Genetic engineering enables the discovery of a gene responsible for the development of the eye, suggesting new possibilities of treating defects of vision
VISION is one, of the most stunning
achievements of evolution. Beginning in
our ancestors with a semi-transparent
membrane that had let in light, the
visual system in higher animals has
evolved to an amazing level of sophistication. The first element in the system,
the eye, is as close to a perfect organ as
anything that has appeared in the course
of the history of life on earth.
We know of at least three kinds of
sophisticated eyes today: the 4nsect eye,
the molluskan eye and the vertebrate
eye. Of these three, the ii*ect eye is
radically different as far as Organisation
is 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 the
eyes of other organisms, the insect eye is
as complex a structure as, for instance,
our own.
Logically, the design and construction of such an organ would seem to
require the coordination of a large
number of elementary steps. And this
makes the recent discovery of a single
'master'gene for eye develp mentinthe
fruitfly Drosophila melanokaster difficult
to understand. The work was carried
out by Georg Haider, Patrick Callaerts
and Walter Gehring of the Biocentre in
Basel, Switzerland (Science, Vol 267, No
5205,1995).
These scientists made use of a previously known gene called'eyeless'. When
mutated, eyeless was known to cause
abnormal development in the compound eye of D melanogaster. There are
at least four other genes that are known
to cause comparable defects when
mutated, but none of them appear to
affect the activity of eyeless. This observation suggested to Haider and his colleaves that eyeless might be at the head
of a hierarchy of genes involved in
development of the eye.
In order to test this hypothesis,
Haider and his team used the methods
of genetic engineering to alter the normal pattern of expression of the eyeless
gene. By this means, they ensured that
the eyeless gene was active in body tissues such as the antenna, the leg and the
wings - where it is normally unexpressed. To their surprise, just this one
gene, when made functional in an
abnormal - 'ectopic' - location on
the body, led to the development of
what appeared to be normal compound
eyes, but this time at the end of the
antenna, or as part of the leg or wing!
The fascinating question @ which
remains unanswered as of now - is
whether such eyes can also help in
seeing. What makes the finding
especially intriguing is that at the level
Of DNA, eyeless Iresembles a, gene in
humans called aniridia. Aniridia is
known to be necessary for the proper
development of the eye. This resemblance opens up the long-range possibility of radical treatments for human
visual problems.
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