FIGURE 1-29 Similar master transcription factors, conserved during evolution, regulate early developmental processes in diverse animals. (a) Urbilateria is the presumed ancestor of all protostomes and deuterostomes that existed about 600 million years ago. The positions of its nerve cord (violet), surface ectoderm (mainly skin; white), and endoderm (mainly digestive tract and organs; light green) are shown. (b) Highly conserved master transcription factors called Hox proteins, which determine the identity of body segments during embryonic development, are found in both protostomes and deuterostomes. Hox genes are found in clusters on the chromosomes of most or all animals, and they encode related master transcription factors that control the activities of other genes. In many animals, different Hox genes direct the development of different segments along the head-to-tail axis, as indicated by corresponding colors. Each gene is activated (transcriptionally) in a specific region along the head-to-tail axis and controls the growth and development of tissues there. For example, in the mouse, a deuterostome, the Hox genes are responsible for the distinctive shapes of vertebrae. Mutations affecting Hox genes in the fruit fly, a protostome, cause body parts to form in the wrong locations, such as legs in lieu of antennae on the head. In both organisms, these genes provide a head-to-tail “address” and serve to direct the formation of structures in the appropriate places.