The Sichuan taimen (Hucho bleekeri) is a flagship species for the Yangtze River and is classified as critically endangered by the IUCN. Successful artificial breeding and conservation efforts are therefore essential for maintaining population stability. The early embryonic stage is the foundation of the entire life cycle and is critical for subsequent survival and growth. Here, we aimed to investigate gene-expression profiles across eight developmental stages through RNA-seq sequencing: fertilized egg, embryonic shield elevation, cleavage, blastula, gastrula, neurula, brain differentiation, and hatching. Time-series analysis revealed remarkable gene-expression changes between the cleavage and embryonic shield elevation, gastrula and blastula, and brain differentiation and hatching stages. The expression levels of cell cycle-related genes—including ccn2d, ccna2, cdk11, cdk17, cdka2, cdkl3, plk1, and others—decreased during embryonic development. Genes associated with muscle development, such as myl9, mylk, and tnnc2, were present in all stages and significantly enriched at hatching, while others were nearly absent during early development. In metabolic pathways, genes related to lipid metabolism and glycolysis were significantly expressed in the hatching stage. Regarding immune-related genes, complement genes were notably enriched at hatching, whereas cfh and cfb were expressed throughout development. Genes involved in adaptive immunity, such as mhc I, mhc II, tcr, and T-cell marker genes, were either not expressed or only weakly expressed in all stages. The results can provide insights into regulatory mechanisms underlying early embryonic development in fishes and provide general knowledge about salmonid development.
Chen et al. (Fri,) studied this question.