Which gene's regulatory region is implicated in controlling plate number variation in freshwater sticklebacks?

• A. SHH
• B. PTEN
• C. EDA
• D. RUNX2

Answer: (C)

The key idea is that changes in when and where a gene is turned on can drive big physical differences, without altering the protein itself. For freshwater sticklebacks, plate number is influenced by a regulatory region of the EDA gene. EDA (ectodysplasin-A) signals during the development of ectodermal structures like armor plates. A regulatory variant in this region reduces EDA expression in the developing plates, so fewer plates form. In contrast, higher EDA activity in marine populations leads to more plates. This shows how modifying gene regulation can produce adaptive morphological changes.

The other genes listed have important developmental or bone-related roles, but they are not the specific regulatory element linked to plate-number variation in this case.