Only some bats and toothed whales rely on sophisticated echolocation, in which they emit sonar pulses and process returning echoes, to detect and track down small prey. Now, two new studies in the January 26th issue of Current Biology, a Cell Press publication, show that bats' and whales' remarkable ability and the high-frequency hearing it depends on are shared at a much deeper level than anyone would have anticipated -- all the way down to the molecular level.
The discovery represents an unprecedented example of adaptive sequence convergence between two highly divergent groups and suggests that such convergence at the sequence level might be more common than scientists had suspected.
"The natural world is full of examples of species that have evolved similar characteristics independently, such as the tusks of elephants and walruses," said Stephen Rossiter of the University of London, an author on one of the studies. "However, it is generally assumed that most of these so-called convergent traits have arisen by different genes or different mutations. Our study shows that a complex trait -- echolocation -- has in fact evolved by identical genetic changes in bats and dolphins."
A hearing gene known as prestin in both bats and dolphins (a toothed whale) has picked up many of the same mutations over time, the studies show. As a result, if you draw a phylogenetic tree of bats, whales, and a few other mammals based on similarities in the prestin sequence alone, the echolocating bats and whales come out together rather than with their rightful evolutionary cousins.
Both research teams also have evidence showing that those changes to prestin were selected for, suggesting that they must be critical for the animals' echolocation for reasons the researchers don't yet fully understand.
"The results imply that there are very limited ways, if not only one way, for a mammal to hear high-frequency sounds," said Jianzhi Zhang of the University of Michigan, who led the other study. "The sequence convergence occurred because the amino acid changes in prestin that result in high-frequency selection and sensitivity were strongly favored in echolocating mammals and because there are [apparently] very limited ways in which prestin can acquire this ability." Prestin is found in outer hair cells that serve as an amplifier in the inner ear, refining the sensitivity and frequency selectivity of the mechanical vibrations of the cochlea, Zhang explained.
Rossiter's team, including Shuyi Zhang of East China Normal University, showed previously that the prestin gene has undergone sequence convergence among unrelated lineages of echolocating bats. These authors, along with Zhang's team at Michigan, now show that convergence extends to echolocating dolphins.
"We were surprised by the strength of support for convergence between these two groups of mammals and, related to this, by the sheer number of convergent changes in the coding DNA that we found," Rossiter said. "We were especially excited to discover that these changes are likely to be adaptive, and also that nonecholocating whales do not group with the bats but instead remain with their true relatives, the even-toed ungulates."
Although they rely on a similar ability, in fact "bats and whales vary greatly in echolocation," Michigan's Zhang pointed out. "For example, bats use echolocation for ranges up to 3 meters, whereas whales use for ranges up to >100 meters. More importantly, the speed of sound in air is about one-fifth that in water, making the information transfer during sonar transmission much slower for bats than for whales. Despite these gross differences, our findings suggest that the high-frequency acoustic sensitivities and selectivities of bat and whale echolocation appear to rely on a common molecular design of prestin."
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