Which pair of loci is commonly cited as major-effect drivers in stickleback adaptive radiation?

• A. Pitx1_Pel and FOXO1
• B. MC1R and SHH
• C. EDA and Pitx1_Pel
• D. GHR and PAX6

Answer: (C)

In stickleback adaptive radiation, a small set of regulatory genes with large effects often drives the major trait shifts seen across freshwater and marine populations. The pair routinely highlighted as major-effect drivers is the locus EDA and the regulatory variant in Pitx1 (Pitx1_Pel). EDA controls the development of lateral armor plates, so marine fish with the typical full plating versus freshwater fish that often have reduced or absent plates due to EDA variants. Pitx1_Pel influences pelvic armor, with regulatory changes leading to reduced pelvic spines in many freshwater populations. Because these two traits—armor plating and pelvic structure—change repeatedly and robustly across independent stickleback lineages adapting to freshwater environments, these loci are consistently identified as major-effect drivers of the adaptive radiation. The other loci listed are associated with different traits and do not show the same repeated, large-effect role in this specific radiation.