Expression of long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis genes and utilization of fatty acids during early development in rabbitfish Siganus canaliculatus

Rabbitfish S. canaliculatus is the first marine fish reported to possess long-chain polyunsaturated fatty acids (LC-PUFA) biosynthetic ability, but the initiation of the LC-PUFA biosynthesis in this species is still unknown. In this study, the gene expression of Δ5/Δ6 fatty acid desaturase (Fad), Δ4 Fad and two fatty acid elongases, four key enzymes for LC-PUFA biosynthesis, were measured during the embryonic and unfed larval development of rabbitfish. Correspondingly, the fatty acid composition of rabbitfish embryos was determined. The results showed that DHA was the primary fatty acid utilized at the zygotic stages. The mRNA expression of the four key genes was detected from 8 hpf (hours post-fertilization) onwards, with a low level of elovl5 mRNA contrary to the high expression of Δ4 fad mRNA. Subsequently, the elevated expression of Δ5/Δ6 fad and two elongase genes was observed after hatching, and then a great increment in the expression of all four key genes was detected at 3-4 dph (days post-hatching). These results indicate the production of DHA from EPA via the “Δ4 Fad” pathway as well as the alternative “Sprecher” pathway at the yolk-sac larval stages. Furthermore, the entire LC-PUFA biosynthetic pathway of converting C18 PUFA precursors to DHA is activated just after the onset of first-feeding. This study provides information on LC-PUFA biosynthesis during early development in rabbitfish, and offers an approach to determine the nutritional requirements of first-feeding larvae for rabbitfish and other marine fish.