In an article/letter published in the Journal "Nature" on July 1st, 2015(paywalled), Gregory S. Gavelis, Shiho Hayakawa, Richard A. White III, Takashi Gojobori, Curtis A. Suttle, Patrick J. Keeling & Brian S. Leander explain that they found a single celled eye in the already complex unicellular Warnowiid Dinoflagellate.
Warnowiid Dinofalgellates are plankton have been previously been studied for their highly elaborate ultrastructural systems: pistons, nematocysts, and ocelloids in a single celled eukaryote. (free full text)
|Single celled organism with an EYE (Ocelloid)|
This Dinoflagellate Warnowiid is a single-celled sea orgaism that has fashioned itself an eye with a basic lens, cornea and retina. That is incredibly complex for a single cell.
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.
The warnowiid is found off the coast of Canada and Japan, but only in tiny numbers. Researchers first stumbled across one more than a century ago, but studying the creature proved difficult because it rapidly disintegrates after being taken out of seawater.
To catch his own warnowiid, Gregory S. Gavelis searched seawater samples under a microscope for a year. When he eventually found one he froze it in plastic resin, preserving it like a fly trapped in amber. He then made a 3-D model by taking snapshots of the warnowiid under an electron microscope – and was stunned by what he saw.
While other single-celled creatures can detect light using “eyespots” – simple structures that allow an organism to tell dark from light – the warnowiid seemed to have repurposed its internal organelles to form what resembled the lens, cornea, iris and retina of a complex eye.
The “cornea”, for example – the transparent outer layer at the front of the eye – was made of mitochondria, the bits of the cell normally responsible for energy production. The mitochondria in the warnowiid interlocked to form a sheet-like layer around the lens, curved to concentrate incoming light on to the “retina”.
|(Left : Electomicrograph Right :Schematic ) of the Eye of the Warnowiid|
The light-sensing “retina” at the back of the eye was made up of plastids, structures normally involved in photosynthesis. The team extracted genetic material from inside the plastids and found it rich in algal DNA. The team suspects that at some point during evolution, a warnowiid ancestor gobbled up some algae and adopted its photosynthetic equipment. When the organism later abandoned photosynthesis for the predatory life, it repurposed its light-capturing plastids into a light-sensing organ.
What is the Warnowiid Eye used for ?
It is not clear how warnowiids use their “eye”, but the authors believe they’re used to hunt. Warnowiids spiral through the water as they swim and Gavelis thinks the eye lets them see flashes of light as it bounces off the single-celled organisms that are their prey. The lens likely concentrates incoming light to increase sensitivity – rather than projecting an image as our own eye does.
What other complex structures do the Warnowiids possess ?
|Image from a previously published study on Warnowiids|
Light micrographs of the investigated warnowiid and polykrikoid taxa. (a)-(f) Images representing isolates 1 and 2 of 'Proterythropsis' sp. (a) Lateral view, median focus showing the large nucleus (n), the ocelloid (double arrowhead), and the posterior cell 'extension' (arrow). (b) Lateral view, showing the nematocysts (arrowheads). (c) Left ventral view showing the posterior cell 'extension' (arrow) and the ocelloid (double arrowhead). (d) Dividing cell with partly reassembled ocelloids/hyalosomes (double arrowheads) in the developing daughter cells. (e) Ocelloid. (f) Nematocysts. (g)-(i) Images representing the isolate of Warnowia sp. (British Columbia). (g) Lateral view of a free swimming cell showing the ocelloid (double arrowhead). (h) Lateral view of a cell in a hyaline cyst (arrow) showing the ocelloid (double arrowhead). (i) Dividing cell in a hyaline cyst (arrow) showing the ocelloids (double arrowheads). (j)-(k) Images showing the isolate of "Warnowia sp." (Florida) used for single cell PCR. (j) Ventral view, surface focus, showing the ocelloid (double arrowhead). (k) Mid cell focus showing the large nucleus (n). (l) An extruded nematocyst of Polykrikos kofoidii. (m)-(p) Images representing the two isolates of 'Nematodinium' sp. (m) Lateral to ventral view of a free swimming cell showing the large nucleus (n), the ocelloid (double arrowhead), and the brownish chloroplast color. (n) Lateral view of a cell in a hyaline cyst (arrow) showing the ocelloid (double arrowhead). (o) Dividing cell in a hyaline cyst (arrow). (p) Image showing recently divided daughter cells within the hyaline cyst (arrow). (q)-(r) Images of Erythropsidinium sp. showing the ocelloid (double arrowheads), and the piston (arrows). (s) Image representing the isolate of 'Pheopolykrikos' hartmannii showing the two large nuclei in the pseudo-colony. (t) Image representing the isolate of Polykrikos kofoidii undergoing division of the pseudo-colony. (u) Images of Polykrikos lebourae showing the two nuclei within the pseudo-colony. Scale bars 10 μm in (a)-(d), (g)-(i), (l)-(p), (t), (u), 20 μm in (j), (k), (q)-(s), 5 μm in (e), (f).
This is just a small bit of the evolution of the eye :
Here’s an abbreviated version of the leading model of the evolution of the eye:
- A mutation resulted in a single photoreceptor cell, which allowed the organism to respond to light and helped to calibrate circadian rhythms by detecting daylight.
- Over successive generations, possessing multiple photoreceptors became the norm in the gene pool, because individuals with mutations encoding for an increased number of photoreceptors were better able to react to their surroundings. An arms race began, fueling the evolution of the new sensory organ.
- Eventually, what was once just a single photoreceptor cell became a light-sensitive patch. At this point, the creature was still only able to distinguish light from dark.
- A slight depression in the patch created a pit, for the first time allowing a limited ability to sense from which direction light or shadow was coming from.
- The pit’s opening gradually narrowed to create an aperture — like that of a pinhole camera — making vision sharper.
- The aqueous humour formed. A colourless, gelatinous mass filling the chamber of the eye, it helped to maintain the shape of the eye and keep the light sensitive retina in place.
- At the front, a transparent tissue with a concave curvature for refracting light formed. The addition of this simple lens drastically improved image fidelity.
- A transparent layer evolved in front of the lens. This transparent layer, the cornea, further focused light, and also allowed for more blood vessels, better circulation, and larger eyes.
- Behind the cornea, a circular ring formed, the iris, with a hole in its centre, the pupil. By constricting, the iris was able to control the amount of light that reached the retina through the pupil.
- Separation of these two layers allowed another gelatinous mass to form, the aqueous humor, which further increased refractive power.
Here is a PDF of an article explaining the Warnowiids .