Spectacular Convergence: A Camera Eye in a Microbe

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They thought it was a joke. A century ago, biologists could not believe that a one-celled creature had an eye. But since the warnowiid dinoflagellate was difficult to find and grow in the lab, detailed research was rare, until now. A team from the University of British Columbia gathered specimens off the coast of BC and Japan for a closer look. They found that the structure, called an ocelloid, has structures that mimic the complex eye of higher animals. PhysOrgsays:

In fact, the ‘ocelloid’ within the planktonic predator looks so much like a complex eye that it was originally mistaken for the eye of an animal that the plankton had eaten.

“It’s an amazingly complex structure for a single-celled organism to have evolved,” said lead author Greg Gavelis, a zoology PhD student at UBC. “It contains a collection of sub-cellular organelles thatlook very much like the lens, cornea, iris and retina of multicellular eyes found in humans and other larger animals.” [Emphasis added.]

New Scientist shares the astonishment:

It is perhaps the most extraordinary eye in the living world — soextraordinary that no one believed the biologist who first described it more than a century ago.

Now it appears that the tiny owner of this eye uses it to catch invisible prey by detecting polarised light. This suggestion is also likely to be greeted with disbelief, for the eye belongs to a single-celled organism called Erythropsidinium. It has no nerves, let alone a brain. So how could it “see” its prey?

The “retina” of this eye, a curved array of chromosomes, appears arranged to filter polarized light. The news item from the Canadian Institute for Advanced Research quotes Brian Leander, co-supervisor of the project:

“The internal organization of the retinal body is reminiscent of the polarizing filters on the lenses of cameras and sunglasses,” Leander says. “Hundreds of closely packed membranes lined up in parallel.”

And that’s not all this wonder of the sea has in its toolkit. It also has a piston and a harpoon:

Scientists still don’t know exactly how warnowiids use the eye-like structure, but clues about the way they live have fuelled compelling speculation. warnowiids hunt other dinoflagellates, many of which are transparent. They have large nematocysts, which Leander describes as “little harpoons,” for catching prey. And some have apiston — a tentacle that can extend and retract very quickly — with an unknown function that might be used for escape or feeding.

Did This Eye Evolve?

Lest anyone think the dinoflagellate’s eye presents an easy evolutionary stepping stone to more complex eyes, the data reveal several problems. The paper inNature claims that the ocelloids are built from “different endosymbiotically acquired components” such as mitochondria and plastids. “As such, the ocelloid is a chimaeric structure, incorporating organelles with different endosymbiotic histories.” We can treat endosymbiosis as a separate issue. For now, we can ask if this complex structure is explainable by unguided natural selection.

The authors did not think this is a clear evolutionary story. “The ocelloid isamong the most complex subcellular structures known, but its function andevolutionary relationship to other organelles remain unclear,” they say. Never in the paper do they explain how organelles with different histories came together into a functioning eye. Most of the paper is descriptive of the parts and how they function individually, or where they might have been derived by endosymbiosis. To explain the eye’s origin as a functioning whole, they make up a phrase, “evolutionary plasticity” —

Nevertheless, the genomic and detailed ultrastructural data presented here have resolved the basic components of the ocelloid and their origins, and demonstrate how evolutionary plasticity of mitochondria and plastids can generate an extreme level of subcellular complexity.

Other than that, they have very little to say about evolution, and nothing about natural selection.

In the same issue of Nature, Richards and Gomes review the paper. They list other microbes including algae and fungi that have light-sensitive spots. Some have the rhodopsin proteins used in the rods and cones of multicellular animals. But instead of tracing eye evolution by common ancestry, they attribute all these innovations to convergence:

These examples demonstrate the wealth of subcellular structures and associated light-receptor proteins across diverse microbial groups. Indeed, all of these examples represent distinct evolutionary branches in separate major groups of eukaryotes. Even the plastid-associated eyespots are unlikely to be the product of direct vertical evolution, because the Chlamydomonas plastid is derived from a primary endosymbiosis and assimilation of a cyanobacterium, whereas the Guillardia plastid is derived from a secondary endosymbiosis in which the plastid was acquired ‘second-hand’ by intracellular incorporation of a red alga. Using gene sequences recovered from the warnowiid retinal body, Gavelis et al. investigated the ancestry of this organelle by building phylogenetic trees for the plastid-derived genes. Their analysis demonstrated that this modified plastid is also of secondary endosymbiotic originfrom a red alga.

Although derived independently, there are common themes in theevolution of these eye-like structures. Many of them involve thereconfiguration of cellular membrane systems to produce anopaque body proximal to a sensory surface, a surface that in four of the five examples probably involves type 1 rhodopsins. Given the evolutionary derivation of these systems, this represents a complex case of convergent evolution, in which photo-responsive subcellular systems are built up separately from similar components to achieve similar functions. The ocelloid example isstriking because it demonstrates a peak in subcellular complexity achieved through repurposing multiple components. Collectively, these findings show that evolution has stumbled on similar solutions to perceiving light time and time again.

But is convergence just a word masquerading as an explanation? We read:

The work sheds shed new light on how very different organisms can evolve similar traits in response to their environments, a process known as convergent evolution. Eye-like structures haveevolved independently many times in different kinds of animals and algae with varying abilities to detect the intensity of light, its direction, or objects.

“When we see such similar structural complexity at fundamentally different levels of organization in lineages that are very distantly related to each other, in this case warnowiids and animals, then you get a much deeper understanding of convergence,” Leander says.

But “convergent evolution” is not a process. It is a post-hoc observation based on evolutionary assumptions. An environment has no power to force an organism to respond to it with a complex function. Light exists, whether or not an organism sees it. Magnetism exists, too; does it contain the power to nudge fish, turtles, and butterflies to employ it for navigation?


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