The human eye has sometimes been held up as a problem for evolutionary theory — how did such a complex structure evolve from simple parts? Evolutionary scientists, beginning with Charles Darwin, have pointed out that the eye could have evolved incrementally, with even basic functions presenting a selective advantage.
A study from UC Davis and UC Santa Barbara published today shows how the light-sensitive pigments which now let our eyes translate light into nerve signals could have played a role in much simpler, eyeless organisms.
David Plachetzki, now a postdoctoral researcher at UC Davis, working with undergraduate Caitlin Fong and Professor Todd Oakley in the Department of Ecology, Evolution and Marine Biology at UC Santa Barbara — studied the stinging mechanism in the freshwater brainless, eyeless freshwater polyp Hydra magnipapillata. Part of a group of animals called cnidarians that includes sea anemones, corals and jellyfish, a hydra is essentially a mouth surrounded by tentacles armed with stinging cells, or cnidocytes.
(Left: A hydra; right, two tentacle bulbs, with stinging cells and neurons in red, muscle fibers in green).
They found a simple nervous system linking the stinging cells and nerve cells that detect light using a process similar to that in the human eye. The nerve cells express a set of genes including opsin, a light-sensitive pigment; cyclic nucleotide gated ion channels; and arrestin. These components are basically the same as those in the light-detecting pathway in animals with eyes, including people.
The hydra fire their stingers less in bright than in dim light, the researchers found. When they blocked one of the pathway’s components, the hydra acted as if they were in dim light and fired their stingers more.
Most of the hydra’s cnidarian relatives lack eyes. But all cnidarians have cnidocyte stinging cells.
“This capacity for stinging cell regulation by light-sensitive neurons could have predated the evolution of eyes in cnidarians,” Plachetzki said. Future work will be aimed at how these findings relate to the evolution of eyes in other groups of animals.
The National Science Foundation funded the work.