Comparative analysis of spleen structure, biochemical parameters, and transcriptome of adult and juvenile yellowfin tuna (Thunnus albacares) in the South China Sea

As one of the top predators in the ocean, yellowfin tuna possesses physiological characteristics that are highly adapted to its high-speed swimming habits, such as high cardiac output and efficient oxygen uptake and transportation systems, which enable it to swim rapidly various diverse layers of the water for feeding activities. These physiological characteristics are intricately associated with the efficient hematopoietic function of its spleen, which plays a crucial role in maintaining its long-distance migration and sustained physical activity in particular. However, there are fewer studies on the developmental biology and function of the spleen in this species. In order to investigate the changes in spleen structure and function during the development of yellowfin tuna, this study compared the histological characteristics, biochemical indexes and transcriptome profiles of the spleen in adult and juvenile yellowfin tuna from the South China Sea. Hematoxylin and eosin (H&E), Masson, and reticular fiber staining revealed that the proportion of white pulp and the mean number of blood sinus in the spleen of adult fish were significantly less than those in juvenile fish (p < 0.05), while the relative area of red pulp displayed no significant difference between the two groups. In addition, the contents of granulocyte-macrophage colony-stimulating factor, erythropoietin, thrombopoietin, and stromal cell-derived factor 1 were significantly lower in the spleen of adult fish than in juvenile fish (p < 0.001), while the contents of bone morphogenetic protein 2 and transforming growth factor β1 were significantly increased in juvenile fish spleens (p < 0.001). Comparative transcriptome analysis revealed that there were 1255 differentially expressed genes (DEGs) between adult and juvenile fish, of which 477 were upregulated and 778 were down-regulated. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses revealed that these DEGs were mainly related to embryonic hematopoiesis, the development of blood and lymphoid organs, and the HIF-1 signaling pathway. Key DEGs associated with hematopoietic function were further identified, such as pik3r3b, gata1a, klf1, epor, and lmo2. In conclusion, this study offers a comprehensive comparison of spleen histology, cytokine activities related to hematopoiesis and cell development, and transcriptomic differences between adult and juvenile yellowfin tuna. These findings provide valuable insights into the spleen hematopoietic development mechanism of decoding yellowfin tuna and other tuna species.