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.
John Easton | Newswise
Subcutaneous Administration of Multispecific Antibody Makes Tumor Treatment Faster & More Tolerable
01.07.2015 | Helmholtz Zentrum München - Deutsches Forschungszentrum für Gesundheit und Umwelt
Why human egg cells don't age well
01.07.2015 | RIKEN
Studentische Industriekooperation zwischen HAW Hamburg und Webasto erarbeitet Ergebnisse für EU-Zertifizierungsprozess von Solardächern zur Verbesserung der Öko-Bilanz von Fahrzeugen.
Unter der Leitung von Dr.-Ing. Volker Skwarek, Professor für technische Informatik an der HAW Hamburg, erarbeiteten sechs Studierende des...
Wind turbines could be installed under some of the biggest bridges on the road network to produce electricity. So it is confirmed by calculations carried out by a European researchers team, that have taken a viaduct in the Canary Islands as a reference. This concept could be applied in heavily built-up territories or natural areas with new constructions limitations.
The Juncal Viaduct, in Gran Canaria, has served as a reference for Spanish and British researchers to verify that the wind blowing between the pillars on this...
Der Markt für Unterhaltungselektronik boomt: Rund 60 Millionen Fernsehgeräte wurden im letzten Jahr in Europa verkauft. Früher oder später werden sie zurückkehren – als Elektroschrott.
Die Recycling-Industrie hat darauf reagiert: Kupfer, Aluminium, Eisen- und Edelmetalle sowie ausgewählte Kunststoffe werden bereits wiederverwertet. Allerdings...
Die Bedrohung im All durch Weltraummüll ist groß. Aktive Satelliten und Raumfahrzeuge können beschädigt oder zerstört werden. Ein neues, nationales Weltraumüberwachungssystem soll ab 2018 vor Gefahren im Orbit schützen. Fraunhofer-Forscher entwickeln das Radar im Auftrag des DLR Raumfahrtmanagement.
Die »Verkehrssituation« im All ist angespannt: Neben unzähligen Satelliten umkreisen Weltraumtrümmer wie beispielsweise ausgebrannte Raketenstufen und...
New technique combines electron microscopy and synchrotron X-rays to track chemical reactions under real operating conditions
A new technique pioneered at the U.S. Department of Energy's Brookhaven National Laboratory reveals atomic-scale changes during catalytic reactions in real...
02.07.2015 | Veranstaltungen
02.07.2015 | Veranstaltungen
02.07.2015 | Veranstaltungen