Abstract
Coral populations across the Great Barrier Reef (GBR) could rapidly adapt to the warming climate if they have standing genetic variation for thermal tolerance. Here, we describe a locus likely involved in latitudinal adaptation of Acropora millepora . This locus shows a steep latitudinal gradient of derived allele frequency increasing at higher latitudes, and harbours a cluster of eight tandemly repeated Δ9‐desaturase genes adjacent to a region in the genome where a hard selective sweep likely occurred. In colonies reciprocally transplanted across 4.5° of latitude, the expression of Δ9‐desaturase is upregulated at the high‐latitude reef. Furthermore, corals from the low‐latitude reef bearing the derived Δ9‐desaturase allele express the gene more and grow faster than their peers when transplanted to the high‐latitude reef. In other organisms ranging from bacteria to fish, Δ9‐desaturase is upregulated under cold conditions to adjust membrane fluidity by introducing double bonds into fatty acid chains of membrane lipids. It is therefore plausible that the signal of latitudinal adaptation at the Δ9‐desaturase locus is due to its involvement in adaptation to cooler temperatures at higher latitudes.