n-3 PUFA biosynthesis by the copepod Apocyclops royi determined by fatty acid profile and gene expression analysis.

The cyclopoid copepod Apocyclops royi (Lindberg 1940) is one of two dominant mesozooplankton species in brackish Taiwanese aquaculture ponds. Especially periodical low n-3 polyunsaturated fatty acids (PUFA) content of seston could potentially be a limiting factor for the zooplankton diversity. A. royi' s potential ability to biosynthesize n-3 PUFA was investigated through a short-term feeding experiment on four species of microalgae. Furthermore, we analyze the expression of genes encoding putative fatty acid elongase ( ELO ) and desaturase ( FAD ) enzymes in A. royi on long-term diets of the PUFA poor D. tertiolecta and the PUFA rich I. galbana The copepods exhibited high contents of docosahexaenoic acid (DHA, C22:6n-3) (> 20% of total fatty acid) even when DHA-starved for two generations, and no significant differences were found in absolute DHA content between treatments. Transcripts correlating to enzymes in the n-3 PUFA biosynthetic pathway were identified: Elovl4, Elovl5, Fad Δ5 and Fad Δ6. Gene expression analysis revealed a significantly higher expression of two desaturases similar to Fad Δ6, in copepods fed PUFA-lacking algae compared to copepods fed algae with high PUFA content. These findings suggest a highly active n-3 PUFA biosynthesis and capability of DHA production of A. royi when fed low-PUFA diets.