. carpio L. (♀), respectively. These allotriploids demonstrated faster growth rates compared to their diploid inbred parents, contributing to the Chinese fisheries industry for several decades. We conducted a systematic comparison of mitochondrial DNA (mtDNA) copy numbers in the liver and muscle tissues of 2nRR, 2nCC, 3nR2C, and 3 nRC2 under different seasons. When entering winter (low water temperature: 13 °C), the triploid fish (3nR2C and 3 nRC2) exhibited lower mtDNA copy numbers in the muscle, indicating a reduction in individual activity and energy expenditure to facilitate weight maintenance when food availability is limited. Furthermore, we analysed the expression levels of three nuclear-regulated mitochondrial genes (tfam, tfb1m, and tfb2m) and observed an imbalance of allelic expression in tfam and tfb1m in the two triploid fish. These findings enhance our understanding of the molecular regulatory mechanisms underlying growth trait differences among fish with different ploidy levels. •Higher weight and a larger muscle fiber were observed in triploids than in diploid parents.•A lower mtDNA copy number during the winter indicates weight maintenance.•Higher expression of tfam and tfb1m in parental 2nRR compared to 2nCC.•Triploids exhibit an allelic imbalance associated with the allele originating from 2nRR.