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Week of February 1, 2010

Research

Echolocating bats and whales share molecular mechanism

With high-pitched squeaks, clicks and chirps and ultra-sensitive hearing, toothed whales and some bats zero in on prey by emitting pulses of sound and interpreting the echoes that bounce back.

Over the course of evolution, the two groups acquired this remarkable ability independently, for use in very different environments, so you’d expect the means by which each accomplishes the feat to differ. Surprisingly, that’s not the case, a new study by researchers at U-M suggests.

“The seemingly different echolocation abilities that evolved independently in whales and bats have a similar underlying molecular mechanism,” says Jianzhi (George) Zhang, professor of ecology and evolutionary biology. The finding overturns conventional thinking that the evolutionary phenomenon known as convergence is rare at the molecular level. The research is reported in the Jan. 26 issue of the journal Current Biology. A separate paper by another research group, published in the same issue of the journal, reports similar results.

In traits such as appearance and behavior, convergence — the acquisition of similar structures or abilities in different lineages — is a well-known biological curiosity. Birds and bats separately developed wings and the ability to fly, for example; elephants and walruses each ended up with tusks. But because similar structures can be built from different blueprints, it’s unusual for these superficial similarities to share molecular underpinnings.

The new research focused on a gene that codes for a protein called prestin, which plays an important role in hearing, amplifying sounds of particular frequencies. Zhang and colleagues searched publicly available databases for prestin gene sequences and found data for 25 mammal species, including pig, cow, dog, cat, mouse, 10 echolocating bats and three non-echolocating bats, but only one toothed whale, for which the hearing gene sequence was incomplete. Fortunately, Zhang had DNA from a bottle-nosed dolphin (a type of toothed whale) left over from a previous project, so his lab used that material to obtain the dolphin’s prestin gene sequence.

Then the researchers constructed an evolutionary tree, considering only prestin sequences of the 25 species, not their entire genomes. On the prestin tree, the dolphin clustered with echolocating bats, rather than with its rightful evolutionary cousins.

“That was shocking,” Zhang said. After ruling out other possibilities, he concluded that molecular-level convergence explained the similarities between dolphin and bat prestin.

To get at the specific amino acid changes involved, the researchers altered the prestin sequences by systematically removing amino acids and then creating new evolutionary trees with the altered sequences, continuing until bats and dolphin clustered with their actual kin instead of together. The results suggested that the same amino acid changes in bats and dolphin led to their misplacement on the prestin tree.

As an additional check, the researchers used the correct evolutionary tree to infer the prestin sequences of various mammalian ancestors; then they mapped differences between the ancestral sequences and the present-day bat and dolphin prestin sequences.

“By this method, too, we found that they share some of the same changes,” Zhang said. The similarity is no coincidence, he believes. It apparently came about due to natural selection operating on prestin.

Zhang’s coauthors on the paper also includes U-M visiting scholar Ying Li.

 

STAFF SPOTLIGHT

Teresa Herzog Mourad, on her favorite part of her job: “I am constantly inspired by changes people make in their alcohol-related attitudes and behaviors.”