Ontogeny of the stomach in winter flounder: characterization and expression of the pepsinogen and proton pump genes and determination of pepsin activity

Abstract

Clones for two forms of pepsinogen A differing in isoelectric point as well as both α and β subunits of the proton pump were isolated from the stomach of winter flounder Pleuronectes americanus . All clones gave positive hybridization signals with DNA from other flatfish species. Reverse transcription‐polymerase chain reaction (RT‐PCR) analysis revealed that the two forms of pepsinogen were sequentially expressed. Expression of pepsinogen IIa was first detected in 13 days post‐hatch (dph) larvae and increased towards the beginning of metamorphosis at 20 dph. Expression of pepsinogen IIb and proton pump genes was first detected at 20 dph and coincided with the appearance of gastric glands and the increase in pepsin activity. The levels of expression of pepsinogen IIb as well as of pepsin activity in 20‐dph larvae were similar to those recorded in metamorphosed larvae (27 dph) and juveniles (41 dph). High pepsin activity and levels of expression of the pepsinogen and proton pump genes observed in 20‐dph larvae indicate an advanced level of development and functionality of the winter flounder stomach at this stage of ontogeny. This advanced level of the stomach development at 1 week before the currently practised time of weaning suggests that feeding formulated feeds may be initiated earlier.