News: International Team Publishes Largest African Genetics Study.
- From: Ye Old One <usenet@xxxxxxxxx>
- Date: Thu, 30 Apr 2009 23:48:48 GMT
International Team Publishes Largest African Genetics Study
April 30, 2009
http://www.genomeweb.com/node/915889?emc=el&m=376121&l=1&v=191bbf7840
NEW YORK (GenomeWeb News) ? An international research team has
published the largest study yet of African genetics ? work that they
say not only offers a peek into human history but also provides the
foundation for future biomedical research in Africa and beyond. The
research appeared in the advanced, online issue of Science today.
The researchers assessed more than 1,300 polymorphic markers in
thousands of individuals from more than 100 African populations, four
African-American populations, and 60 non-African populations. The
results confirmed relationships between some populations with shared
language and culture, but uncovered shared ancestry in other groups
that were previously thought to be unrelated.
In a conference call with reporters yesterday, lead author Sarah
Tishkoff, a geneticist affiliated with the Universities of Maryland
and Pennsylvania, called the work the "culmination of a nearly 10
years effort."
Tishkoff emphasized the remarkable genetic diversity both within and
between the African populations they tested, noting that no single
African population alone represents the overall diversity present on
the continent.
Modern humans arose some 200,000 years ago in Africa. Linguistic
studies have placed African populations into four main language
groups: Niger-Kordofanian, Nilo-Saharan, Afroasiatic, and Khoesan. But
relatively little was known about the nuclear genetic variation on the
continent.
"Characterizing the pattern of genetic variation among ethnically
diverse African populations is critical for reconstructing human
evolutionary history, the population history of African Americans, and
for the proper design and interpretation of genetic disease
association studies, since sub-structure can cause spurious results,"
the authors explained.
To begin untangling African genetics, the researchers evaluated 848
microsatellites, 476 insertions and deletions, and three SNPs in 2,551
individuals: 2,432 Africans from 113 populations; 98 African Americans
from Chicago, Baltimore, Pittsburgh, and North Carolina; and 21
Yemenites. They integrated these findings with data on 952 Human
Polymorphism Study Center-Human Genome Diversity Panel individuals
from around the world, as well as 432 individuals of Indian descent,
and 10 Native Australians.
The researchers detected 14 main ancestral population clusters in
Africa, which corresponded fairly well with language and cultural
information. They speculated that genetic differences between
populations likely arose through a combination of factors ? from
ethnicity, language, and cultural differences to geographical,
environmental, and climate differences that "contributed to population
size fluctuations, fragmentations, and dispersals in Africa."
The researchers also uncovered previously unrecognized relationships
between geographically distinct populations. For example, their
genetic analysis suggests southern African Khoesan, Hadza, Sandawe,
and Pygmy populations ? hunter-gatherer populations currently living
in different parts of Africa ? might be "remnants of an historically
more widespread proto-Khoesan-Pygmy population of hunter-gatherers."
The work also provided a window on historical human migrations within
and out of Africa, suggesting modern human migration originated in
southern Africa near what is now the Namibia-Angola border ? the
current homeland of click-speaking San populations. Even so, the
researchers noted, current geographic distribution of populations may
not represent the population distributions present long ago.
The work provided new insights into language patterns in Africa as
well. By looking at gene flow, for instance, the team could determine
whether language spread into new areas via large groups of individuals
or via small but influential groups. The current findings included
examples of both, co-author Christopher Ehret, an African history
researcher at the University of California at Los Angeles, told
reporters.
By assessing particular markers, the researchers were also able to
trace the ancestry of the African Americans tested. They found that,
as a group, African Americans most often shared ancestry with the
Niger-Kordofanian language group, mainly from different parts of
western Africa. Although genetic evidence indicated that the African
Americans had genetic contributions from more than one
Niger-Kordofanian population, the markers used couldn't accurately
pinpoint individuals' ancestry to a particular West African tribe.
"[M]ost African Americans are likely to have mixed ancestry from
different regions of western Africa," the authors noted. "This
observation, together with the subtle substructure observed among
Niger-Kordofanian speakers, will make tracing ancestry of African
Americans to specific ethnic groups challenging, unless considerably
more markers are used."
Overall, the African Americans tested had a mean European ancestry of
13 percent, though this varied widely from one individual to the next,
with some having no European ancestry and others having more than 50
percent European ancestry, Tishkoff said. Researchers also detected a
lower degree of ancestry from other African populations as well as
some East Asian and Indian ancestry in the African Americans tested.
Along with its potential for offering insights into human history,
Vanderbilt University Center for Human Genetics Research molecular
physiology and biophysics researcher Scott Williams, senior author on
the study, said the research also has implications for biomedical
research, providing a "critical piece of the puzzle" for researchers
designing disease-gene and drug-response studies.
"Because of the extensive level of sub-structure in Africa, ethnically
and geographically diverse populations need to be included in
re-sequencing, genome-wide association (GWAS), and pharmacogenomic
studies, to identify population or regional-specific functional
variants associated with disease or drug response," the authors wrote.
Consequently, even though the current study has provided information
about more than a hundred African populations, Tishkoff stressed the
need for even more genetic research in Africa to learn more about the
nearly 2,000 other ethno-linguistic populations on the continent.
"I truly hope that the study will set the stage for future research in
Africa," Tishkoff said. "We don't want to see African programs left
behind in this genomic revolution."
--
Bob.
.
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