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Long-chain polyunsaturated fatty acids (LC-PUFAs) such as arachidonic (ARA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids are essential components of biomembranes, particularly in neural tissues. Endogenous synthesis of ARA, EPA and DHA occurs from precursor dietary essential fatty acids such as linoleic and α-linolenic acid through elongation and  Δ5  and  Δ6  desaturations. With respect to desaturation activities some noteworthy differences have been noted in vertebrate classes. In mammals, the  Δ5  activity is allocated to the  Fads1  gene, while  Fads2  is a  Δ6  desaturase. In contrast, teleosts show distinct combinations of desaturase activities (e.g. bifunctional or separate  Δ5  and  Δ6  desaturases) apparently allocated to  Fads2-type  genes. To determine the timing of  Fads1-Δ5  and  Fads2-Δ6  evolution in vertebrates we used a combination of comparative and functional genomics with the analysis of key phylogenetic species. Our data show that  Fads1  and  Fads2  genes with  Δ5  and  Δ6  activities respectively, evolved before gnathostome radiation, since the catshark  Scyliorhinus canicula  has functional orthologues of both gene families. Consequently, the loss of  Fads1  in teleosts is a secondary episode, while the existence of  Δ5  activities in the same group most likely occurred through independent mutations into  Fads2  type genes. Unexpectedly, we also establish that events of  Fads1  gene expansion have taken place in birds and reptiles. Finally, a fourth  Fads  gene ( Fads4 ) was found with an exclusive occurrence in mammalian genomes. Our findings enlighten the history of a crucially important gene family in vertebrate fatty acid metabolism and physiology and provide an explanation of how observed lineage-specific gene duplications, losses and diversifications might be linked to habitat-specific food web structures in different environments and over geological timescales.

Functional Desaturase Fads1 (Δ5) and Fads2 (Δ6) Orthologues Evolved before the Origin of Jawed Vertebrates