Sean B. Carroll Closely related to the rattlesnake,the fer-de-lance is a pit viper,a member of a group of venomous snakes named for the deep sensory pits between the nostrils and eyes. These specialised pits enable the snakes to detect infrared light in the form of heat. Humans and other warm-blooded animals emit heat as infrared radiation. Pit vipers are so adept at infrared sensing that some can detect potential prey a metre away. To understand how snakes evolved their infrared detection systems,a group of scientists led by Professor David Julius at the University of California-San Francisco,searched for potential infrared-sensing proteins in the western diamondback rattlesnake. They looked in particular at genes active in the nerve cells that are connected to the pits,called trigeminal neurons. They found one gene,known as TRPA1,that was 400-fold more active in rattlesnake trigeminal neurons than in other kinds of neurons. Moreover,they found that the TRPA1 gene was not highly active in the trigeminal neurons of snakes lacking pits. These two pieces of evidence suggested that TRPA1 might encode a protein involved in infrared sensing. The TRPA1 protein was very familiar to the scientists. A few years earlier,Julius group had identified TRPA1 as the receptor that drives our response to the molecules that give wasabi its punch,as well as to other chemical irritants,like tear gas. The TRPA1 gene encodes a type of receptor protein known as an ion channel. In humans and other mammals,when the protein is exposed to and binds specific chemicals,the channel opens,allowing ions to flow into nerve cells and setting off a sequence of events that produces a nerve impulse. In pit vipers,however,Julius and his collaborators discovered that the TRPA1 has evolved to be especially heat-sensitive. While the receptor is not activated in most snakes by temperatures approaching 37 degrees Celsius (98 degrees Fahrenheit,our normal body temperature),the western diamondback rattlesnake TRPA1 receptor is stimulated around 27 degrees Celsius (80 degrees Fahrenheit),creating a thermal image of the heat source in the snakes brain that is used to aim its strike. Pit vipers are not the only animals or even the only snakes to have evolved infrared sensing. Pythons and boas have also evolved heat-sensing pit organs on their faces,although of a different structure. Julius and his team found that TRPA1 was also highly expressed in the trigeminal neurons of python and boa pit organs,about 65-fold and 170-fold higher,respectively,than in the trigeminal neurons of other snakes lacking pits. Similarly,their TRPA1 receptors were 5 to 8 degrees Celsius more heat-sensitive than typical snakes. In both groups of snakes,changes in the structure of the TRPA1 receptor,and the evolution of very high levels of expression in their sensory pits,endowed the animals with sensitive infrared detectors. The large evolutionary distance between pit vipers and pythons and boas indicates that the two groups of snakes separately evolved infrared sensing. What is especially noteworthy about TRPV1 is that it is the very same receptor that detects capsaicins,the active ingredients in chili peppers,in our neurons. It causes the familiar burning sensation when prompted. The scientists discovered that the particular form of the receptor expressed in vampire bat trigeminal neurons was much more heat-sensitive (by about 9 degrees Celsius) than either our TRPV1 receptor or the TRPV1 expressed in fruit bat neurons. The vampire bat receptor is thus tuned to detect heat in ways that other bats and mammals cannot. Both the TRPA1 and TRPV1 genes are hundreds of millions of years old,having arisen deep in evolutionary history,while vampire bats,pit vipers,and pythons and boas are much younger species. The histories of these genes and animals,and the repeated invention of infrared sensing,demonstrate how the evolution of new abilities does not necessarily require new genes,but new variations of very old genes and new ways of using them.
