Bats and dolphins are about as different as mammals get. Yet, both home in on their prey by emitting sound waves and sensing the reflections, a process called echolocation. And a new study shows that in both groups the same protein evolved in the same way to make that possible. Researchers say it's surprising to discover a molecular convergence in these very distantly related groups of animals.
The protein, called prestin, exists in all mammals and helps so-called outer hair cells in the inner ear amplify incoming sound waves. Because people with mutations in the prestin gene often can't hear high frequencies, Shuyi Zhang, an ecologist at East China Normal University (ECNU) wondered whether prestin had evolved to make possible the high-frequency hearing that bats use for echolocation. With student Liu Yang of ECNU, James Cotton and Stephen Rossiter of Queen Mary, University of London, and colleagues, he analyzed the sequence of the prestin gene in distantly related bats that had independently evolved echolocation. In 2008, the researchers discovered that the functional parts of prestin had come to look the same in both groups of bats but not in other bats that do not use echolocation, indicating convergent evolution of the protein.
The team has now looked even farther afield, examining whether prestin in dolphins and other toothed whales has the same makeup. They sequenced the prestin gene in several dolphin species, in a sperm whale, and in baleen whales, which do not use echolocation, and then compared the sequences with those of bats. Fourteen sites had evolved to be exactly the same, six of which proved most likely to be due to convergent evolution, they report in the 26 January issue of Current Biology. They then used computers to simulate how the prestin gene might change over time to rule out that these six genes converged by chance through random mutations. They concluded that natural selection likely drove the changes to be the same. The sperm whale had some of the same prestin gene sequence but was not completely the same. That difference might be because it seems to use lower frequencies for echolocation than other toothed whales, says Rossiter.
Intrigued by what Shuyi Zhang and colleagues had seen in bats, evolutionary geneticist Jianzhi Zhang of the University of Michigan, Ann Arbor, also chased down prestin sequences in 25 other species. He, Ying Li and Zhen Liu of the Chinese Academy of Sciences in Kunming, and colleagues also find that prestin has come to look quite similar in bats and toothed whales, and report their results as well in another paper in Current Biology. "It is likely that in echolocating mammals, prestin is specifically tuned to amplify very high-frequency sound," says Jianzhi Zhang.
Many researchers assume that similarities among organisms in very different environments would occur by evolution of different genes, or at least different mutations of the same genes. So the new finding was "very unexpected," says Gareth Jones, an evolutionary biologist at the University of Bristol in the United Kingdom. "The next big question will be to determine what the functional significance of these changes are." Shozo Yokoyama, an evolutionary biologist at Emory University in Atlanta agrees that verifying prestin's role in echolocation is a critical next step. "Until then," says Yokoyama, "the results in the two papers should be interpreted with caution."