DNA Science Workshop for VCE Biology Teachers
The eye has always been a tough case for evolutionary theory. Its technological perfection gave Darwin the cold shudders . Its one thing to conceptualize the gradual lengthening of the giraffe's neck through selection of ever longer-necked individuals; its quite another to see the technological perfection of the eye arising this way.
As Darwin himself put it, "that the eye , with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection seems, I freely confess, absurd in the highest possible degree.
Worse still, nature has seemingly accomplished this feat, not just once, but many times over. If you've ever encountered a squid in the deep, you might have found yourself staring into an eye not much different than your own. Squid and man have been proceeding down separate evolutionary pathways for over half a billion years. Our common ancestor was probably a sponge. Yet, we have evolved almost the same model of eye.
In 1976, the famous classificationist Ernst Mayr and his colleague Salvini-Plawen , decided to survey the animal world to see how many times eyes had evolved independantly. They identified image-forming eyes in at least 40 separate lineages of animals. Like man and squid, these lineages diverged from each other at a time when multicellular creatures were very primitive (and presumably devoid of optical apparatus). The astonishing conclusion was that the eye had evolved from scratch somewhere between 40 and 65 times.
Apparently, despite human incredulity, its no big deal to evolve an eye. Sight is obviously a huge survival advantage for a mobile animal. And perhaps the number of ways of efficiently producing an image is limited. So animals, through trial and error end up converging on the same basic designs over and over again. Just another case of convergent evolution, like wings in bats and birds, or fins in dolphins and fish.
Yet in 1976, Salvini-Plawen and Mayr were not entirely convinced by the conclusions they'd drawn from their paper. Why would something as important as an eye have had to evolve some 65 times? Surely it would have been as important for the prototypical animals of the Pre-Cambrian seas, as for animals in later eras.
With rather little conviction, they offerered an explanation. These ancestors were probably filter feeders with no need to chase their prey and many lived in dark and murky spaces or tunnelled, so they did not need eyes. Nevertheless, the authors concluded their paper thus. " At present it does not seem probable that these two factors are sufficient to account for the apparent absence of photoreceptors in the ancestral types of so many of the phyletic lines. More thinking about this problem is surely needed.
'' From Darwin's time till now, that's been pretty much the enigma. On one hand, the seeming impossibility of ever evolving an eye by chance. On the other hand, the apparent triflingness of the feat. Nature did it at least 40 times and last year a computer following the rules of Darwinian evolution, did it in a twinkling 400,000 generations.
O.K. Now I'm going to tell you about a bizarre experiment carried out earlier this year that adds a new dimension to the thinking on eye evolution.
You might have heard about the flies with extra eyes on their wings, legs and antennae. Professor Walter Gehring's team in Basel found a gene that can single-handedly mastermind the formation of an eye. They proved this by using genetic tricks to turn the gene on in the wrong places and flies sprouted rosy red eyes on legs and wings and at the tips of their antennae like teeny crabs. These ectopic eyes were at least partly functional too, as they responded to light beamed at them. The gene, called eyeless, is the first to earn the title of master control gene. Embyrologists have long believed that genes like this would exist. Forming a leg, wing or eye requires the orchestrated performance of many thousands of genes. Early in development when the embryo is still a mass of identical cells, master control genes, like architects, were believed to step in and mastermind the development of the embryo's body plan.
Genes that mastermind the development of the fly body plan have been identified before, though nothing quite as masterful as eyeless. Amongst these are the homeotic genes. When there's a problem with these genes, the flies sport some bizarre mistakes in their body layout - like a leg where their antenna should be or four wings instead of two.. Fruit fly geneticists have been hot on the trail of these genes for the last twenty years. Perhaps that's why they missed the eyeless gene.
Unlike the homeotic genes, whose bizarre mutations make it quite clear that their job is to lay down the body plan, eyeless mutations did not betray the true master status of the gene. Flies simply developed with small eyes. The gene responsible for the defect could have could have been something trivial like an enzyme involved in energy metabolism. So no one was too excited about spending several years chasing what might have been a rather dull gene.
