Homeotic Genes and Segment Identity

Once segmentation is established, another important and remarkable set of genes turns on: the homeotic selector genes. These genes control development within each segment. For instance, the thorax region of the fly contains three segments, each with one pair of legs (the reason insects are six-legged). The homeotic selector gene antennapedia is normally expressed only in the thoracic segments, leading to the creation of a pair of legs.

Note that antennapedia does not itself "code for" legs. Instead, its protein product is a transcription factor. By regulating expression of many other transcription factor protein that increases the rate of transcription of a gene

In place of its antennae this fly grew legs. Normally expressed in the thoracic region of a fly, the homeotic selector gene antennapedia "kick starts" a series of events leading to leg growth. A mutated form of this gene allowed for its expression in the head region of the fly.

genes, it sets off a cascade of events that results in the creation of legs. Remarkably, however, this single gene is sufficient by itself to turn on the leg-producing program, and its absence keeps the program silent. It can even turn it on in other segments. For instance, when antennapedia is mutated to allow it to be expressed in head segments, a pair of legs develops in place of the normal appendages, antennae (hence the gene name, which means "antenna foot").

Intriguingly, the sequence of homeotic selector genes along the fly chromosome matches the order of segments in which each is expressed. That is, the genes expressed in head segments come first, followed by those expressed in thoracic segments, then the abdomen, then the tail. The way in which this correspondence is exploited during development is still unknown, but the arrangement is clearly not accidental. Related genes have been found in vertebrates, including humans, and the same pattern holds: Genes expressed more anteriorly precede those expressed toward the posterior.

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