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DNA Discovery Reveals Surprising Dolphin Origins

DNA Discovery Reveals Surprising Dolphin Origins

January 12, 2014 - A well-known dolphin species, the clymene dolphin, arose from mating between two separate and distinct dolphin species, report genetics researchers.

Also known as the "short-snouted spinner dolphin," the clymene dolphin (Stenella clymene) grows to nearly seven feet (2.1 meters) long and dwells in deep waters in tropical and temperate parts of the Atlantic Ocean.

Evolutionary biologists have seen other such hybrid species elsewhere in the animal kingdom. The new discovery, reported in the journal PLOS ONE by a team led by marine biologist Ana Amaral of Portugal's University of Lisbon, adds to increasing evidence of such cross-breeding commonly leading to new species, even in the wide-open oceans.

Clymene dolphins feed mostly at night when squid and fish come to the surface of the water. The short-snouted dolphin gets its name from the ocean nymph Clymene of Greek mythology. (See "Dolphins Have 'Names,' Respond When Called.")

Researchers initially thought the clymene dolphin was a subspecies of the spinner dolphin (Stenella longirostris). However, in 1981, a closer look at the clymene's anatomy revealed it was a distinct species.

But experts remained uncertain about the clymene's relationship with its close relatives. Although its outward appearance and behavior are more similar to those of the spinner dolphin, its skull features closely resemble those of the striped dolphin (Stenella coeruleoalba).

To help solve this mystery, the study scientists analyzed skin samples from 15 clymene dolphins, as well as from 21 spinner and 36 striped dolphins. They collected the DNA from free-ranging dolphins—using special tissue-collecting darts—and from dead, stranded dolphins.

The investigators looked at nuclear DNA, which is found in the cell's nucleus and comes from both the mother and father, as well as DNA from their mitochondria—the cell's powerhouse—which possesses its own genes and is passed down solely from the mother.

"When I was first analyzing the data I collected, it was very confusing," said Amaral.

That's because Amaral and colleagues discovered that while the nuclear DNA of the clymene dolphin most resembled that of the spinner dolphin, the mitochondrial DNA was most similar to that of the striped dolphin. (See pictures of other hybrid animals.)

This is strong evidence that the clymene dolphin is a naturally occurring hybrid of the spinner and striped dolphins, said Amaral. Continued hybridization may still occur between the clymene dolphin's parent species, although at low levels, the study authors added.

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