In a reversal of traditional genetics, the fly eyeless gene finally received some attention after a number of eye genes had been cloned in mice and men. Any gene that causes a developmental defect in mammals is important and generally cloned as soon as possible. When the mouse small eye gene is defective, the unfortunate mice develop with shrunken eyes. The gene responsible was cloned several years ago. Soon after, researchers tracked down the gene for the human condition Aniridia, where those afflicted have defects of the iris, lens, cornea and retina. These days as soon as the genetic code of a gene is deciphered, it gets entered into a universal database where its kinship with other genes is rapidly established. It turned out that the mouse small eye gene and the human Aniridia gene were closely related
Meanwhile back in the fly, Rebecca Quiring, a student from the University of Basel had been scanning the fruit fly gene library for proteins that interfaced with homeotic genes. Quiring was working on the premise that homeotic genes are part of a molecular network and that by tracing who they were talking to, she would find other body plan genes. She picked up the eyeless gene. Into the database went the genetic sequence and now things began to get very interesting.
It turned out that the "eye genes" in these three species: flies, mice and man, had extremely similar DNA sequences, particularly at two locations - the homeoebox and the pax box. These well known motifs were over 90% identical in the three species. Like heraldic shields of the gene world, these motifs proclaim ancestry to the clan of "body plan " genes.
The researchers were on to a gene that appeared to play a key role in the design of the eye in flies, and might play a similar role in mice and men. But there are probably thousands of genes involved in the formation of the fly's eye. Where in the hierarchy did the eyeless gene lie? The answer was dramatically answered by the experiment I described before. Turn the eyeless gene on in the leg, wing or head and behold, wherever the gene acts there grows an eye.
Eyeless is a master control gene that sits at the top of the hierarchy of eye development. With the top gene identified, it should now be possible to systematically work down through the other genes in the hierarchy and indeed several have already been identified. The fly people may soon be able to do for development, what biochemists earlier this century did for metabolism. That is, describe the development of the eye in terms of a biochemical pathway -- gene A turns on B and C. B and C turn on D, E, F, G and so on.
This is fascinating stuff, the holy grail of developmental biologists, no less. But, it's not the punchline of this story. Georg Halder and Patrick Callaerts, also at the University of Basel decided to do the cheeky experiment of seeing whether the mouse gene could also make ectopic eyes in the fly. And the mouse gene worked; it made ectopic eyes in the fly. What do you think? Mouse eyes or fly eyes? What a thought! Fly eyes!
What is one to make of this? A gene from a mouse can mastermind the development of a fly's eye. So not only do the two genes have similar tracts of code, the mouse gene can actually take over as architect in the fly and work with the fly construction team. That raises all sorts of inferences. Somehow, despite the very gross differences between the fly's compound eye and the vertebrate camera eye, there is something fundamentally similar about the way flies, mice and presumably human eyes are made.
As geneticist David Bowtell at Melbourne's Peter Macallum Institute points out, it's hard to argue this as a case of convergent evolution. You could argue this for the rhodopsins, the molecules that are the basis of light reception in all types of eyes. But that's because there are extreme structural constraints on how to make a light sensitive molecule. But why should this be the case for a (pardon the pun) designer gene, whose primary job is to signal other genes? If animals evolved the design of the eye independently, then they would have evolved their own signalling systems, like different languages.
Put it this way, you would not be all that surprised to find that a Martian had evolved legs for roaming the surface of his not too dissimilar terrain. But I bet you'd be astonished if he spoke English!
So let's revisit the quandry that enmeshed Darwin, Mayr and their descendants. Has the eye actually evolved from scratch 40 - 65 times over?. We will never know the history of our earliest ancestors. They were soft-bodied creatures that left scant traces in the fossil record. To answer questions of evolutionary origin, Mayr and Plawen used the tools of anatomy, microscopy and biochemisry to explore the origins of the eyes amongst the different phyla. They concluded separate origins. But the gene hunters have provided a new tool. So far, all the phyla from nemerteans( ribbon worms), the most primitive of all metazoans with a visual system to cephalopods (squids) , arthropods (flies) and vertebrates (man) carry a relative of the eyeless gene. That very strongly points to the common ancestor of these phyla having possessed the gene and bequeathing it to its descendants.
Evolution is a potent force, but perhaps it did not have to start from scratch to evolve the eye each time. The groundwork may have already been laid.