EVOLUTION CAUGHT IN THE ACT
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Friday, January 1, 2010
Evolution caught in the act
US-German team measures how quickly genomes change
December 31st, 2009, Max Planck Society
Mutations are the raw material of evolution. Charles Darwin already
recognized that evolution depends on heritable differences between
individuals: those who are better adapted to the environment have
better chances to pass on their genes to the next generation. A
species can only evolve if the genome changes through new mutations,
with the best new variants surviving the sieve of selection.
Scientists at the Max Planck Institute for Developmental Biology in
T�bingen, Germany, and Indiana University in Bloomington have now
been able to measure for the first time directly the speed with which
new mutations occur in plants. Their findings shed new light on a
fundamental evolutionary process. They explain, for example, why
resistance to herbicides can appear within just a few years.
(Science, January 1, 2010)
Fig.: Different mutants of Arabidopsis thaliana.
Image: Detlef Weigel
"While the long term effects of genome mutations are quite well
understood, we did not know how often new mutations arise in the
first place," said Detlef Weigel, director at the Max Planck
Institute in Germany. It is routine today to compare the genomes of
related animal or plant species. Such comparisons, however, ignore
mutations that have been lost in the millions of years since two
species separated. The teams of Weigel and his colleague Michael
Lynch at Indiana University therefore wanted to scrutinize the
signature of evolution before selection occurs. To this end, they
followed all genetic changes in five lines of the mustard relative
Arabidopsis thaliana that occurred during 30 generations. In the
genome of the final generation they then searched for differences to
the genome of the original ancestor.
The painstakingly detailed comparison of the entire genome revealed
that in over the course of only a few years some 20 DNA building
blocks, so called base pairs, had been mutated in each of the five
lines. "The probability that any letter of the genome changes in a
single generation is thus about one in 140 million," explains Michael
Lynch. To put it differently, each seedling has on average one new
mutation in each of the two copies of its genome that it inherits
from mum and dad. To find these tiny alterations in the 120 million
base pair genome of Arabidopsis was akin to finding the proverbial
needle in a haystack, says Weigel: "To ferret out where the genome
had changed was only possibly because of new methods that allowed us
to screen the entire genome with high precision and in very short
time." Still, the effort was daunting: To distinguish true new
mutations from detection errors, each letter in each genome had to be
checked 30 times.
The number of new mutations in each individual plant might appear
very small. But if one starts to consider that they occur in the
genomes of every member of a species, it becomes clear how fluid the
genome is: In a collection of only 60 million Arabidopsis plants,
each letter in the genome is changed, on average, once. For an
organism that produces thousands of seeds in each generation, 60
million is not such a big number at all.
Apart from the speed of new mutations, the study revealed that not
every part of the genome is equally affected. With four different DNA
letters, there are six possible changes�but only one of these is
responsible for half of all the mutations found. In addition,
scientists can now calculate more precisely when species split up.
Arabidopsis thaliana and its closest relative, Arabidopsis lyrata,
differ in a large number of traits including size and smell of
flowers or longevity: Arabidopsis lyrata plants often live for years,
while Arabidopsis thaliana plants normally survive only for a few
months. Colleagues had previously assumed that only five million
years had passed by since the two species went their separate ways.
The new data suggest instead that the split occurred already 20
million years ago. Similar arguments might affect estimates of when
in prehistory animals and plants were first domesticated.
On a rather positive note, the results of the US-German team show
that in sufficiently large populations, every possible mutation in
the genome should be present. Thus, breeders should be able to find
any simple mutation that has the potential to increase yield or make
plants tolerate drought in a better manner. Finding these among all
the unchanged siblings remains nevertheless a challenging task. On
the other hand, the new findings easily explain why weeds become
quickly resistant to herbicides. In a large weed population, a few
individuals might have a mutation in just the right place in their
genome to help them withstand the herbicide. "This is in particular a
problem because herbicides often affect only the function of
individual genes or gene products," says Weigel. A solution would be
provided by herbicides that simultaneously interfere with the
activity of several genes.
Turning to the larger picture, Weigel suggests that changes in the
human genome are at least as rapid as in Arabidopsis: "If you apply
our findings to humans, then each of us will have on the order of 60
new mutations that were not present in our parents." With more than
six billion people on our planet, this implies that on average each
letter of the human genome is altered in dozens of fellow citizens.
"Everything that is genetically possible is being tested in a very
short period," adds Lynch, emphasizing a very different view than
perhaps the one we are all most familiar with: that evolution reveals
itself only after thousands, if not millions of years.
[SD]
Original work:
Stephan Ossowski, Korbinian Schneeberger, Jos� Ingnacio Lucas-Lled�,
Norman Warthmann, Richard M. Clark, Ruth G. Shaw, Detlef Weigel and
Michael Lynch
The rate and molecular spectrum of spontaneous mutations in
Arabidopsis thaliana.
Science, January 1, 2010
PDF (174 KB)
Contact:
Detlef Weigel
Max Planck Institute for Developmental Biology, T�bingen
Tel.: +49 179 676 9032
E-mail: wei...@tue.mpg.de
Susanne Diederich (Public Relations officer)
Max Planck Institute for Developmental Biology, T�bingen
Tel.: +49 170 6304946
E-mail: pre...@tue.mpg.de
End of forwarded message from S. Kalyanaraman
Jai Maharaj, Jyotishi
Om Shanti
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