( The repeated copying of a small segment...)Mobile DNA part of evolution's toolbox

The repeated copying of a small segment of DNA in the genome of a primeval fish may have been crucial to the transition of ancient animals from sea to land, or to later key evolutionary changes in land vertebrates. The discovery is "tantalizing evidence" that copied DNA elements known as retroposons could be an important source of evolutionary innovations, says the director of the research, Howard Hughes Medical Institute investigator David Haussler.

"The big question is whether this is a special case or whether it's the tip of the iceberg," says Haussler, who is at the University of California, Santa Cruz. A report on the research is published in the May 4, 2006, issue of the journal Nature.

Haussler and his colleagues were led to the discovery through their work on what they call "ultraconserved elements" -- segments of DNA hundreds of nucleotides long that are almost exactly the same in a wide variety of vertebrate organisms. Haussler and postdoctoral fellow Gill Bejerano discovered the ultraconserved elements in 2003, and since then they have been trying to figure out how they arose and what function they serve.

One ultraconserved element in particular caught their eye. "We were very interested in this sequence, because it had a number of copies elsewhere in the genome," says Bejerano, who is the first author of the study. Close copies of the sequence were ubiquitous in amphibians, birds, and mammals, indicating that it served an important function. "We found it in every species for which we have genomes, from frogs to humans," says Bejerano.

Comparing the sequence to other species also turned up a big surprise. When the researchers compared the human ultraconserved element to all the DNA sequences in the public database GenBank, the closest match was to DNA from the coelacanth -- an ancient fish thought to have gone extinct millions of years ago until a live specimen was caught in 1938 off the east coast of South Africa
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Source:Howard Hughes Medical Institute

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