Platynereis has one type of light-sensing cells in its eyes but another type in its brain.© ScienceDarwin famously realized that the eye would be a key test for his theory of evolution by natural selection. He suggested gradual steps from an "imperfect and simple" form, and modern scientists have no trouble believing that the eye evolved from a single light-detecting cell. But they disagree over whether it evolved just once, or many times.
Now the miniscule marine worm Platynereis dumerilii, whose crude light perception seems to have stood it in good stead for millennia, hints at an answer to this question.
Its few light-sensing cells come in two types: one is of a type seen almost exclusively in vertebrates, and one is seen in insects, according to a paper in this week's Science1. Could a worm like Platynereis have been the father of the eye?
Fingers or umbrellas
Insect eyes are known to consist of an array of compound lenses, whereas vertebrate eyes contain a single lens. But they are also made of different types of cells: insects' eyes are built up with cells called rhabdomeric photoreceptors; vertebrates use ciliary photoreceptors.
“What was in the brain in the worm ends up in our eye.”
Joachim Wittbrodt
European Molecular Biology Laboratory in Heidelberg, Germany.
The most obvious difference between these cells is in the way the cells increase their surface area to accommodate large numbers of light sensors. Rhabdomeric cells are covered in little finger-like protrusions. Ciliary cells sport hair-like cilia that extend outwards and branch out like tiny umbrellas.
Joachim Wittbrodt of the European Molecular Biology Laboratory in Heidelberg, Germany, and his colleagues have found that Platynereis has rhabdomeric receptors in its tiny eyes, and ciliary cells in its equally tiny brain. The ciliary cells perhaps regulate its daily activity cycle by sensing light, Wittbrodt guesses. "We think they are related to circadian rhythms. We have found that there is a direct connection to the area used for locomotion."
Single origin?
So if this worm has both kinds of photoreceptor, does that mean that the two types of eyes, insect and vertebrate, both originated in an ancestor of this species? If the animal had two copies of the genes needed to make one kind of photoreceptor, speculates Wittbrodt, then the extra set would have been free to evolve into the other photoreceptor. Different animals would subsequently evolve to use the two options in different ways.
"In the beginning we had a toolbox, and we took what was there. What was in the brain in the worm ends up in our eye," says Wittbrodt, who goes on to make careful caveats about the need for more research. This finding, he says, does not by itself prove that the eye only evolved once.
Joram Piatigorsky, chief of molecular and developmental biology at the National Eye Institute in Bethesda, Maryland, is just as careful. " It is consistent with the idea of a single origin," he says. "But it doesn't nail the issue that everything came from one animal. It leaves open a lot of evolutionary questions."
European Molecular Biology Laboratory in Heidelberg, Germany.
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