In Defense of Evolution: That Persistent Homeobox
By Patrick Wright and John Wielgus, Professor of Biology
Drosophila melanogaster has been the model of genetic research for years due to the large numbers of offspring and the ease with which mutations can be made in the genome of the fruit fly. It was only a matter of time before the monumental discovery of the homeobox occurred. Yes, we believe the homeobox to be a monumental discovery, and if you have heard anything about the homeobox, you know why. The homeobox is a sequence of 180 base pairs that codes for 60 amino acids in a fruit fly, and it has been found in eight different loci on the third chromosome, which is known to contain the homeotic genes responsible for development. So, what are the homeotic genes? Imagine the third chromosome of Drosophila stringing from the top of the page to the bottom. The homeotic genes at the top of the page guide the development of the head and as you go down the page, the genes guide the development of sections successfully posterior until you reach the bottom of the page, which controls development in the abdomen. Because scientists have seen this recurring sequence of nucleotides in this set of genes crucial for proper development, they came to the obvious conclusion that the homeobox, a genetic sequence conserved in many different genes through evolution, must perform a very important task.
As is usually the case when a new gene is being studied, the first thing to do is figure out what amino acid sequence the gene codes for and derive the function from the protein. In this case, the homeobox sequence codes for a protein with a distinctive shape called the helix-turn-helix. This shape is usually seen in DNA-binding proteins, which can control gene activity by literally grabbing onto the DNA to either inhibit or encourage gene transcription. Recent research supports the hypothesis that the homeobox guides the development of the fruit fly by selectively turning on genes that produce proteins responsible for cellular specialization. Therefore, it is the homeobox that guides the fly through development after the maternal transcripts have performed their tasks. This is a key piece of information when studying flies, but this information alone does not classify this discovery as monumental.
Curiosity fueled further research and a very similar gene sequence to the homeobox was found in frogs. This sequence was so similar that of the 60 amino acids coded for by the fly, 55 were identical in the frog. More organisms were studied and the same DNA sequences were found in earthworms, beetles, mice, chickens, cows, sea urchins, yeast, plants, and even though humans are separated from insects by over 600 million years of evolution, people. This is what makes the discovery of the homeobox monumental. Drosophila scientists have uncovered another piece of evidence to support the hypothesis of a common ancestor and have fueled new research that could ultimately unravel the mysteries of development. We can’t wait to see what happens next.