Which gene is identified as involved in the formation of the pelvic spines in stickleback fish?

• A. Pitx1
• B. Pitx2
• C. Pax6
• D. Shh

Answer: (A)

In sticklebacks, the formation of pelvic spines is driven by the Pitx1 gene, a transcription factor that promotes hindlimb/pelvic development. A regulatory change near Pitx1 reduces its expression specifically in the pelvic region, leading to loss or reduction of the pelvic spines. This example shows how changing when and where a gene is turned on can reshape anatomy without altering the protein itself. Other genes listed have different roles: Pitx2 is involved in other aspects of development, Pax6 is crucial for eye development, and Shh is a morphogen involved in broad patterning. Pitx1 is the gene linked to pelvic spine formation.