The curious case of ectopic eyes in Drosophila

The model organism Drosophila melanogaster is famous for its elaborate genetic experiments. Scientists have learned a great deal about animal development — even human development — by creating countless mutations in D. melanogaster and observing their effects on fly development. One such famous experiment is the case of ectopic eyes in Drosophila melanogaster, which taught us a great deal about the genes that regulate body development.

Is the eyeless gene a master regulator of eye development?

The authors of this classic study investigated the eyeless (ey) gene, which encodes a protein containing a homeobox (an evolutionarily conserved DNA-binding domain important for animal development).1 The eyeless gene was first described in 1915 and already known to be required for compound eye development in the fly, but these researchers wanted to go further and determine if eyeless is a master control gene for eye development — in other words, if eyeless is sufficient to create the eyes, anywhere it is expressed. How can this be done?

Gene overexpression: an essential research tool

This brings us to the concept of gene “overexpression,” a common research technique whereby a particular gene is expressed (i.e. transcribed into mRNA) at a higher level than normal, or at a time or place where it is not normally active. For example, one could overexpress an embryonic patterning gene in the adult fly brain — where it is not normally expressed — and then observe what happens. This technique can help us infer the normal function of a gene by observing the effects when it is expressed in some tissue or developmental stage it is not supposed to be (i.e. expressed ectopically). Many such overexpression experiments are done using the Gal4-UAS system, a widely used tool for inducible and targeted gene expression in flies.

Indeed, the authors of this study used the Gal4-UAS system to overexpress the eyeless gene in different locations in the antennal, leg, and wing imaginal discs of developing D. melanogaster larvae. You can see the design of their eyeless Gal4-UAS expression system below:

Schematic explaining how the eyeless gene was overexpressed in a targeted manner in the developing larva.1

Expression of eyeless is sufficient to form eye structures

After expressing eyeless in the larval tissues described above, the authors observed compound eye structures on the antenna, wing, and leg of adult flies, as you can see in the striking images below (the top panel of images were taken with a light microscope, the bottom panel is a closer view using a scanning electron microscope). A close analysis of these eye structures revealed the normal complement of facets, bristles, and photoreceptors, so it appeared that ectopic eyeless expression induced the formation of fully normal and functional eyes. This demonstrates that the activity of the eyeless gene is sufficient to induce the development of compound eyes wherever it is expressed, suggesting it is indeed a “master control gene.”

Drosophila ectopic eyes
(A) An adult head in which both antennae have formed eye structures. (B) Dissected wing with a large outgrowth of eye tissue. (C) Dissected antenna in which most of the third segment is replaced by eye tissue. (D) Dissected leg with an eye-tissue outgrowth.
Drosophila ectopic eyes
Scanning electron micrographs of ectopic eye structures in different locations of the adult fly. (A) Ectopic eye in the head region formed by the antennal disc. (B) Ectopic eye structures under the wing and on the antenna. (C) and (D) provide closer views of the fine structures of these ectopic compound eyes.

Ectopic eyes in Drosophila: What did we learn?

Through the use of some ingenious genetic experiments, the authors demonstrated that eyeless is a master control gene for compound eye development in D. melanogaster — meaning that expression of eyeless is sufficient to induce formation of eye structures, wherever it is expressed. Given that eyeless is evolutionarily conserved among insects and vertebrates, in the closing section of the paper the authors reflect on the significance of their findings, and speculate that the eyeless gene could be a common master regulator of eye development shared by mammals and insects for over 500 million years:

“The high degree of sequence conservation between the human, the mouse, and the Drosophila genes… and the similarity of the expression patterns suggested to us that ey might be a master control gene for eye morphogenesis that is shared by vertebrates and invertebrates… The observation that mammals and insects, which have evolved separately for more than 500 million years, share the same master control gene for eye morphogenesis indicates that the genetic control mechanisms of development are much more universal than anticipated.”

References

  1. Halder G, et al. Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila. Science. 1995;67(5205):1788-1792. DOI: 10.1126/science.7892602

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