In sticklebacks, variation at the Eda locus is most commonly associated with differences in what trait?

• A. Plate number
• B. Body length
• C. Tail color
• D. Gill morphology

Answer: (A)

Plate number is the trait most closely linked to variation at the Eda locus in sticklebacks. Eda encodes a signaling molecule that drives the development of bony armor plates along the body's sides. Changes in Eda expression, often in the regulatory region, lead to fewer plates forming during development. Marine populations typically show many plates, while freshwater populations often have reduced or absent plates because of these Eda-driven changes. Other traits like body length, tail color, or gill morphology aren’t governed by Eda in the same strong, direct way, and are influenced by different genes and factors. So variation at Eda mainly alters how many plates the fish has, making plate number the best fit for what this locus influences.