Temporal expression analyses of pancreatic and gastric digestive enzymes during early development of the olive flounder (Paralichthys olivaceus)

The olive flounder (Paralichthys olivaceus) is one of the most important aquaculture species in Korea. Previous studies focused on the morphological changes and only analysed digestive enzyme expression patterns of certain digestive systems. A comprehensive analysis of the pancreatic and gastric digestive enzymes of the olive flounder in its early developmental stages has not been performed. In this study, we determined the mRNA expression levels of pancreas and stomach digestive enzymes by extracting total RNA from the whole body of olive flounders from hatching to post-metamorphosis and conducting real-time PCR analysis with gene-specific primers for each isoform. The pancreatic digestive enzymes were initially and strongly expressed from the first feeding period to pre-metamorphosis. The expression of trypsinogen 3 was detected from the early stage of development to metamorphosis, whereas trypsinogen 2 was significantly increased only in the post-metamorphosis period. The mRNA expression of pepsinogen was only detected at metamorphosis and corresponded to stomach differentiation. The lipid digestive-related enzymes had already reached a certain level at the beginning of hatching, then increased during the early developmental stage and gradually decreased before metamorphosis. These results suggest that food ingestion is exclusively digested by pancreatic digestive enzymes and trypsinogen 3 during the early developmental stage until pre-metamorphosis, and it then fully switches to digestion by pepsinogen and trypsinogen 2 in mid-metamorphosis. In conclusion, understanding the expression profiles of digestive enzymes in the pancreas and stomach during the early developmental stages is an important consideration when developing formulated larval diets for aquacultured olive flounder.