Which loci are recognized as major-effect in stickleback adaptive divergence?

Which loci are recognized as major-effect in stickleback adaptive divergence?

• A. EDA and Pitx1_Pel
• B. MC1R and EDNRB
• C. Pax6 and Sox9
• D. SRY and SHH

Answer: (A)

In sticklebacks, a small number of genes can produce large, repeatable changes as populations adapt to freshwater environments. The two classic major-effect loci are the gene EDA, which controls the development of lateral armor plates, and the Pitx1 pelvic enhancer (Pel), which governs pelvic structure. EDA variation leads to substantial differences in plating: freshwater sticklebacks often evolve toward reduced plate numbers because regulatory changes reduce EDA activity, yielding a clear fitness benefit in the freshwater setting. This effect is strong and has been observed repeatedly across many independent populations, making EDA a quintessential example of a major-effect locus in stickleback adaptation. Pitx1_Pel is a regulatory element that drives the development of the pelvic girdle and spines. Changes in this enhancer—particularly deletions or alterations that reduce pelvic structures—produce a pronounced pelvic reduction in freshwater populations, another pattern seen again and again in parallel across land-locked sticklebacks. Other gene pairs listed are primarily associated with traits like coloration or general development in various species, or with sex determination, but they do not show the same consistent, large-scale impact on stickleback adaptive divergence as EDA and Pitx1 Pel do.

In sticklebacks, a small number of genes can produce large, repeatable changes as populations adapt to freshwater environments. The two classic major-effect loci are the gene EDA, which controls the development of lateral armor plates, and the Pitx1 pelvic enhancer (Pel), which governs pelvic structure.

EDA variation leads to substantial differences in plating: freshwater sticklebacks often evolve toward reduced plate numbers because regulatory changes reduce EDA activity, yielding a clear fitness benefit in the freshwater setting. This effect is strong and has been observed repeatedly across many independent populations, making EDA a quintessential example of a major-effect locus in stickleback adaptation.

Pitx1_Pel is a regulatory element that drives the development of the pelvic girdle and spines. Changes in this enhancer—particularly deletions or alterations that reduce pelvic structures—produce a pronounced pelvic reduction in freshwater populations, another pattern seen again and again in parallel across land-locked sticklebacks.

Other gene pairs listed are primarily associated with traits like coloration or general development in various species, or with sex determination, but they do not show the same consistent, large-scale impact on stickleback adaptive divergence as EDA and Pitx1 Pel do.