RESUMEN
Vertebrates have repeatedly modified skeletal structures to adapt to their environments. The threespine stickleback is an excellent system for studying skeletal modifications, as different wild populations have either increased or decreased the lengths of their prominent dorsal and pelvic spines in different freshwater environments. Here we identify a regulatory locus that has a major morphological effect on the length of stickleback dorsal and pelvic spines, which we term Maser (major spine enhancer). Maser maps in a closely linked supergene complex that controls multiple armor, feeding, and behavioral traits on chromosome IV. Natural alleles in Maser are differentiated between marine and freshwater sticklebacks; however, alleles found among freshwater populations are also differentiated, with distinct alleles found in short- and long-spined freshwater populations. The distinct freshwater alleles either increase or decrease expression of the bone growth inhibitor gene Stanniocalcin2a in developing spines, providing a simple genetic mechanism for either increasing or decreasing spine lengths in natural populations. Genomic surveys suggest many recurrently differentiated loci in sticklebacks are similarly specialized into three or more distinct alleles, providing multiple ancient standing variants in particular genes that may contribute to a range of phenotypes in different environments.