Evolution of a horizontally acquired fatty acid desaturase enables the biosynthesis of ω-3 polyunsaturated fatty acids in Collembola

Marine ecosystems are rich in essential dietary fatty acids, particularly ω-3 long-chain polyunsaturated fatty acids (PUFAs) including eicosapentaenoic acid (EPA). Various marine microorganisms including microalgae produce ω-3 long-chain PUFAs through the action of methyl- and front-end desaturases. However, most terrestrial organisms cannot produce ω-3 long-chain PUFAs due to the absence of methyl- and/or front-end desaturases. One notable exception is de novo EPA biosynthesis in springtails (Collembola), the non-insect soil hexapod lineage serving as a nutrient source for predatory animals; however, Collembola EPA biosynthesis enzymes remain unknown. Here, we provide the first evidence of the key desaturases required for EPA biosynthesis in Collembola. Mass spectrometric analysis suggested that EPA is synthesized through arachidonic acid biosynthesis and following ω-3 desaturation, which requires front- and methyl-end desaturases, respectively. RNA-sequencing of Collembola transcripts isolated multiple candidate genes encoding front-end desaturases; however, no clear orthologous sequence of a methyl-end desaturase was retrieved. Verification of the activity of the isolated enzymes using a yeast expression system revealed that Collembola front-end desaturases catalyzed arachidonic acid biosynthesis. Furthermore, a subgroup of Collembola front-end desaturase sequences catalyzed the ω-3 desaturation step, facilitating the bioconversion of arachidonic acid to EPA. Phylogenetic analysis further revealed that the Collembola front-end desaturase sequences clustered closely to those of marine microorganisms producing ω-3 long-chain PUFAs. Given that Collembola is derived from a marine Crustacea ancestor, we propose that the horizontal acquisition of a front-end desaturase gene from a marine microbe, followed by duplication and neofunctionalization, empowered Collembola to become an EPA producer.