Methyl-end desaturases determine the capability for de novo biosynthesis of polyunsaturated fatty acids in bivalves

Abstract

Recent studies have shown that many invertebrate species possess methyl-end desaturases (herein referred to as ‘ωx’), enabling biosynthesis of polyunsaturated fatty acids (PUFA). However, the phylogenetic distribution of these enzymes across the animal kingdom remains puzzling, possibly due to horizontal gene transfer (HGT) and/or independent large-scale gene loss in certain invertebrate lineages. In molluscs, ωx genes have been identified in various cephalopods and gastropods but remain barely explored in bivalves. The increasing availability of genomic and transcriptomic resources enables a comprehensive exploration of the ωx gene repertoire in bivalves. To elucidate the distribution of ωx in bivalves, we conducted a broad homology search across existing genome and transcriptome assemblies, followed by functional characterisation of ωx in lineage representative species. Our results revealed no ωx-like sequences in any of the 65 Pteriomorphia species, suggesting gene loss in this clade. However, ωx-like sequences were found in Protobranchia, Palaeoheterodonta and Imparidentia. We analysed ωx from Solemya pusilla (Protobranchia), Lanceolaria oxyrhyncha and Margaritifera margaritifera (Palaeoheterodonta), and Ruditapes philippinarum and Tridacna crocea (Imparidentia). Except for M. margaritifera, which had two ωx genes, each species had a single ωx gene. Functional analysis showed Δ15Δ17Δ19 desaturase activity in the R. philippinarum and T. crocea ωx, while the L. oxyrhyncha ωx exhibited Δ15Δ17 activity but not Δ19. Both ωx from M. margaritifera showed no detectable activity in yeast. Interestingly, the S. pusilla ωx exhibited Δ12 desaturase activity. These findings highlight the diversity of ωx desaturation capabilities in bivalves, with significant gene loss in Pteriomorphia.