Origin and evolution of fatty acid desaturase genes in oil crop Brassica napus

Fatty acid (FA) desaturases, as the key enzymes in lipid metabolism, are responsible for biosynthesis of the unsaturated fatty FAs, which play important roles in maintaining cell membrane integrity and multiple stress responses. Although attention has been drawn to some plant FA desaturase genes, their global landscape in oil crops is still lacking. Here, we performed systematic characterization and phylogenomic synteny network analyses of the FA desaturase gene family in polyploid oil crop B. napus and other 54 species covering major streptophyte lineages. A total of 1653 FA desaturase genes were identified from these plant genomes. Based on the broad-scale family phylogeny and functional domains, we proposed a unified eight-group classification system for angiosperm FA desaturases, and found that the origin of genes responsible for FA desaturation evolved early and some genes were absent in different species. Phylogenomic analyses revealed deeply conserved syntenic relationships within each of the eight FA desaturase groups. B. napus contains up to 93 FA desaturase genes from the eight groups. Recurrent duplication events in Brassicaceae contributed to the expansion of FA desaturase genes in B. napus, leading to further functional diversification. These FA desaturase genes exhibited spatio-temporal specific expression patterns in different tissues of B. napus, and a set of FA desaturase genes seem to be orchestrated by key transcriptional factors during seed development, such as zf-HD, B3, GATA3, PEI1, NFYA7, YAB1 and YAB2. Altogether, our data have inferred the evolutionary trajectory of this important gene family across distinct plant lineages, providing theoretical basis for future manipulation of FA desaturase genes to improve the seed oil quality of B. napus.