The Whale's Tale

Science News,  Nov 6, 1999  by Richard Monastersky

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Usually harmless, these segments have an uncanny ability to co-opt the DNA copying apparatus in cells, and they may have played a role in spawning some viruses. "Retroviruses, such as the AIDS viruses, were believed to have been generated from LINEs," says Okada.

Though SINEs and LINEs may fulfill no useful mission in the genome, these self-serving molecules turn out to have unique value in mapping the twists and turns of evolution. By looking for particular examples of these copies in specific sites of the genome, researchers can determine when various species split off from related ones.

In the recent study, Okada and his colleagues found that ail artiodactyls are not equally related to whales, as paleontologists have long maintained. Instead, hippos and whales share four SINEs not present in the other artiodactyls tested--camels, pigs, deer, giraffes, and cows. Since hippos are clearly artiodactyls, whales would also deserve bona fide membership in the same order, rather than simply being cousins to artiodactyls, as paleontologists would have it.

Molecular biologists have found evidence of a link between whales and hippos before, but Okada's technique has much more power than the conventional analysis of gene sequences, says Daniel Graur of Tel Aviv University in Israel. "Okada's ruining my livelihood. He'll put me out of business," jokes Graur, who analyzes genes.

In the standard method of constructing genetic family trees, molecular biologists compare the DNA sequences of genes in several species of organisms. Because the sequences change slowly over time, they should look more similar in closely related species--deer and antelope--than they do in more distant species--deer and pig.

This technique isn't foolproof. It goes awry when the same mutation happens independently in two different species. This kind of event, a molecular version of convergent evolution, makes two animals' sequences look similar even though they may be only distantly related. What's more, a mutation in one spot can change again or even correct itself, further muddling comparisons of sequences.

Scientists who analyze gene sequences try to get around this problem by considering many different genes, each of which represents a string of hundreds to thousands of individual DNA bases--the four letters in the genetic alphabet.

In contrast to standard sequencing, analysis of SINEs and LINEs presents a nearly perfect record of evolutionary change, says Okada. In 10 years of study, he has never found an example of these elements appearing independently in the same spot or, once inserted, extracting themselves from the DNA. "I am 100 percent confident with the conclusion that the most closely related species to whales, among extant mammals, is the hippo," Okada says.

Other molecular biologists agree that the SINE and LINE data add weight to the idea of a close relationship between whales and hippos, but many say that the connection was already firmly established by conventional genetic data. "Every gene I've ever sequenced says the same thing. The molecular data is ail fairly consistent," says John Gatesy of the University of Wyoming in Laramie, one of the first who reported the whale-hippo connection. Some researchers have taken to calling this the whippo hypothesis.