Among these, genes involved in lipid metabolism and, particularly, in LC PUFA biosynthesis, were found to be together up regulated in fish fed VD. Not surprisingly, delta 6 desaturase, steroyl CoA desaturase 9 and NADH cytochrome b5 reductase, involved in long chain fatty acid desaturation and or elongation, were up regulated in VD fed fish. It is indeed established that in most fish species that the LC PUFA biosynthetic pathway is posi tively regulated in response to the use of a diet poor in LC PUFA but rich in PUFA, although this regulation depends on fish species, degree of fish oil substitution, nature of the vegetable oil, and environmental para meters. The positive regulation of the LC PUFA biosynthetic pathway is in agreement with results obtained by tran scriptomic approaches in salmonids fed vegetable oil.

Altogether, the expression data obtained in mar ine fish species and salmonids indicate that the different capacity of marine and fresh water species to grow on a LC PUFA deprived diet does not seem to be due to a dif ferent transcriptional regulation of key genes involved in lipid synthesis, such as fads2 or scd9. Indeed, the level of induction of fads2 expression found in this experiment is of similar amplitude to that observed in the liver and intestine of Atlantic salmon. The stimulation of this biosynthetic pathway in fish fed a diet poor in LC PUFAs can be explained by the fact that LC PUFAs play several key physiological roles in vertebrates, particularly in fish.

For example, fish are poikilothermic organisms and therefore need a high degree of unsaturation of LC PUFA included in mem brane phospholipids to maintain phospholipid bilayer fluidity at reduced temperature. LC PUFA, espe cially arachidonic acid and eicosapentaenoic acid, are also precursors of eicosanoids, which are involved in pro and anti inflammatory pathways. This hypothesis is reinforced by our data, indicating a stimulation of genes involved in phospholipids bio synthesis when fish were fed the VD. Despite this stimulation of LC PUFA and the phos pholipid biosynthesis pathway at the transcriptional level, our investigation of fatty acid profiles indicated that the amounts of LC PUFA, particularly eicosapen taenoic acid and docosahexaenoic acid, were still considerably lower in the flesh of fish from both half sibfamilies fed VD in comparison with fish fed FD.

This finding is in agreement with those previously obtained, which revealed that the stimulation of fads2 expression in fish fed a vegetable diet was not associated with an induction of its enzymatic activity, suggesting a post transcriptional regulation of fads2 expression. Such EPA and DHA deficiency can notably explain the growth deficiency observed in fish fed VD, as well as effects Drug_discovery observed on immune function.