I saw a documentary where they inserted the gene of a mouse that basically is the starting "build an eye" command into a fruit fly, and a fruit fly eye grew. My question is, if eyes of different types of animal had different evolutionary beginnings (I don't know much about arthropod eye evolution, but for a different example, as I understand it, cephalopod eyes evolved from skin cells while vertebrate eyes evolved from brain cells), then how is it that the same gene controls the growth of two such evolutionary different eyes?
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3 Answers
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Both fruit flies (Drosophila) and mice (Mus) are classified under Bilateria.
The presence of such a highly conserved sequence in both species suggests that they share a urbilaterian ancestor who also used a similar gene to turn on eye formation (not create the whole eye, just start the process).
Even though the structure of insect and mammalian eyes diverged evolutionarily after the common ancestor, the switch that starts eye development worked so well there was little need to change it. I suppose one could say that it remains backwards compatible.
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$\begingroup$ Okay so to clarify one point, going back to the example of cephalopods, which are also Bilateria, if one eye evolved from skin and one eye evolved from brain tissue, how can they have the same origin of the eye? Was the brain and skin part of the same thing in the common ancestor? $\endgroup$– A LCommented Aug 19, 2013 at 17:29
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2$\begingroup$ Yes, in a way. The outermost layer of a gastrula, the ectoderm, gives rise to both skin and neural tissues. $\endgroup$– GossarCommented Aug 19, 2013 at 21:15
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$\begingroup$ So the first bilateral animals had a mush of quasi-skin neurons for an eye or what? $\endgroup$– A LCommented Aug 20, 2013 at 1:16
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$\begingroup$ Perhaps something like we see in the chiton, not quite an eye but definitely light-sensitive. But as a soft-bodied animal without the mollusc's shell, there was usually very little left of early bilaterians for fossil records. Another intriguing example of the skin--brain conection is the parietal eye still seen in some ampibians, fish, and reptiles. $\endgroup$– GossarCommented Aug 29, 2013 at 6:33
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The documentary was probably talking about Halder's paper in Science (Science, Halder et al., v.267). If you'll read the abstract (especially the bottom part) You'll notice that the gene is a control gene that is responsible for starting a chain of interactions at the end of which the real fly-specific 'eye-building' process begins. Basically this is like using the same kind of electrical switch to turn on the light in your room (mouse) or an air conditioner (fly).
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$\begingroup$ I understand that part. My question is how is it that they both use the same genetic switch if arthropod and vertebrate eyes had completely different origins? Wouldn't they each have their own unique switch? $\endgroup$– A LCommented Aug 18, 2013 at 18:23
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2$\begingroup$ To extend the metaphor, in evolutionary history it wasn't a light or an air conditioner, but something much simpler, like a bare light bulb or something. The light bulb needed to be upgraded, but the switch was fine. $\endgroup$ Commented Aug 19, 2013 at 14:32
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So the gene that was implanted from the mouse to the fly is called a homologue/homeotic/hox gene which doesn't actually build the eye it just switches on the genes that build the eye. The reason that this worked is because hox genes are found in most or all multicellular organisms. In terms of evolution this is most likely evidence for a common ancestor.
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1$\begingroup$ thanks for your effort... can you add some references with hyperlinks so that others can follow you answer? $\endgroup$ Commented May 25, 2017 at 16:05