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Residential Life Goes
West • Chewing
on Evolution •
Atmospheric Ambiguities
Lobsters Play Biological Violins •
Remembering Wannamaker
When is a Platypus Not a Kangaroo? •
Solution for Smokers
• In
Brief
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When is a
Platypus Not a Kangaroo?
 lassifying
kangaroos and platypuses together on the evolutionary family tree
is as absurd as adding your neighbors to your own family ancestral
line simply because they share your love of the opera, according to
Duke scientists.
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| DNA decides: e platypus
unum |
But the current molecular method of using mitochondrial
DNA to classify how mammals evolved is so flawed that it might have
erroneously linked very different mammals, the scientists say. The
mitochondrial DNA method of analyzing mammals has turned on its head
the common-sense approach of linking mammals by similar anatomical
traits or “morphology.”
Using a more comprehensive method to analyze the genetic
material of fifteen types of mammals, Duke researchers have shown
that the mitochondrial DNA method that links disparate animals (hippo
and whale, kangaroo and platypus) is statistically unreliable when
it comes to evolutionary genetics, according to Randy Jirtle, professor
of radiation oncology at Duke Medical Center. Their own research using
nuclear genes (genes from the nucleus or core of cells) has shown
a nearly 100 percent statistical likelihood that the Duke results
are correct.
Mitochondria are the cell’s power plants and possess
their own genes that are inherited through the maternal line. Scientists
use this method because mitochondrial DNA is more accessible and easier
to sequence, and all multi-cellular animals have mitochondria, whereas
all animals do not share the same nuclear genes.
Results of the study are published in a July issue of
the journal Mammalian Genome.
Such conclusive results led the researchers to strongly
support the Theria hypothesis of classifying the three major groups
of mammals. The Theria hypothesis holds that eutherians (humans, rats,
pigs, whales, etc.) and marsupials (kangaroos, wallabies, koalas,
etc.) have evolved from a common ancestor, and monotremes (platypus,
echidna) have evolved from a different ancestor and on a separate
land mass. The mitochondrial method of studying evolution, however,
supports the Marsupionta hypothesis, which places the platypus and
kangaroo together. This controversy has lasted for more than two centuries
since the discovery that the platypus lays eggs.
“Our study is the first to provide statistically
unambiguous results in favor of classifying mammals using the Theria
hypothesis, as paleontologists have long done through studying fossils,”
Jirtle says. “Now we need to retest the results generated by
scientists who have used mitochondrial DNA sequences to link mammals
such as hippos to whales.”
The Duke scientists generated their results by isolating
a whole nuclear gene from the genetic material of fifteen different
mammals, and then determining the unique genetic code or sequence
that distinguishes each gene from the others in the respective mammals.
Then, by plugging molecular traits of the gene into a computer software
program—similar to entering eye color and earlobe structure into
a family tree software program—they identified which animals
shared common DNA traits and which did not. The data they derived
from studying nuclear genes clearly identified marsupials as having
a common evolutionary background
with eutherians and monotremes as having evolved separately.
Aside from its purely academic value, the scientists say
that classifying mammals correctly is critical because it helps biologists
apply the information learned from one mammal to others in the same
class, without having to conduct identical molecular studies on each
mammal.
“The family tree is a crucial evolutionary roadmap,”
says Keith Killian, a Duke researcher in molecular development and
evolution. “If you are trying to trace, for example, the evolutionary
steps of fetal heart development
to better understand how fetal defects occur, it helps to know which
mammals are related so that you can make accurate inferences about
one mammal from another mammal’s development.”
In recent years, scientists have increasingly relied on
using mitochondrial DNA to make comparisons among mammals and thereby
link those that are related on the evolutionary tree.
But Killian says mitochondrial DNA provides misleading
results for a variety of reasons. Most importantly, it requires more
human input to decide which information is fed to the computer, thereby
raising the risk of human bias. When the data were given to three
different laboratories for analysis, they generated three different
family trees, he says.
“This is the first molecular evolutionary study that
seriously and powerfully says the paleontologists have been right
all along in grouping mammals the way they did,” says Killian.
“It turns out that common sense is correct.”
The study was funded by grants from the National Institutes
of Health, Department of Defense, Sumitomo Chemical Company Ltd.,
and AstraZeneca Pharmaceuticals Ltd.
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