Sonar links bats and whales

In a striking example of evolutionary convergence, bats and whales appear to have at least two things in common: their ability to use biosonar to navigate and explore their environments and the molecular sequence of a protein that helps them do so, according to two new papers published online today (January 25) in Current Biology.

An echolocating bat (Myotis bechsteinii)
avoiding collision with a plant
Image: Wikimedia commons,
PLoS Computational Biology

"It's a nice example" of convergence at the molecular level, said evolutionary biologist linkurl:David Pollock;http://www.evolutionarygenomics.com/Pollock.html of University of Colorado School of Medicine, who was not involved in the research, "and you've got this unusual convergence of function that they're both using sonar. That's a sort of ready explanation [for the strong sequence similarity], if you will." The protein in question, Prestin, is thought to play a role in allowing mammals to detect minute differences in timing and frequency of the outgoing and incoming signals of echolocation. Previous work showed that Prestin has undergone convergent evolution in unrelated clades of bats that echolocate. By adding another distantly related group of echolocating mammals -- toothed whales -- to the picture, two groups of researchers working independently demonstrate a more widespread convergence of Prestin among echolocating organisms, suggesting that molecular convergence may be more common than previously believed. Evolutionary biologist linkurl:Stephen Rossiter;http://www.sbcs.qmul.ac.uk/staff/stephenrossiter.html of Queen Mary, University of London and his colleagues sequenced the Prestin genes of echolocating toothed whales -- the sperm whale and four species of dolphins -- and non-echolocating baleen whales, and compared them with the gene and protein sequences of 18 bat species. (Some of the researchers, including Rossiter, were also authors of the linkurl:original paper on bats;http://www.pnas.org/content/105/37/13959.abstract only.) Building evolutionary trees based on these Prestin protein sequences, the team found that dolphin species formed a cohesive group with two different families of echolocating bats, suggesting a striking similarity in their amino acid sequences. "It's spectacular really," Rossiter said. "I would never have imagined convergence to such an extent" that the protein sequences would group such distantly related organisms. Working entirely independently, linkurl:Jianzhi Zhang;http://www.umich.edu/%7Ezhanglab/ of the University of Michigan and his colleagues got a nearly identical result, with bottlenose dolphins being lumped in with the same two families of bats. The fact that the two groups' analyses returned such similar findings "is reassuring," Zhang said. These findings, combined with a linkurl:previously identified example of molecular convergence;http://www.pnas.org/content/106/22/8986 in snake and lizard mitochondrial genomes, suggest that molecular convergence may be more common than scientists realize -- "it's [just] not always easy to detect," Pollock said. "And the more we see this kind of thing, [the more it] gives us an idea of how proteins really do evolve," he added. One echolocating whale species, the sperm whale, however, was not grouped with echolocating bats on the basis of its Prestin sequence. "It shares some [amino acid] sites with the dolphin, but it hasn't undergone this dramatic conversion with bats," Rossiter said. This may be because sperm whales echolocate at much lower frequencies than the other species, he added. To determine if this remarkable convergence of the Prestin protein is a result of the convergent ability to hear high frequency sounds, "we need to look at bats that produce very low frequency signals when they're echolocating," said linkurl:Brock Fenton;http://www.uwo.ca/biology/Faculty/fenton/people.htm of the University of Western Ontario, who did not participate in the research. Further clues might come from other species, he added. "It would be really exciting to see what the echolocating birds are doing, because they're also using very low frequency signals. If they also have the same Prestin, it's not high frequency [hearing that was selected for]; it's just echolocation." Either way, it is clear that Prestin's extraordinary convergence is associated with receiving the echolocating signals, as opposed to producing them. "Whales are using a completely different mechanism for sound production," said Fenton, who published a linkurl:paper online yesterday;http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08737.html in Nature detailing the structure of the larynx of different bat species, many of whom use the larynx to echolocate. In whales, "the whole [bone structure] is different." But regardless of which aspect of echolocation drove the convergence of the Prestin sequences, this example of molecular convergence with a clear functional significance "is important [for] understanding how proteins work and how they move through the adaptive landscape," Pollock said. While it seems there could be many different ways for proteins to achieve the same functional goal, this work supports the idea that "there are specific reasons the proteins will change in a specific way," Pollock said. "That's sort of fundamentally interesting."
**__Related stories:__***linkurl:Primate evolution claim challenged;http://www.the-scientist.com/blog/display/56110/
[21st October 2009]*linkurl:Ancient organism, modern immunity;http://www.the-scientist.com/blog/display/55736/
[27th May 2009]*linkurl:Evolution of innate immunity;http://www.the-scientist.com/article/display/22271/
[8th July 2004]