Cloning and expression analysis of three digestive enzymes from Atlantic halibut (Hippoglossus hippoglossus) during early development: Predicting gastrointestinal functionality

The objective of the present study was to describe the histological and physiological development of the gastrointestinal system in Atlantic halibut from the time of first-feeding until metamorphosis. At first-feeding, the gastrointestinal (GI) tract is divided into anterior, mid and hindgut regions. The liver is present, as is the pancreas. During development the pancreas changes from a compact organ to a diffuse tissue interspersed through much of the mesentery surrounding the GI tract. Functional gastric glands are not present until approximately 66 days post-hatch (dph). Using primers based upon winter flounder digestive enzyme gene sequences, we were able to amplify partial sequences for bile salt-activated lipase (BAL), trypsinogen (TRP), and pepsinogen (PEP) from RNA extracted from whole larvae and juveniles using Reverse Transcription-PCR (RT-PCR). PCR products were sequenced and the sequences used to design halibut gene-specific primers for BAL and PEP. RT-PCR analysis revealed that BAL and TRP gene expression was evident at least from the time of first-feeding but PEP gene expression was not detectable until 80 dph. In situ hybridization using molecular probes from winter flounder sequences localized expression of BAL and TRP to the exocrine pancreas. PEP expression was only localized to the glandular regions of the stomach. These data provide a first step toward understanding the molecular events governing the ontogeny of digestive capacity in Atlantic halibut.