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

<div><p>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 <em>Δ5</em> and <em>Δ6</em> desaturations. With respect to desaturation activities some noteworthy differences have been noted in vertebrate classes. In mammals, the <em>Δ5</em> activity is allocated to the <em>Fads1</em> gene, while <em>Fads2</em> is a <em>Δ6</em> desaturase. In contrast, teleosts show distinct combinations of desaturase activities (e.g. bifunctional or separate <em>Δ5</em> and <em>Δ6</em> desaturases) apparently allocated to <em>Fads2-type</em> genes. To determine the timing of <em>Fads1-Δ5</em> and <em>Fads2-Δ6</em> evolution in vertebrates we used a combination of comparative and functional genomics with the analysis of key phylogenetic species. Our data show that <em>Fads1</em> and <em>Fads2</em> genes with <em>Δ5</em> and <em>Δ6</em> activities respectively, evolved before gnathostome radiation, since the catshark <em>Scyliorhinus canicula</em> has functional orthologues of both gene families. Consequently, the loss of <em>Fads1</em> in teleosts is a secondary episode, while the existence of <em>Δ5</em> activities in the same group most likely occurred through independent mutations into <em>Fads2</em> type genes. Unexpectedly, we also establish that events of <em>Fads1</em> gene expansion have taken place in birds and reptiles. Finally, a fourth <em>Fads</em> gene (<em>Fads4</em>) 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.</p>