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UBC study sheds light on bizarre plankton with ‘eyes’

The warnowiid, a single-celled organism, (photo, top right) sports an eye-like structure called an ocelloid

Greg Cavelis

In biology there is nothing quite as elegant as the eye, an organ so impressive in its design that two centuries ago Christian philosopher William Payley held it up as evidence for a divine creator.

God only knows what Rev. Payley would have made of the ocelloid, an unusual feature that has turned up in a few species of single-celled plankton. Though it measures only one hundredth of a millimetre across, the ocelloid appears to be doing for the plankton what eyes do for animals. It may even help the plankton hunt down prey.

Now, a study conducted at the University of British Columbia has bolstered this extraordinary claim. Not only does the ocelloid act like an eye, the study reports, it's built like one too – complete with the microscopic equivalents of a cornea, lens and retina, made from components that were originally acquired and adapted from other microbes.

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"It's an incredible example of convergent evolution," said Brian Leander, a professor of marine zoology and a co-author on the study, published Wednesday in the journal Nature.

Convergent evolution means unrelated species arriving at similar solutions to common problems. For example, bird wings did not give rise to bat wings. Instead, bat wings evolved independently by converging toward the same function – flight.

Building on earlier work, the UBC study suggests how a group of microbes known as warnowiids may have converged on something as biologically complex as an eye by using building blocks that were close at hand.

Those building blocks include mitochondria, the energy-producing units within cells. The UBC team found that assembled rows of mitochondria are used to form the cornea of the ocelloid. The cornea encases a lens of unknown material, which has the right shape to bend and concentrate light onto a retinal body.

The UBC team also used genetic markers to show that the retinal body is made of plastids, light-sensitive structures that have their own DNA and were originally imported from red algae.

During cell division, when a warnowiid splits into two daughter cells, the retina copies itself while the cornea and lens dissemble. Then each warnowiid reassembles its own ocelloid from its share of the pieces.

Dr. Leander noted that the structure of the retinal body features a "highly ordered arrangement of tightly packed membranes" that resemble a polarized light filter. This may offer a clue to the ocelloid's purpose.

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Warnowiids survive by feeding on other single-celled creatures that are mostly transparent except for their chromosomes, which can polarize light. The UBC researchers speculate that ocelloids allow warnowiids to "see" the polarized light streaming through their prey.

Dr. Leander and his colleagues found a network of plastid "arms" that radiate away from the retinal body and might convey information to the part of the cell that can move the warnowiid toward its prey.

"If this is true, then the plastid network would essentially function like a nerve net – however, this is pure speculation at this point," he said.

Fernando Gomez, a plankton expert at the University of Sao Paulo in Brazil who was not involved in the study, praised the UBC team members but cautioned that it would be some time before their theory could be tested. Warnowiids are rare in marine samples and, so far, have not been successfully cultured in a laboratory setting.

The UBC project was a three-year effort, led by Greg Gavelis, a PhD student, and involved a painstaking search for warnowiids off the coasts of Japan and British Columbia.

Dr. Gomez noted that ocelloid-bearing micro-organisms are more common in tropical ocean waters, "where the light penetration allows using this visual competitive advantage," but that laboratories best equipped to study the creatures are located in the temperate north.

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Dr. Leander, whose work on microbial biodiversity is supported by the Canadian Institute for Advanced Research said the ocelloid study was motivated by researchers' curiosity about the "beautiful and bizarre" and the broader goal of trying to understand the evolution and interrelationships of life.

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