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Columns::October 14, 2002
Always and forever: Annual reunion celebrates good times, homecoming
Celebration of life: Memorial service for Eugene Odum scheduled for Oct. 16
UGA, consortium dedicate beamline at Argonne National Laboratory
President names search committee to identify candidates for provost
Taste of college life
Vet med professor endures ‘trying’ times to complete his education
Retirees
Kudos
Review, revise, revisions
The world’s a classroom
Campus News
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| John McDonald has been studying retroelements for more than a decade. (Photo by Paul Efland) |
Divergent paths
Study suggests retroelements played role in shift from more rudimentary primates to modern human beings
By Phil Williams
phil@franklin.uga.edu
Scientists in the past decade have discovered that remnants of ancient germ line infections called human endogenous retroviruses make up a substantial part of the human genome. Once thought to be merely “junk” DNA and inactive, many of these elements, in fact, perform functions in human cells.
Now, a study by John McDonald of UGA and King Jordan at the National Center for Biotechnology Information at the National Institutes of Health suggests for the first time that a burst of transpositional activity occurred at the same time humans and chimps are believed to have diverged from a common ancestor--six million years ago. These new results implicate retroelements, a particular type of transposable elements that are abundant in the human genome, in the actual shift from more rudimentary primates to modern human beings.
Jordan received his doctoral degree at UGA, working with McDonald. The research was published earlier this year in the journal Genome Letters.
“There is a growing body of evidence that transposable elements have contributed to the evolution of genome structure and function in many species,” says McDonald, a molecular evolutionist and head of the genetics department. “Our results suggest that a burst of transposable-element activity may well have contributed to the genetic changes that led to the emergence of the human species.”
There has been a molecular arms race going on between transposable elements and their host genomes for millions of years. Host genomes are continually evolving new regulatory mechanisms to silence the mutagenic effects associated with the replication of these elements. Those mechanisms, in turn, place selective pressure on the elements to evolve their own new mechanisms to escape such controls. The result is an internal drive mechanism to increase biological complexity.
Just as new technologies generated by the military arms races between rival countries get spun off and used for non-military purposes, so the new regulatory mechanisms resulting from the arms race between transposable elements and host genomes generate new molecular mechanisms that can be used to accelerate evolution on the organismic level.
The idea of a relatively sudden genetic change that alters evolution isn’t new. The late Stephen Jay Gould and other scientists proposed a mechanism called “punctuated equilibrium” more than two decades ago. The idea, still a matter of argument, proposes that evolution has depended more often on sudden and unexpected changes in genomes than on a simple Darwinian paradigm of gradual evolutionary change due to extremely long-term natural selection.
Analyses of DNA using modern tools have demonstrated that human and chimpanzee DNA are more than 95 percent identical, a clue to a mutual origin. Finding real evidence for sudden genetic changes, however, has been slow. By using phylogenetic surveys, however, McDonald and King were able to distinguish between the youngest HERVs (human endogenous retroviruses) and more ancient lineages.
The discovery that human-specific retroviruses emerged at the same time other researchers believe humans and chimps diverged was startling. Equally interesting, however was the discovery that the oldest subfamily of HERV elements is closely related to--and gave rise to--the youngest and most recently active group of these elements. This suggests, the authors say, that “ancient families of HERVs may be capable of retaining the potential for biological activity over long spans of evolutionary time.”
McDonald has been studying retroelements for more than a decade, and interest has been growing recently. Two researchers from Tufts University identified 23 new HERVs last year and reported that at least 16 percent of those elements had undergone rearrangements that resulted in large-scale “deletions, duplications and chromosome reshuffling during the evolution of the human genome.”
Just how these retroviral elements have moved around in the human genome, possibly changing organisms at the morphological level, remains a matter of speculation. Until 50 years ago, scientists thought all genes worked from a stable position along a chromosome. That idea, however, began to change dramatically in the 1970s, when it became clear that transposable elements are pervasive in plant and animal genomes. It simply made no sense that such elements would be conserved over thousands of millennia if they had no real function.
They may have been--and may still be--a driving force of evolution at the cellular and organismal levels. Jordan and McDonald’s research goes further, suggesting that retroviral elements may actually be implicated in the leap from apes to humans.
McDonald says it is increasingly clear that organisms need the viral elements and that their apparent continual backdoor assaults on normal genes may, in truth, be more like a vast, sophisticated chess game on an enormously complex board.
This is the first evidence, however, that suggests they may have made humans what they are today.
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