Hox Genes Keep Legs Off of Heads

Mutations that change body parts, if the changes are too extreme, might kill their bearers before the bearers of these new mutants achieved reproductive maturity. Enter Hox genes. Hox genes are 'super' genes that apparently reside at the top of a hierarchy of genes to control many other genes lower down.

All of an animal's genes, working together, ultimately determine the animal's form. One gene, having the beautiful name of ultrabithorax for example, regulates where limbs form on the body. Usually limbs do not grow on heads or on abdominal segments. In insects this gene, shortened to ubx, suppresses formation of limbs and wings in the abdominal segments and also ensures that wings and legs develop only in the thorax. We know this because if we remove the ubx gene from fruit flies, legs sprout from every abdominal segment. Ubx thus suppresses abdominal legs. Then if we place the ubx gene inappropriately into the thorax where it is not usually active, ubx suppresses development of legs where legs normally occur. Thus, ubx genes turn off and turn on genes that more specifically and minutely control development of the complex structures of the legs and wings. If we take a ubx protein from brine shrimp — these crustaceans carry legs on their abdominal segments — and we place this brine shrimp protein into the thorax of fruit flies, only fifteen percent of the limb development of a fruit fly's legs gets turned off or suppressed. This change suggests that over the interval of geological time between when brine shrimp arose to when fruit flies emerged, the ubx gene evolved and even changed its function.

These and other genetic findings suggest that altering just one small part of a tightly organized genetic system may, in turn, modify a much larger pattern. Not only are individual genes not immutable, whole patterns of genes are not fixed through time, as they can adjust their collective responses to evolutionary demand. It would be useful if human designers could develop devices having at least some of the sophistication of insect bodies, so that devices might evolve according to the demands of their changing environments.

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