The evolution of animal eyes has caused much discussion among scientists. Most classical textbooks dealing with this topic come to the conclusion that the eye has been invented many times independently in different animal phyla, due to the dramatic differences in eye structure observed between, for example, arthropods, molluscs, and vertebrates. A few years ago, however, a study published by the lab of Walter Gehring at the Biozentrum in Basel/Switzerland, showed that a gene which is required for eye formation in the fruitfly is also required for the normal development of eyes in humans and mice. When these researchers took the mouse gene and activated it in the wrong location in fruitflies, they were able to induce extra eyes in these tissues, strongly arguing that the mechanism of eye induction is conserved between flies and vertebrates.
While these results clearly suggested that the trigger for vertebrate and invertebrate eye formation is evolutionarily conserved, they left open the question whether the ancestral structure from which vertebrate and invertebrate eyes evolved was already a fully-formed eye, or whether it was a simpler structure that was then independently developed into an eye in different animal lineages.
This question has now been addressed by geneticists Carl Neumann and Christiane Nuesslein-Volhard, working at the Max-Planck Institute for Developmental Biology, Tuebingen, Germany. In a report published in the September 22, 2000 issue of Science they show that a gene called Sonic Hedgehog is used to control the formation of nerve cells called ganglion cells in the eye. These nerves form in a wave that spreads from the center to the periphery of the retina. The Sonic Hedgehog gene encodes a protein which is secreted by cells and acts as a signal to other cells. Using the tools of molecular genetics, Neumann and Nuesslein-Volhard show that the first nerves in the retina activate the Sonic Hedgehog gene, which then instructs nearby cells to become nerves as well. These nerve cells then also activate the Sonic Hedgehog gene, which signals to nearby cells, and so on, thereby propagating the wave of nerve-cell formation.
The intriguing aspect of this mechanism is that it is highly reminiscent of the way in which nerve cells form in the fruitfly eye. This process is controlled by a gene called Hedgehog, which is very similar in structure and function to the vertebrate Sonic Hedgehog gene. Here, too, nerve cells form in a wave, and the first nerve cells activate Hedgehog, which then instructs nearby cells to become nerves, etc.
The mechanisms used to control nerve cell formation in the zebrafish and fruitfly eyes thus appear to be exact copies of each other. Since this mechanism is specific to the eye in both organisms, this strongly argues that the last common ancestor of vertebrate and invertebrate lineages already had a well-formed eye. The animal eye may only have evolved once, and the bewildering differences in structure would then be due to subsequent evolutionary radiation. Apart from this insight, the work of Neumann and Nuesslein-Volhard also raises the possibility that further similarities may exist between fruitfly and vertebrate eyes. Since a lot of information is available on the development of fruitfly eyes, this promises to help our understanding of vertebrate eye development.
Journal
Science