Which gene is most directly linked to the loss of lateral plates in freshwater sticklebacks?

• A. Pitx1
• B. BMP4
• C. SHH
• D. EDA

Answer: (D)

The trait being tested is how plate formation is controlled genetically. In freshwater sticklebacks, the notable reduction in lateral plates is driven by changes at the EDA gene, which encodes ectodysplasin A, a signaling protein important for developing ectodermal structures like plates. A regulatory mutation reduces EDA activity in the flank, producing many fewer plates, a change that has evolved repeatedly in freshwater environments because lighter armor can be advantageous or less costly. While other genes like Pitx1 are associated with different armor traits (pelvic reduction) and BMP4/SHH have broader developmental roles, they are not the primary drivers of lateral plate loss in these fish.