The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution

by Richard Dawkins

A Hox gene, then, is a gene whose mission in life is to know whereabouts in the body it is, and so inform other genes in the same cell. We are now armed to understand homeotic mutations. When things go wrong with a Hox gene, the cells in a segment are misinformed about which segment they are in, and they fashion the segment they ‘think’ they are in. So, for instance, we see a leg growing in the segment that would normally grow an antenna. This makes perfect sense. The cells in any segment are perfectly capable of assembling the anatomy of any other segment. Why should they not? The instructions for making any segment lurk in the cells of every segment. It is the Hox genes, under normal conditions, that call forth the ‘correct’ instructions for making the anatomy appropriate to each segment. As William Bateson rightly suspected, homeotic abnormality opens a revealing window on how the system normally works. Recall that flies, unusually among insects, normally have only one pair of wings, plus a pair of gyroscopic halteres. The homeotic mutation Ultrabithorax misleads cells in the third thoracic segment into ‘thinking’ they are in the second thoracic segment. They therefore collaborate to make an extra pair of wings, instead of a pair of halteres (see plate 36). There is a mutant flour beetle (Tribolium) in which all 15 segments develop antennae, presumably because the cells all ‘think’ they are in segment 2.

The sublime beauty of nature and evolution is reflected in the Hox gene.