VISION is one, of the most stunningachievements of evolution. Beginning inour ancestors with a semi-transparentmembrane that had let in light, thevisual system in higher animals hasevolved to an amazing level of sophistication. The first element in the system,the eye, is as close to a perfect organ asanything that has appeared in the courseof the history of life on earth.
We know of at least three kinds ofsophisticated eyes today: the 4nsect eye,the molluskan eye and the vertebrateeye. Of these three, the ii*ect eye isradically different as far as Organisationis concerned. Also, it is a compound eye,made up of many hundred smaller eyes,each of which points in a different direction. In spite of its presence in an evolutionary more primitive form than theeyes of other organisms, the insect eye isas complex a structure as, for instance,our own.
Logically, the design and construction of such an organ would seem torequire the coordination of a largenumber of elementary steps. And thismakes the recent discovery of a single'master'gene for eye develp mentinthe fruitfly Drosophila melanokaster difficultto understand. The work was carriedout by Georg Haider, Patrick Callaertsand Walter Gehring of the Biocentre inBasel, Switzerland (Science, Vol 267, No5205,1995).
These scientists made use of a previously known gene called'eyeless'. Whenmutated, eyeless was known to causeabnormal development in the compound eye of D melanogaster. There areat least four other genes that are knownto cause comparable defects whenmutated, but none of them appear toaffect the activity of eyeless. This observation suggested to Haider and his colleaves that eyeless might be at the headof a hierarchy of genes involved indevelopment of the eye.
In order to test this hypothesis,Haider and his team used the methodsof genetic engineering to alter the normal pattern of expression of the eyelessgene. By this means, they ensured thatthe eyeless gene was active in body tissues such as the antenna, the leg and thewings - where it is normally unexpressed. To their surprise, just this onegene, when made functional in anabnormal - 'ectopic' - location onthe body, led to the development ofwhat appeared to be normal compoundeyes, but this time at the end of theantenna, or as part of the leg or wing!
The fascinating question @ whichremains unanswered as of now - iswhether such eyes can also help inseeing. What makes the findingespecially intriguing is that at the levelOf DNA, eyeless Iresembles a, gene inhumans called aniridia. Aniridia isknown to be necessary for the properdevelopment of the eye. This resemblance opens up the long-range possibility of radical treatments for humanvisual problems.