Body plan: from genes to embryo.

A recently discovered segment of DNA, called a "homeo box," is thought to direct the development of the body plan in a wide variety of animals (SN: 7/14/84, p. 21). Walter Gehring of the University of Basel in Switzerland and colleagues are examining fruit fly embryos to determine when and where genes containing homeo boxes are active. They have studied three fruit fly genes containing homeo boxes and find that each is active in specific periods and locations in the early embryo. Gehring reports that these sites of activity correspond to the recognized roles of the genes. For example, flies with a defective form of one of these genes have legs instead of antennae extending from their heads. In normal embryos, the gene is active only in the region destined to become the head. Similar experiments on genes that influence segmentation produced stripes of genes activity. The gene necessary for development of the normal number of body segments, instead of half the normal number, is present in stripes the width of a segments. In another case, the gene is found to be active later in embryonic development and in the posterior, but not the antually seeing how the body plan is structured."