Genome of wild olive and the evolution of oil biosynthesis

Here we present the genome sequence and annotation of the wild olive tree (<i>Olea europaea</i> var. <i>sylvestris</i>), called oleaster, which is considered an ancestor of cultivated olive trees. More than 50,000 protein-coding genes were predicted, a majority of which could be anchored to 23 pseudochromosomes obtained through a newly constructed genetic map. The oleaster genome contains signatures of two Oleaceae lineage-specific paleopolyploidy events, dated at ∼28 and ∼59 Mya. These events contributed to the expansion and neofunctionalization of genes and gene families that play important roles in oil biosynthesis. The functional divergence of oil biosynthesis pathway genes, such as <i>FAD2</i>, <i>SACPD, EAR</i>, and <i>ACPTE</i>, following duplication, has been responsible for the differential accumulation of oleic and linoleic acids produced in olive compared with sesame, a closely related oil crop. Duplicated oleaster <i>FAD2</i> genes are regulated by an siRNA derived from a transposable element-rich region, leading to suppressed levels of <i>FAD2</i> gene expression. Additionally, neofunctionalization of members of the <i>SACPD</i> gene family has led to increased expression of <i>SACPD2</i>, <i>3</i>, <i>5</i>, and <i>7</i>, consequently resulting in an increased desaturation of steric acid. Taken together, decreased <i>FAD2</i> expression and increased <i>SACPD</i> expression likely explain the accumulation of exceptionally high levels of oleic acid in olive. The oleaster genome thus provides important insights into the evolution of oil biosynthesis and will be a valuable resource for oil crop genomics.