Solea senegalensis is an unusual marine teleost with very low dietary requirement for longchain polyunsaturated fatty acids (LC-PUFA) during early development. Aquaculture is rapidly becoming the main source of health-beneficial fish products for human consumption. This, associated with limited supply of LC-PUFA-rich ingredients for fish feeds, render S. senegalensis a highly interesting species in which to study the LC-PUFA biosynthesis pathway. We have cloned and functionally character -3 series) and elovl5 with the potential to catalyze docosahexaenoic acid (DHA) biosynthesis from eicosapentaenoic acid (EPA). Changes in expression of both transcripts were determined during embryonic and early larval development, and transcriptional regulation in response to dietary n-3 LC-PUFA was assessed during larval and post-larval stages. There was a marked pattern of regulation during early ontogenesis, with transcripts showing peak expression coinciding with the start of exogenous feeding. Although elovl5 transcripts were present in fertilized DHA/EPA ratio (~ 11) to meet the high demands for early embryonic development. Fatty acid profile was significantly up-regulated in response to LC-PUFA-poor diets, which may suggest biological relevance of this pathway in reducing LC-PUFA dietary requirements in this species, compared to other marine teleosts. These results indicate that sole is capable of synthesizing DHA from EPA through a Sprecher-independent pathway. Suggested Reviewers: Xiaozhong Zheng xzheng@staffmail.ed.ac.uk Dr Zheng has done considerable work involving cloning and functional characterization of Fads and Elovls in fish and is therefore a very specific expert in this field.Johnathan A. Napier jon.napier@bbsrc.ac.uk Prof. Napier is an expert in Fad molecular biology and has considerable expertise in pathways of DHA biosynthesis.Chantal Cahu Chantal.Cahu@ifremer.fr Dr. Chantal Cahu is an expert in marine larvae nutrition including fatty acid metabolism. In addition, she has also been involved in molecular and genomic studies, including the cloning and Fad expression in fish.Gabriel Mourente gabriel.mourente@uca.es Dr. Gabriel Mourente is an expert in marine larvae nutrition including fatty acid metabolism. He has additionally performed several larval nutrition studies in S. senegalensis and has also worked specifically with enzymes of the pathway of LC-PUFA biosynthesis.Manuel Yúfera manuel.yufera@icman.csic.es Dr. Manuel Yúfera is an expert in marine larvae nutrition including fatty acid metabolism and has additionally performed several larval nutrition studies in S. senegalensis. Fatty acyl desaturase and elongase cDNAs corresponding to a 4fad (with some 5 activity for PUFA of the n-3 series) and elovl5 were cloned from a marine teleost (Solea senegalensis) and functionally characterized by heterologous expression in yeast. Historically, it was believed that the pathway for DHA synthesis would involve a 4 desaturase, as this would be the most direct route, but the presence of 4Fad enzymes cou...

Dietary fatty acid supply can affect stress response in fish during early development. Although knowledge on the mechanisms involved in fatty acid regulation of stress tolerance is scarce, it has often been hypothesised that eicosanoid profiles can influence cortisol production. Genomic cortisol actions are mediated by cytosolic receptors which may respond to cellular fatty acid signalling. An experiment was designed to test the effects of feeding gilthead sea-bream larvae with four microdiets, containing graded arachidonic acid (ARA) levels (0·4, 0·8, 1·5 and 3·0 %), on the expression of genes involved in stress response (steroidogenic acute regulatory protein, glucocorticoid receptor and phosphoenolpyruvate carboxykinase), lipid and, particularly, eicosanoid metabolism (hormone-sensitive lipase, PPARa, phospholipase A 2 , cyclo-oxygenase-2 and 5-lipoxygenase), as determined by real-time quantitative PCR. Fish fatty acid phenotypes reflected dietary fatty acid profiles. Growth performance, survival after acute stress and similar whole-body basal cortisol levels suggested that sea-bream larvae could tolerate a wide range of dietary ARA levels. Transcription of all genes analysed was significantly reduced at dietary ARA levels above 0·4 %. Nonetheless, despite practical suppression of phospholipase A 2 transcription, higher leukotriene B 4 levels were detected in larvae fed 3·0 % ARA, whereas a similar trend was observed regarding PGE 2 production. The present study demonstrates that adaptation to a wide range of dietary ARA levels in gilthead sea-bream larvae involves the modulation of the expression of genes related to eicosanoid synthesis, lipid metabolism and stress response. The roles of ARA, other polyunsaturates and eicosanoids as signals in this process are discussed.

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Lipid nutrition of marine fish larvae has focused on supplying essential fatty acids (EFA) at high levels to meet requirements for survival, growth and development. However, some deleterious effects have been reported suggesting that excessive supply of EFA might result in insufficient supply of energy substrates, particularly in species with lower EFA requirements such as Senegalese sole (Solea senegalensis). This study addressed how the balance between EFA and non-EFA (better energy sources) affects larval performance, body composition and metabolism and retention of DHA, by formulating enrichment emulsions containing two different vegetable oil sources (olive oil or soybean oil) and three DHA levels. DHA positively affected growth and survival, independent of oil source, confirming that for sole post-larvae it is advantageous to base enrichments on vegetable oils supplying higher levels of energy, and supplement these with a DHA-rich oil. In addition, body DHA levels were generally comparable considering the large differences in their dietary supply, suggesting that the previously reported ∆4 fatty acyl desaturase (fad) operates in vivo and that DHA was synthesized at physiologically significant rates through a mechanism involving transcriptional up-regulation of ∆4fad, which was significantly up-regulated in the low DHA treatments. Furthermore, data suggested that DHA biosynthesis may be regulated by an interaction between dietary n - 3 and n - 6 PUFA, as well as by levels of LC-PUFA, and this may, under certain nutritional conditions, lead to DHA production from C18 precursors. The molecular basis of putative fatty acyl ∆5 and ∆6 desaturation activities remains to be fully determined as thorough searches have found only a single (∆4) Fads2-type transcript. Therefore, further studies are required but this might represent a unique activity described within vertebrate fads