What genetic mechanism underlies pelvic spine reduction in freshwater sticklebacks?

• A. Mutations in Pitx1 pelvic enhancer (Pel)
• B. Deletions in Gdf6
• C. Mutations in EDA promoter
• D. Amplification of Shh

Answer: (A)

This question tests how regulatory changes in developmental genes can drive morphological evolution, illustrated by pelvic spine loss in freshwater sticklebacks. The key idea is that Pitx1 controls the development of pelvic structures, and its activity in the pelvis is governed by a cis-regulatory element known as the pelvic enhancer, called Pel. A deletion in this pelvic enhancer reduces or eliminates Pitx1 expression specifically in the developing pelvis, so the pelvis fails to form fully and the pelvic spines are reduced or lost. This kind of regulatory mutation can produce tissue-specific effects without disrupting Pitx1 function elsewhere, which fits observed patterns of repeated pelvic reduction in different freshwater populations.

Other proposed mechanisms would affect different pathways or produce broader changes. For example, alterations in Gdf6, the EDA promoter, or Shh signaling would lead to different morphological outcomes or more widespread patterning effects and are not the established cause of pelvic spine reduction in this system. The Pel enhancer deletion best accounts for the targeted pelvic phenotype.