Ontogeny of Expression and Activity of Digestive Enzymes and Establishment of gh/igf1 Axis in the Omnivorous Fish Chelon labrosus

SIMPLE SUMMARY: Thick-lipped grey mullet (Chelon labrosus) feeds on the lowest trophic levels during adult stages, for which it is considered a viable candidate for an economically and environmentally sustainable aquaculture. Similar to most of marine fish species, C. labrosus produce a large number of eggs, leading to morphologically and anatomically larvae that are not completely mature and have to pass through substantial differentiation and development in their functional systems to acquire adult features. Therefore, the study of the development of digestive tract and of the growth regulation can provide useful information to adapt the feeding protocols and rearing conditions to the physiological requirements at each stage. This work aimed to evaluate the early ontogeny of key digestive enzymes and somatotropic factors at biochemical and/or transcriptional levels. Our results evidenced that maturation of the digestive system and acquisition of the adult mode of digestion occurs around 60 to 70 days post hatch (dph), when starch or other low-cost carbohydrate-based compounds could be used in formulated diets at increasing levels. Furthermore, our results implied an independent expression of the studied somatotropic genes during the first 40 dph and establishment of a functional growth hormone/insulin-like growth factor 1 axis from 50 dph onward. ABSTRACT: Thick-lipped grey mullet (Chelon labrosus) is a candidate for sustainable aquaculture due to its omnivorous/detritivorous feeding habit. This work aimed to evaluate its digestive and growth potentials from larval to early juvenile stages. To attain these objectives the activity of key digestive enzymes was measured from three until 90 days post hatch (dph). Expression of genes involved in digestion of proteins (try2, ctr, pga2, and atp4a), carbohydrates (amy2a), and lipids (cel and pla2g1b), together with two somatotropic factors (gh and igf1) were also quantified. No chymotrypsin or pepsin activities were detected. While specific activity of trypsin and lipase were high during the first 30 dph and declined afterward, amylase activity was low until 57 dph and increased significantly beyond that point. Expression of try2, ctr, amy2a, and cel increased continuously along development, and showed a peak at the end of metamorphosis. Expression of pla2g1b, pga2 and atp4a increased until the middle of metamorphosis and decreased afterwars. Most of these trends contrast the usual patterns in carnivorous species and highlight the transition from larvae, with high protein requirements, to post-larvae/juvenile stages, with omnivorous/detritivorous feeding preferences. Somatotropic genes, gh and igf1, showed approximately inverse expression patterns, suggesting the establishment of the Gh/Igf1 axis from 50 dph.