| Homeotic Genes |
At the end of this cascade is a class of genes that have a long history among Drosophila researchers. Decades before Watson and Crick ascertained the structure of DNA, and even more decades before geneticists understood the principles of gene expression, biologists were using Drosophila melanogaster as a model system for studying the transmission of genetic traits from parent to offspring. Let's go back to 1915 at Columbia University: In a small laboratory, crowded with thousands of milk bottles containing stocks of the tiny fruitfly Drosophila melanogaster, Thomas Hunt Morgan, the father of Drosophila genetics, and his students worked. They examined this fruitfly, focusing on ones that looked different in their quest to find and map genes.
One day Calvin Bridges, one of Morgan's graduate students, discovered a most unusual fly. One of the hallmark features of flies is that they have two wings; Diptera, the insect order to which flies belong, means "two wings." The fly Bridges found had one pair of normal wings and one pair of somewhat developed wings. Four wings! Bridges found that this "four wing" phenotype was a genetic mutation which mapped to the third chromosome. After closer inspection, Bridges noted that the third segment of the thorax in these flies looked a good deal like a normal, second segment of the thorax (where wings normally grow). He consequently named the gene associated with this mutant phenotype "bithorax." (Genes in Drosophila are traditionally named for their mutant phenotype, not for what they do in normal flies.)
Drosophila geneticists would later find other, similar mutations. One, named ultrabithorax, caused the fly to form two, completely developed pairs of wings. Another, seemingly different mutation (antennapedia), caused legs to grow where the fly's antennae should have been (Fig.5). These mutant genes became referred to collectively as homeotic genes, named after homeosis. Homeosis, a term coined by William Bateson (a prominent zoologist and one of the early geneticists), refers to "cases in which structures belonging to one body segment were transformed in identity to those belonging to another segment2. Mutants in these genes appeared to change the characteristics of one segment of the fly into those of another segment. Interestingly, all of these genes would map very close together in two clusters on the third chromosome.
Recall the cascade that led to these homeotic genes. The maternal effect coordinate genes laid down the anterior-posterior and dorsal-ventral gradients, which influenced the expression of genes further along the cascade. These genes turned other genes on or off and, as a result, formed the segmented pattern of the Drosophila embryo. The homeotic genes, having been turned on or off by genes above in the cascade, are also transcription factors. They influence the expression of numerous other genes and, by doing so, determine the identity of the segment they are in. Certain homeotic genes, such as bithorax, are expressed in what would become the thorax; other genes are expressed only in the head or abdomen. It's interesting to note that genes expressed in similar regions are also located near each other on the chromosome (Fig. 6).