Which genetic element is primarily responsible for pelvic spine variation in sticklebacks?

• A. Hox gene cluster.
• B. Pitx1 pelvic enhancer.
• C. Shh coding region.
• D. EDA promoter.

Answer: (B)

Regulatory changes can have big effects on where and when a gene is active, shaping anatomy without changing the gene’s protein sequence. In sticklebacks, the pelvic spine variation is driven mainly by a regulatory element that controls Pitx1 expression specifically in the pelvic region—the pelvic enhancer. When this enhancer is altered or deleted in freshwater populations, Pitx1 is not turned on in the developing pelvic buds, so the pelvic spines don’t form or are reduced, while Pitx1’s coding sequence and other tissues remain normal. This shows how evolution often uses changes in gene regulation to produce localized morphological differences. Other candidates would affect broader aspects of development or different traits (for example, Hox genes influence body plan more generally; Shh coding changes would alter signaling more broadly; the EDA promoter relates to armor plates), but they are not the primary drivers of pelvic spine variation in this case.