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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 that look very much like the lens, cornea, iris and retina of multicellular eyes found in humans and other larger animals." [Emphasis added.]
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 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."
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 a piston -- a tentacle that can extend and retract very quickly -- with an unknown function that might be used for escape or feeding.
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.
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 Chlamydomonasplastid 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 origin from 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 is striking 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.
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 atfundamentally 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.