The role of cell-signalling proteins in the morphology of birds' feet.

This figure shows how changes in developmental gene expression can produce new traits. (a) Expression of the BMP4 gene in the developing limbs. BMP4 protein is stained blue here. (b) Expression of the gremlin gene in the developing limbs. Gremlin protein is stained brown here. Note that gremlin is expressed in the interdigit region only in the duck. Gremlin inhibits BMP4, which causes programmed cell death. (c) Because BMP4 is not inhibited in the interdigit regions in the chicken, the cells in this region die, and the foot is not webbed. By comparison, inhibition of BMP4 in the interdigit regions in the duck results in a webbed foot.