The flies' extraordinary view
A genetically altered fruit fly, Drosophila, with extra eyes on the antennae. The eyes were induced through switching on the single gene Pax6 (Foto: U. Kloter)
(July 28th 2008) Developmental biologists have reported new insights into ectopic eyes. They equipped fruit flies with additional eyes on legs, antennae and wings - the extra eyes are capable of vision.
Thirteen years have passed since Walter Gehring, a Swiss developmental biologist of the University of Basle, first induced numerous extra eyes on body parts of the fruit fly
Drosophila, not normally involved in vision (
Science, 1995). The multi-eyed flies were generated by targeted expression of
Pax6, a gene of the homeobox gene family, which encodes transcription factors that regulate development by switching on cascades of other genes.
Pax6 is the master control gene involved in eye formation in flies as well as vertebrates and humans.
Those flies equipped with supernumerary peepers evoked quite a stir. They resembled misshapen creatures postulated by the German geneticist Richard Goldschmidt about a century ago. According to Goldschmidt, radically transformed mutants that he affectionately termed "hopeful monsters" arise from macromutations, which drive evolution. Goldberg's "hopeful monster theory" was never quite accepted by the scientific community but neither was it ever disproved.
However, Gehring and his co-workers were less interested in the possible destiny of their mutant bugs undergoing selective pressures of evolution. They wanted to figure out how, and to what extent, genes control the development of eye formation. Would additional eyes fulfil their task?
In a current study, Gehring and his colleague Patrick Callaerts of the University of Leuven in Belgium, present evidence that the artificial ectopic eyes located on antennae or legs indeed seem to function, of which the antennae eyes operate the best (
PNAS,105: 8968).
Conducting morphological studies involving immunocytochemistry combined with light and electron microscopy, the researchers were able to demonstrate that axonal projections protruded from antennae and leg eyes into the insect's brain. The axons formed functional synapses as shown by electroretinograms, a technique also applied in humans to check eyesight by measuring electrical responses of retina cells to light.
But interestingly the signals coming from the extra eyes were not processed in the optical lobe of
Drosophila's brain as signals from normal complex eyes. Instead, the axons of the additional eyes entered regions of the CNS reflecting their ectopic location. Thus antennae eyes were connected to the antennae lobe, normally receiving olfactory signals, and leg eyes to the thoracic ganglion, respectively. Moreover, the supplementary eyes' synapses were similar, but not identical, to regular vision processing nerve junctions. Hence, the
Pax6 expression seems sufficient enough to kick off the cascade necessary to induce functional eye development in fruit flies, although routing and fine tuning of the neuronal circuits are not yet perfect. Gehring and Callaerts propose that regional factors in the target tissue of the brain might be necessary for correct eye formation, which may not be accessible for the supernumerous eye neurons if they are implemented in the wrong target tissues.
Gehring and colleagues are now designing mutant flies with impaired compound eyes and ectopic eyes, to ultimately elucidate how the man-made add-on eyes function. Maybe the mutant flies smell rather than see with their antennae-eyes? 'Smelling' sunshine and blue summer skies could truly be an enviable sensory perception.
Melanie Estrella
