The first glimpse of the answer to this problem came over 80 years ago, with the discovery of a set of mutations in Drosophila that cause bizarre disturbances in the organization of the adult fly. In the Antennapedia mutant, for example, legs sprout from the head in place of antennae, whereas in the Bithorax mutant, portions of an extra pair of wings appear where normally there should be the much smaller appendages called halteres (Figure 21–23). These mutations transform parts of the body into structures appropriate to other positions, and they are called homeotic mutations (from the Greek “homoios,” meaning similar) because the transformation is between structures of a recognizably similar general type, changing one kind of limb, or one kind of segment, into another. It was eventually discovered that a whole set of genes, the homeotic selector genes, or Hox genes, serve to permanently specify the A-P characters of the whole set of animal segments. These genes are all related to one another as members of a multigene family.
There are eight Hox genes in the fly, and they all lie in one or the other of two gene clusters known as the Bithorax complex and the Antennapedia complex.
wild type loss of Ubx gain of Ubx
Figure 21–22 The pattern of expression of Engrailed, a segment-polarity gene.
the Engrailed pattern is shown in a 10hour embryo and an adult (whose wings have been removed in this preparation). the pattern is revealed by constructing a strain of Drosophila containing the control sequences of the Engrailed gene coupled to the coding sequence of the reporter LacZ, whose product is detected histochemically through the brown product generated by immunohistochemistry against LacZ (10-hour embryo) or through the blue product generated by a reaction that LacZ catalyzes (adult). Note that the Engrailed pattern, once established, is preserved throughout the animal’s life. (courtesy of tom Kornberg.)
Figure 21–23 Homeotic mutations.
Ultrabithorax, or Ubx, is one of three genes in the Bithorax gene complex (a Hox gene cluster). Ubx is responsible for all of the differences between the second and third thoracic segments. (a, B) Ubx loss-of-function mutations transform the haltere-bearing segment (a) into a wing-bearing segment, resulting in four-winged flies (B). (c) Ubx gain-of-function in the second thoracic segment transforms this wing-bearing segment into a halterebearing segment, resulting in wingless flies. (courtesy of richard Mann.)
The genes in the Bithorax complex control the differences among the abdominal and thoracic segments of the body, while those in the Antennapedia complex control the differences among thoracic and head segments. Comparisons with other species show that the same genes are present in essentially all animals, including humans. These comparisons also reveal that the Antennapedia and Bithorax complexes are the two halves of a single entity, called the Hox complex, that has become split in the course of the fly’s evolution, and whose members operate in a coordinated way to exert their control over the head-to-tail pattern of the body.
The products of the Hox genes, the Hox proteins, are transcription regulators, all of which possess a highly conserved, 60-amino-acid-long DNA-binding homeodomain (see p. 376). The corresponding motif in the DNA sequence is called a “homeobox,” from which, by abbreviation, the Hox complex takes its name. There are many homeobox-containing genes, but only those located in a Hox complex are Hox genes.
The Hox proteins can be viewed as molecular address labels possessed by the cells of each segment: these labels give the cells in each region a positional value—that is, an intrinsic character that differs according to a cell’s location. If the address labels in a developing Drosophila segment are changed, the segment behaves as though it were located somewhere else; if all the Hox genes in an embryo are deleted, the body segments in the larva will all be alike.