Expression of fatty acyl desaturase and elongase genes, and evolution of DHA:EPA ratio during development of unfed larvae of Atlantic bluefin tuna (Thunnus thynnus L.)

Abstract

The concentration of n-3 long-chain polyunsaturated fatty acids (LC-PUFA) in neural tissues is known to be crucial for effective prey capture from the time of first feeding in marine fish larvae. Furthermore, tissues of tunids, including Atlantic bluefin tuna, have relatively high levels of DHA (docosahexaenic acid, 22:6n-3) and a high ratio of DHA:EPA (eicosapentaenoic acid; 20:5n-3) compared to most other species. Although the lipid biochemistry underpinning the high DHA:EPA ratio in tuna is unclear, it has been generally assumed that they must selectively accumulate and retain DHA in their tissues. In the present study, we investigated lipid and fatty acid metabolism during early development of Atlantic bluefin tuna and determined the changes in lipid content, lipid class composition and total, phospholipid and neutral lipid fatty acid profiles in unfed larvae during yolk sac utilisation. In addition, we studied the LC-PUFA biosynthesis pathway by quantifying expression of fatty acyl desaturase and elongase genes. To this end, we cloned and functionally characterized two cDNAs by heterologous expression in yeast, showing them to code for a Δ6 desaturase and Elovl5 elongase, respectively, that could both be involved in the conversion of EPA to DHA. The level of DHA was maintained, but the proportion of EPA declined, and so the DHA:EPA ratio increased in bluefin tuna larvae during yolk sac utilization. Although this could be the result of relative retention of DHA during a period of generally high fatty acid oxidation and utilization, there was also a great increase in desaturase and elongase expression with larval development. This suggests that increased activity of these enzymes is important for the normal development of tuna larvae related to the provision of adequate DHA for the formation of biomembranes, particularly in neural (brain and eye) tissues.