Chriswant Jintin
Light-sensing proteins are found in all areas of life. Even single-celled microorganisms have proteins that respond to light. Animals also have light-sensitive organs of various shapes and structures. They all seem to work along the same principle. Photons are absorbed by proteins that respond by allowing ions to flow across the membrane. In single cells, this causes regional differences in ion concentrations, enabling responses. In more complex organisms, these ions flow into nerve cells and emit signals.
But scientists are describing a strange exception this week: centipedes. These organisms clearly respond to light. This is known to anyone who tries to stomp before returning under a rock or wall. However, many species appear to have no eyes (and many species with eye-like structures do not sense light with their eyes). And studies of their genomes show that they don’t have the usual light-sensitive proteins. So how do these arthropods do that?
see heat
Many species of centipedes have something like eyes and contain cells that respond to light. However, studies of these organs show no significant effect on the animal’s response to light. Light-trigger proteins tend to have similar structures because they need to form channels through membranes to allow the passage of ions. Therefore, selecting genes for these proteins is usually relatively straightforward when the genomic sequences become available.
In any case, many centipede species have eyes. They also tend to live under rocks and debris when they’re not sneaking into their homes. That is, you don’t necessarily need to see much in normal circumstances.
However, when one of these animals is exposed to light, it tends to get out of it quickly, indicating that the animal can sense light. what happened?
To find out, a team of Chinese researchers built a system that allows centipedes to be exposed to light to test their reactions. Imaging with this system showed that the antennae had a distinctive pattern in the infrared. They were heated by the light. The temperature of the antennae increased by more than 8 degrees Celsius in about 10 seconds, much faster than any other part of the body.
Another appendage (of which many legs are imaged) was not. Don’t ask me how they did this, the researchers put a tin foil cap on the centipede to cover off its antennae. This greatly reduced our ability to find dark areas. Overall, this has led researchers to suspect that whatever is happening in light sensing might be happening in these organs.
rare gene
The researchers next decided to identify the genes involved. To do so, they isolated all gene copies active in the antennae. Next, proteins need to allow ions to pass through, so we identified those with segments that could pass through membranes. This reduced the list of possible genes from over 8,000 to over 1,000. At that point, researchers were just beginning to put genes individually into human cells, finding one that puts ions into cells at high temperatures.
The gene that worked was called BRTNaC1 and is somewhat distantly related to the known family of ion channels that move sodium ions in and out of cells. But that family is not temperature sensitive. Also, BRTNaC1 is not picky about which ions it lets into the cell. Fortunately, calcium can also cross membranes.
After testing various chemicals, they found that testosterone inhibited the activity of BRTNaC1. Insects do not make testosterone, so this is biologically irrelevant. So the researchers treated centipedes with testosterone to test if they were still sensitive to light. they were not
All of this indicates that these animals sense light without directly recording photons. Instead, they sense it very indirectly, using organs that selectively heat up when exposed to light, allowing the animal to sense that temperature change. This is the only example that has been These centipedes may be unique in this biology.
There are two surprising things about this. One is that there are many known proteins that organisms use to sense temperature that might cause them to take their hands off the hot stove. yeah. As such, it seems to have evolved completely independently. Second, I don’t understand how the antenna is structured and selectively heated.
PNAS, 2019. DOI: 10.1073/pnas.2218948120 (About DOI).
