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from ancestors that survived last ice age, scientists say
Barlow, Life Sciences Editor
CHAMPAIGN, Ill. —
by Bill Wiegand
New research based on a
mitochondrial DNA analysis of 244 chipmunks has
revealed surprising results. From left, doctoral
student Kevin C. Rowe led the study with co-author
Ken N. Paige, head of animal biology at Illinois.
chipmunks (Tamias striatus) have upset the apple cart of assumptions
on glacier-driven population migrations. Based on a mitochondrial DNA
analysis of 244 chipmunks, it seems the majority of them living in Illinois
and Wisconsin today descend from ancestors who survived the last North
American ice age in what researchers believe were isolated pockets of
forestland amid the cold tundra.
The findings – reported online this week ahead of regular publication
by the Proceedings of the National Academy of Sciences – came
as a surprise to researchers at the University of Illinois at Urbana-Champaign
and the Illinois Natural History
They found that 78 of the 95 haplotypes (groups of individuals with
similar sequences of base pairs of genetic material) identified in mostly
the Wisconsin and Illinois populations clearly descend from survivors
in the west and north, closer to the Wisconsin glaciation. Over time,
these chipmunks migrated south from the colder region, merging with
chipmunks that migrated into the region from the warmer east and south.
“This is counter-intuitive given that organisms would be expected
to respond to glacial expansion by shifting their ranges to more suitable
climates most often in a southern refuge followed by a northward recolonization
as the glaciers receded,” said Kevin C. Rowe, lead author and
doctoral student in the evolutionary/molecular biology laboratory at
the U. of I.
“It also is particularly surprising that while chipmunks in Illinois
and Wisconsin are closely related, they are distantly related to chipmunks
in Indiana and Michigan,” he said. “There really is no clear
geographical barrier at present that should lead to their isolation,
so chipmunk history may be responsible. From our data, this history
appears to include colonization of the Midwest from multiple sources
such as separate ice-age refugia.”
Unlike nuclear DNA that passes along vital genetic information, mitochondrial
DNA is found in organelles and generates energy. It has been used to
trace genetic lineage. In this case, researchers focused on a mitochondrial
region called the D-loop, a highly variable one that opens a window
on geographical time and distribution of organisms.
DNA was taken from tissue from the very tip of an ear of each chipmunk
after the animals were captured in live traps. The chipmunks were released
at the same sites of their capture. The DNA, from which 964 unambiguous
base pairs were identified, was then analyzed for evolutionary relationships,
with 95 unique haplotypes being identified.
Only 17 of the haplotypes were identified as from eastern clades, or
groups descending from multiple eastern and southern populations, that
included northern Michigan through Indiana and eastern and southern
Illinois. The other 78 haplotypes, whose genetic sequences showed little
divergence, were found primarily in chipmunks from northern Wisconsin
to southern Illinois.
The data, the researchers write, “indicate that T. striatus from
Wisconsin and Illinois are descended from a population that has recently
expanded out of a small glacial refugium.”
That refugium refers to northern regions up against the ice sheets “in
an area known as the ‘driftless region,’ from where the
chipmunks that currently inhabit Wisconsin and Illinois emerged from
surviving deciduous forest areas and moved south as the glaciers receded,”
The Wisconsin glaciation occurred about 18,000 years ago during the
Late Pleistocene, with the Laurentide Ice Sheet gradually receding from
its southern reaches. Researchers had long thought that the driftless
region near the ice sheets were all tundra, but emerging geological
evidence now suggests that small areas of deciduous forests may have
persisted, allowing for some animals to survive.
“Overall, in light of global climate change occurring today, our
results indicate that predictions of movements and where organisms may
or may not end up will not be as straightforward as the literature up
to now has led one to believe,” said co-author Ken N. Paige, head
of the animal biology department at the U. of I.
Other co-authors were Edward J. Heske of the Center for Wildlife Ecology
unit of the Illinois Natural History Survey and Patrick W. Brown, formerly
at the Natural History Survey and now of Michigan State University Extension.
The American Museum of Natural History, American Society of Mammalogists
and National Science Foundation funded the project.