According to a research team headed by Prof Brian Leander of the University of British Columbia’s Departments of Botany and Zoology, single-celled organisms called warnowiid dinoflagellates evolved a tiny version of a multi-cellular eye, possibly to help see their prey better.
Light micrograph of a warnowiid dinoflagellate: double arrowhead points to the ocelloid, and arrow to the posterior cell extension. Scale bar – 10 μm. Image credit: Mona Hoppenrath et al., doi: 10.1186/1471-2148-9-116.
“It’s an amazingly complex structure for a single-celled organism to have evolved. It contains a collection of sub-cellular organelles that look very much like the lens, cornea, iris and retina of multicellular eyes found in humans and other larger animals,” said Gregory Gavelis, a PhD student at the University of British Columbia and first author on the study published online in the journal Nature.
In fact, the ocelloid (eye-like structure) 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.
Marine biologists still don’t know exactly how warnowiid dinoflagellates use the eye. These organisms use small harpoon-like structures to hunt prey cells in the plankton, many of which are transparent.
The researchers speculate that the eye helps warnowiid dinoflagellates detect shifts in light as it passes through their transparent prey. The structure could then send chemical messages to other parts of the cell, showing them in which direction to hunt.
Illustration of a warnowiid dinoflagellate showing the basic components of the ocelloid with their putative organellar origins. Image credit: Gregory S. Gavelis et al.
“The internal organization of the retinal component of the ocelloid is reminiscent of the polarizing filters on the lenses of cameras and sunglasses. It has hundreds of closely packed membranes lined up in parallel,” said Prof Leander, who is the senior author on the study.
Prof Leander, Mr Gavelis and their colleagues gathered samples of warnowiid dinoflagellates off the coasts of British Columbia and Japan. They then analyzed the eye-like structure using state of the art microscopy.
The study sheds new light on how very different organisms – in this case warnowiids and animals – can evolve similar traits in response to their environments, a process known as convergent evolution.
“When we see such similar structural complexity at fundamentally different levels of organization in lineages that are very distantly related, then you get a much deeper understanding of convergence,” Prof Leander said.
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Gregory S. Gavelis et al. Eye-like ocelloids are built from different endosymbiotically acquired components. Nature, published online July 01, 2015; doi: 10.1038/nature14593
