History-Hunting Geneticists Can Still Follow Familiar Trail

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History-Hunting Geneticists Can Still Follow Familiar Trail

http://www.medicalnewstoday.com/medicalnews.php?newsid=59474

As the world's first explorers branched away from humanity's

birthplace in east Africa some 65,000 years ago, distinct mutations

accumulated in the DNA of each population, essentially providing a

genetic trail for modern researchers to follow.

Recently some scientists have raised doubts about this classic

genetic system to study ancient migrations of people and to estimate

the populations of people or animals as they existed tens of

thousands of years ago.

But University of Florida researchers writing this month in an

online edition of Science validate the approach, which involves

tracking sequences of mitochondrial DNA, also known as mtDNA.

" The study of mtDNA has helped to demonstrate the African origin of

our species and the relationship between living humans and the

Neanderthals, " said Connie Mulligan, an associate professor of

anthropology in the College of Liberal Arts and Sciences and an

assistant director of the UF Genetics Institute. " MtDNA data have

also been used to establish the time and route of major events in

human history, such as the expansion of Neolithic farmers into

Europe, and the settlement of the New World. "

MtDNA has made headlines recently because of initiatives such as the

National Genographic Project, a multimillion-dollar endeavor to

reconstruct humanity's ancient migrations, and because of well-

publicized efforts to track the ancestral roots of Oprah Winfrey and

other personalities.

Located within the hundreds of energy-producing mitochondria that

lie outside the nucleus of our cells, mtDNA is unlike the DNA inside

the nucleus of a cell that contains genes from both of our parents --

in people and animals mtDNA is exclusively passed from mothers to

their children.

For humans, this means that all of the mtDNA in our cells are copies

of our mothers' mtDNA, which in turn were copies of their mothers'

mtDNA. In this way, mtDNA progresses through the ages, springing

from what many scientists believe was a common ancestral mother.

But over the eons, random mutations enter the genetic code of all

species. By tracking similarities and differences of mtDNA,

scientists gain insight about the size of groups and how they moved

around the world.

" When you look at ancient migration, you're always asking the

question, 'How big was the population, how many were there?' " said

Miyamoto, a professor and associate chairman of zoology in

UF's College of Liberal Arts and Sciences. " The field has worked

from the premise that the more mtDNA variation you saw, the larger

the population was that carried that variation, just like there

would be a greater diversity of T-shirts or shoes within a larger

population than a smaller population. "

However, the connection between mtDNA and population size was

questioned this year when French scientists analyzed vast groups of

gene sequences from more than 3,000 animal species. They speculated

that an evolutionary tendency for species to keep helpful genes and

sift out detrimental ones, called " natural selection, "

preferentially affects mitochondrial diversity, making mtDNA less

useful for population size estimates.

" From a conservation perspective, when scientists look at census

counts of animals and how the population size may be increasing or

decreasing, the study of mtDNA tells us about the level of genetic

diversity in a population, which is important in making conservation

decisions on endangered species, " Mulligan said. " If this approach

were not credible, it could potentially have a bearing on future

policy decisions, as well as affect literally hundreds of previous

studies on humans and other mammals. "

UF Genetics Institute scientists analyzed publicly available mtDNA

datasets of 47 species of mammals -- a subset of the animals that

were in the French study -- as well as associated data on protein

diversity in the same species. A greater variety of proteins

indicates more diverse DNA, because DNA contains a species'

blueprints for manufacturing protein -- and the French scientists

agreed that protein diversity did correlate with population size.

All that remained for UF researchers to do to reinstate mtDNA

diversity as indicative of population size was to determine that

protein diversity and mtDNA diversity were correlated.

" The researchers showed a correlation between mitochondrial DNA and

genetic variation in a way that has never been done before, " said

Marc Allard, an associate professor of biology at Washington

University who was not involved in the study. " Population

geneticists have used mitochondria for all kinds of work for 20

years, and to think that mtDNA didn't correlate with population size

was clearly going against the dogma. This study shows the dogma is

safe in mammals and probably in vertebrates, as well. "