Which locus is associated with pelvic reduction in sticklebacks?

• A. EDA
• B. GHR
• C. MC1R
• D. Pitx1_Pel

Answer: (D)

Pelvic reduction in sticklebacks comes from a regulatory change in the Pitx1 gene, specifically in the pelvic enhancer Pitx1_Pel. This enhancer drives Pitx1 expression in the developing pelvic region, and when its function is disrupted or deleted, the pelvic structures fail to develop or are greatly reduced. The result is a loss of pelvic girdle and spines without altering Pitx1’s other roles elsewhere in the body. Pitx1 is a transcription factor important for hindlimb development, and the pelvic enhancer Pitx1_Pel is what confines its pelvic expression.

The other loci correspond to different traits: EDA affects armor plating, MC1R affects pigmentation, and GHR relates to growth. None of these explain pelvic reduction, which is specifically linked to Pitx1_Pel.