Spine loss is due to the mutation of a regulatory sequence

Deletions within a Pitx1 cis-regulatory element underlie the adaptive evolution of the pelvic skeleton of stickleback fish. (a) One form of the three-spine stickleback fish inhabits shallow water, and a different form inhabits open water. (b) The shallow-water form has a reduced pelvic skeleton (left) relative to the open-water form (right). (c) This reduction is due to the selective loss of expression of the Pitx1 gene (orange) from the pelvic fin bud during development of the stickleback larvae (compare left and right stickleback larvae). (d) The loss of Pitx1 expression in turn is due to the mutation of an enhancer of the Pitx1 gene specific to the pelvic fin (X marks the mutated enhancer). Other enhancers of the Pitx1 gene, which control expression of the gene elsewhere in the developing body, are unaffected and function similarly in both forms of the fish.