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Investigating fatty acid metabolism for sustainable farming of cobia Rachycentron canadum L., a promising candidate for diversifying European aquaculture

Final Report Summary - COBIAGENE (Investigating fatty acid metabolism for sustainable farming of cobia Rachycentron canadum L., a promising candidate for diversifying European aquaculture)

The investigation of the long-chain polyunsaturated fatty acid (LC-PUFA) biosynthetic pathway in fish has attracted great interest due to the crucial role that fish play as the primary source of healthy n-3 LC-PUFA in the human food chain. With declining fisheries worldwide, farmed fish constitute an ever-increasing proportion of the fish in the human food basket amounting to almost 50 % in 2008. Until now, high n-3 LC-PUFA levels in farmed fish have been obtained by the use in the feeds of fish oils (FOs), paradoxically themselves derived from industrial marine fisheries, but this is not sustainable and is constraining continued growth of aquaculture activities. In consequence, alternatives to FOs are urgently required, with vegetable oils (VOs) as the prime candidates. However, VOs are rich in C18 PUFA, but devoid of the C20-22 LC-PUFA abundant in FOs. Use of VOs in aquafeeds can therefore compromise the provision of essential n-3 LC-PUFA if fish fed on them have limited enzymatic complement required for conversion of C18 PUFA to C20-22 LC-PUFA. Two types of enzymes are involved in the LC-PUFA biosynthetic pathway, the fatty acyl desaturases (Fad), which introduce double bonds, and elongases of very long-chain fatty acids (Elovl), which account for the condensation of 2C to the pre-existing fatty acid substrate.

The overall objective of COBIAGENE project was to investigate the basis of regulation of LC-PUFA synthesis in cobia, Rachycentron canadum, an excellent candidate for diversifying marine aquaculture. The primary objectives were:
1) the molecular cloning of genes encoding Fad and Elovl;
2) their functional characterisation in yeast
3) their quantitative expression profiles among tissues;
4) to determine the gene structure and promoter analyses.

The huge success of the COBIAGENE project is demonstrated by the number of scientific papers (eight) published in international peer-reviewed journals, including key papers presenting the molecular cloning, functional characterisation and tissue expression analysis of three genes involved in the LC-PUFA pathway in cobia (a ?6-like Fad, and elongases Elovl4 and Elovl5). Additionally, the structure of the Fad encoding gene was determined, demonstrating that it consists of 13 exons coveringing 5 219 bp of genomic DNA. Moreover, the fad promoter activity was assayed using a luciferase reporter gene expression system and a region of approximately 300 bp upstream of the transcription start site (TSS) was identified that contained the putative promoter region.

The enormous and rapid progress of COBIAGENE activities allowed the fellow to participate in further studies with similar aims and objectives on other fish species. Among all, investigations of two Fad genes isolated from rabbitfish have shown extraordinary functions. On one hand, rabbitfish was found to have a dual ?6/?5 activity similar to a Fad previously found in zebrafish. Even more outstanding was the fact that a second Fad had ?4-desaturation ability, this being the first desaturase ever reported in vertebrates with such an activity. Other notable achievements include the cloning of two Fad in Atlantic salmon, one in the Asian sea bass and another one in cobia.

COBIAGENE also made great advances in the elongases (Elovl), with the cloning and functional analysis of different members of the Elovl protein family (Elovl2-, Elovl4- and Elovl5-like). Particularly noteworthy were the investigations performed on fish Elovl4, a type of elongase that had not been previously investigated in fish, and whose functions indicate critical roles in the production of the very long-chain polyunsaturated fatty acids (VLC-PUFA), a group of compounds with lengths > C24. These findings highlighted the importance of VLC-PUFAs and their biosynthesis in farmed fish in which altered visual acuity, fertility issues of broodstock, and disruptions of brain functioning can affect growth performance and the economical profit of the farm.