Let There Be Light!
Evolution and the Genesis of Light Sensitivity in Animals
I'll discuss our lab's work to try to learn about how light sensitivity, which today allows vision, first originated in animals. Some of our work is published, and I've discussed some of it before on the blog here.
Namely, we have been studying the genetic basis of photosensitivity in the cnidarian Hydra magnipapillata. Cnidarians are the most distant relatives of humans that possess (type-II) opsin-based photosensitivity. As such, components of the mechanisms of light sensitivity shared in humans and cnidarians may have been present at the very origin of this type of photosensitivity, and can inform us about how this amazing sense originated during evolution.
To test whether particular genes are used in sensing light in hydra, we are taking advantage of a behavioral response to light. Hydra have no eyes - they do not have pigmented spots. But they do respond to light. I've today been working on a video to illustrate a contraction response that hydra undergoes when light is shined on them. We can knock out candidate photo genes, and if the animal doesn't contract, this supports the hypothesis that the knocked out gene could be used in sensing light.
The video is time-lapse, shot through a dissecting microscope. The animal is small, less than half an inch long. Its base, to the right, is attached to a petri dish, and its tentacles are out to the left. The hydra does a little stretch, and then the contraction. This happens over about a minute or two, but is condensed to a few seconds here.
This type of behavior was noted in a different species way back in 1744 by Abraham Trembly. At that time, people thought polyps were plants. Trembly saw a hydra polyp move, and decided it was probably an animal. To test this hypothesis, he cut one in half: animals should die, and plants could regenerate. But hydra can regenerate even though they are animals. So Trembly discovered animal regeneration, and (sort of) stem cells.