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

Clones for two forms of pepsinogen A differing in isoelectric point as well as both alpha and beta subunits of the proton pump were isolated from the stomach of winter Bounder Pleuronectes americanus. All clones gave positive hybridization signals with DNA from other flatfish species. Reverse transc ription-polymerase chain reaction (RT-PCR) analysis revealed that the two f orms of pepsinogen were sequentially expressed. Expression of pepsinogen II a was first detected in 13 days post-hatch (dph) larvae and increased towar ds the beginning of metamorphosis at 20 dph. Expression of pepsinogen IIb a nd proton pump genes was first detected at 20 dph and coincided with the ap pearance 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 lar vae were similar to those recorded in metamorphosed larvae (27 dph) and juv eniles (41 dph). High pepsin activity and levels of expression of the pepsi nogen and proton pump genes observed in 20-dph larvae indicate an advanced level of development and functionality of the winter flounder stomach at th is stage of ontogeny. This advanced level of the stomach development at 1 w eek before the currently practised time of weaning suggests that feeding fo rmulated feeds may be initiated earlier. (C) 1999 The Fisheries Society of the British Isles.