Longer or shorter spines: Reciprocal trait evolution in stickleback via triallelic regulatory changes in Stanniocalcin2a

Across a broad range of species, evolution has modified a common vertebrate body plan to produce endless forms most beautiful. A key unanswered question is whether diverse morphological changes in a common structure arise from modifying different genes or the same genes in different ways. Many natural populations of threespine stickleback have evolved either longer or shorter dorsal and pelvic spines. Here, we identify reciprocal regulatory changes in an ancient enhancer of the bone growth inhibitor, Stanniocalcin2a, as an underlying genetic cause. Many other stickleback loci similarly show three or more major classes of variants across populations; we suggest that diverse alleles at key loci may represent a common mechanism for producing diverse phenotypes from a smaller toolkit of genes.