What is a QTL hotspot, and why is it relevant to stickleback armor evolution?

• A. A genomic region that harbors multiple QTL affecting a trait; clustering of armor-related loci like Eda can accelerate adaptation.
• B. A region with a single gene that determines armor.
• C. A region with no recombination that prevents adaptation.
• D. A region that is never under selection.

Answer: (A)

A QTL hotspot is a genomic region that contains several quantitative trait loci influencing the same trait. In sticklebacks, armor variation—especially the number of lateral plates—is not governed by a single gene but by a cluster of loci near each other, including the major Eda gene. When these linked loci are favored by selection in freshwater environments, they tend to be inherited together. This linkage keeps beneficial allele combinations intact across generations, allowing a faster and more coordinated shift in armor traits than if the loci were scattered. Sometimes these regions also sit in inversions that further suppress recombination, strengthening the rapid, repeatable adaptation seen in different stickleback populations.