Web address:
http://www.sciencedaily.com/releases/2013/02/
130214141608.htm
Humans and Chimps Share Genetic Strategy in Battle Against Pathogens
Feb. 14, 2013 A genome-wide analysis searching for evidence of long-lived
balancing selection -- where the evolutionary process acts not to select the
single best adaptation but to maintain genetic variation in a population -- has
uncovered at least six regions of the genome where humans and chimpanzees share
the same combination of genetic variants.
The finding, to be published Feb. 14 in the journal Science, suggests that in
these regions, human genetic variation dates back to a common ancestor with
chimpanzees millions of years ago, before the species split. It also highlights
the importance of the dynamic co-evolution of human hosts and their pathogens
in maintaining genetic variation.
Balancing selection allows evolution to keep all hereditary options open. The
classic human example is the persistence of two versions of the hemoglobin
gene: a normal version and hemoglobin S., a mutation that distorts the shape
and function of red blood cells. Those who inherit two normal hemoglobin genes
are at high risk for malaria, a parasitic disease that infects more than 200
million people each year. Those who inherit one normal gene and one hemoglobin
S. gene are partially protected from malaria -- a potentially life-saving
benefit. Those with two copies of the gene suffer from sickle-cell anemia, a
serious and lifelong circulatory disease.
"When we looked for genetic clues pointing to other, more ancient, examples of
balancing selection, we found strong evidence for at least six such regions and
weaker evidence for another 119 -- many more than we expected," said study
author Molly Przeworski, PhD, professor of human genetics and of ecology and
evolution at the University of Chicago.
"We don't yet know what their functions are," she said. None of the six regions
codes for a protein. There are clues that they are involved in host-pathogen
interactions, "but which pathogens, what immune processes," she said, "we don't
know."
The researchers used genetic data from 10 chimpanzees from Western Africa and
59 humans from sub-Saharan Africa who were part of the 1,000 Genomes Project.
The scientists looked for cases in which genetic variations that arose in the
ancestor of humans and chimpanzees have been maintained through both lines. The
fact that variation in these regions of the genome has persisted for so long
argues that they "must have been functionally important over evolutionary
time," said Ellen Leffler, a graduate student in Przeworski's laboratory and
first author of the study.
The researchers, from the University of Chicago and Oxford University, designed
the study to be very conservative. "We wanted to find the cases we believed the
most, rather than the most cases," Przeworski said.
Computers sorted snippets of the genetic data from humans and chimps into
clusters depending on how similar the subjects were to each other. For almost
every snippet, they found a cluster of humans and a separate cluster of
chimpanzees, as expected. But there were a few segments of the genomes in which
each cluster included both chimpanzees and humans; in those regions, some
humans were more closely related to some chimpanzees than to other humans.
"Instances in which natural selection maintains genetic variation in a
population over millions of years are thought to be extremely rare," the
authors wrote. The oldest and best known example of balanced polymorphism
shared between humans and chimpanzees is the major histocompatibility complex
(MHC), a group of genes that help the immune system distinguish between the
body and potential invaders, such as bacteria or viruses.
Last year, a team led by Przeworski found that humans and gibbons shared
genetic variation related to the ABO blood-group system from a common ancestor.
The six new examples of balanced selection described in this study appear to
play a role, like the MHC, in fending off infectious disease. This requires a
variety of evolutionary tools, including balancing selection. When a population
moves to a new environment -- for example the exodus out of Africa to northern
Europe -- they face many one-time adjustments, such as adapting to less intense
sunlight and decreased ultraviolet radiation. Over many generations, their
offspring manage to decrease melanin production -- a static adaptation for a
static environment.
Fighting off pathogens is more dynamic, a constant arms race. Balancing
selection may have enabled humans and chimps to retain multiple lines of
defense that can be called on when a pathogen evolves new weapons.
"Our results imply that dynamic co-evolution of human hosts and their pathogens
has played an important role in shaping human variation," Przeworski said.
"This highlights the importance of a different kind of selection pressure in
human evolution."
The National Institutes of Health, The Royal Society, the Wellcome Trust and
the Howard Hughes Medical Institute funded this study. Additional authors
include Ziyue Gao, Laure Segurel and Guy Sella from the University of Chicago;
Susanne Pfeifer, Adam Auton, Oliver Venn, Rory Bowden, Peter Donnelly and
Gilean McVean from Oxford University; Ronald Bonstrop from the Biomedical
Primate Research Centre in the Netherlands; and Jeffrey Wall from the
University of California at San Francisco.
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Story Source:
The above story is reprinted from materials provided by University of
Chicago Medical Center.
Note: Materials may be edited for content and length. For further
information, please contact the source cited above.
Journal Reference:
E. M. Leffler, Z. Gao, S. Pfeifer, L. Segurel, A. Auton, O. Venn, R.
Bowden, R. Bontrop, J. D. Wall, G. Sella, P. Donnelly, G. McVean, M.
Przeworski. Multiple Instances of Ancient Balancing Selection Shared Between
Humans and Chimpanzees. Science, 2013; DOI: 10.1126/science.1234070
Need to cite this story in your essay, paper, or report? Use one of the
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University of Chicago Medical Center (2013, February 14). Humans and chimps
share genetic strategy in battle against pathogens. ScienceDaily. Retrieved
February 16, 2013, from http://www.sciencedaily.com
/releases/2013/02/130214141608.htm
Note: If no author is given, the source is cited instead.
Disclaimer: This article is not intended to provide medical advice, diagnosis
or treatment. Views expressed here do not necessarily reflect those of
ScienceDaily or its staff.
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