Which gene is most famously associated with armor plating variation in sticklebacks, and what phenotype does its freshwater variant cause?

• A. Eda; freshwater variants reduce the number of lateral plates
• B. Pitx1; freshwater variants reduce pelvic girdle
• C. Fgf8; freshwater variants increase jaw size
• D. Sox9; freshwater variants alter fin rays

Answer: (A)

The key idea is that a single gene can drive a big, repeatable adaptation when a population moves into a new environment. In sticklebacks, the gene Eda (ectodysplasin A) sets how many bony armor plates grow along the fish’s sides. When freshwater populations evolve, regulatory changes reduce Eda signaling, leading to fewer lateral plates. That produces the freshwater phenotype: a low-plated morph with fewer armor plates.

Pitx1 is known for reducing the pelvic girdle in freshwater sticklebacks, which is a different trait. Fgf8 and Sox9 influence other aspects of development (jaw size and cartilage/bone formation, respectively) and are not the primary drivers of armor plate variation in this context.