Today.Az » Weird / Interesting » Are genes our destiny? Scientists discover 'hidden' code in DNA evolves more rapidly than genetic code
20 September 2011 [11:30] - Today.Az
A "hidden" code linked to the DNA of plants allows them to develop and pass down new biological traits far more rapidly than previously thought, according to the findings of a groundbreaking study by researchers at the Salk Institute for Biological Studies.
The study, published September 16 in the journal Science,
provides the first evidence that an organism's "epigenetic" code -- an
extra layer of biochemical instructions in DNA -- can evolve more
quickly than the genetic code and can strongly influence biological
traits.
While the study was limited to a single plant species called
Arabidopsis thaliana, the equivalent of the laboratory rat of the plant
world, the findings hint that the traits of other organisms, including
humans, might also be dramatically influenced by biological mechanisms
that scientists are just beginning to understand.
"Our study shows that it's not all in the genes," said Joseph Ecker, a
professor in Salk's Plant Molecular and Cellular Biology Laboratory,
who led the research team. "We found that these plants have an
epigenetic code that's more flexible and influential than we imagined.
There is clearly a component of heritability that we don't fully
understand. It's possible that we humans have a similarly active
epigenetic mechanism that controls our biological characteristics and
gets passed down to our children. "
With the advent of techniques for rapidly mapping the DNA of
organisms, scientists have found that the genes stored in the
four-letter DNA code don't always determine how an organism develops and
responds to its environment. The more biologists map the genomes of
various organisms (their entire genetic code), the more they are
discovering discrepancies between what the genetic code dictates and how
organisms actually look and function.
In fact, many of the major discoveries that led to these conclusions
were based upon studies in plants. There are traits such as flower shape
and fruit pigmentation in some plants that are under the control of
this epigenetic code. Such traits, which defy the predictions of
classical Mendelian genetics, are also found in mammals. In some strains
of mice, for instance, a tendency for obesity can pass from generation
to generation, but no difference between the genetic code of fat mice
and thin mice explains this weight difference.
Scientists have even found that identical human twins exhibit
different biological traits, despite their matching DNA sequences. They
have theorized that such unexplained disparities could be the work of
epigenetic variation.
"Since none of these patterns of variation and inheritance match what
the genetic sequence says should happen, there is a clearly a component
of the 'genetic' heritability that is missing," Ecker said.
Ecker and other scientists have traced these mysterious patterns to
chemical markers that serve as a layer of genetic control on top of the
DNA sequence. Just as genetic mutations can arise spontaneously and be
inherited by subsequent generations, epigenetic mutations can emerge in
individuals and spread into the broader population.
Although scientists have identified a number of epigenetic traits,
very little was known about how often they arose spontaneously, how
quickly they could spread through a population and how significant an
influence they could have on biological development and function.
"Perception of the extent of epigenetic variation in plants from
generation to generation varies widely within our scientific community,"
said Robert Schmitz, a post-doctoral research in Eckers' laboratory and
the lead author on the paper. "We actually did the experiment, and
found that overall there is very little change between each generation,
but spontaneous epimutations do exist in populations and arise at a rate
much higher than the DNA mutation rate, and at times they had a
powerful influence over how certain genes were expressed."
In their study, the Salk researchers and collaborators at Scripps
Research Institute mapped the epigenome of a population of Arabidopsis
plants then observed how this biochemical landscape had changed after 30
generations. This mapping consisted of recording the state of all
locations on the DNA molecule that could undergo a chemical modification
known as methylation, a key epigenetic change that can alter how
certain underlying genes are expressed. They then watched how
methylation states of these sites evolved over the generations.
The plants were all clones of a single ancestor, so their DNA
sequences were essentially identical across the generations. Thus any
changes in how the plants expressed certain genetic traits were likely
to be a result of spontaneous changes in their epigenetic code --
variations in the methylation of the DNA sites- not the result of
variations in the underlying DNA sequences.
"You couldn't do this kind of study in humans, because our DNA gets
shuffled each generation," Ecker said. "Unlike people, some plants are
easily cloned, so we can see the epigenetic signature without all the
genetic noise."
The researchers discovered that as many as a few thousand methylation
sites on the plants' DNA were altered each generation. Although this
represents a small proportion of the potentially six million methylation
sites estimated to exist on Arabidopsis DNA, it dwarfs the rate of
spontaneous change seen at the DNA sequence level by about five orders
of magnitude.
This suggests that the epigenetic code of plants -- and other
organisms, by extension -- is far more fluid than their genetic code.
Even more surprising was the extent to which some of these changes
turned genes on or off. A number of plant genes that underwent heritable
changes in methylation also experienced substantial alterations in
their expression -- the process by which genes control cellular function
through protein production.
This meant that not only was the epigenome of the plants morphing
rapidly despite the absence of any strong environmental pressure, but
that these changes could have a powerful influence on the plants' form
and function.
Ecker said the results of the study provide some of the first
evidence that the epigenetic code can be rewritten quickly and to
dramatic effect. "This means that genes are not destiny," he said. "If
we are anything like these plants, our epigenome may also undergo
relatively rapid spontaneous change that could have a powerful influence
on our biological traits."
Now that they have shown the extent to which spontaneous epigenetic
mutations occur, the Salk researchers plan to unravel the biochemical
mechanisms that allow these changes to arise and get passed from one
generation to the next.
They also hope to explore how different environmental conditions,
such as differences in temperature, might drive epigenetic change in the
plants, or, conversely, whether epigenetic traits provide the plants
with more flexibility in coping with environmental change.
"We think these epigenetic events might silence genes when they
aren't needed, then turned them back on when external conditions
warrant," Ecker said. "We won't know how important these epimutations
are until we measure the effect on plant traits, and we're just now to
the point where we can do these experiments. It's very exciting."
The research is supported by the National Science Foundation, the
National Institutes of Health, the Howard Hughes Medical Institute, the
Gordon and Betty Moore foundation and the Mary K. Chapman Foundation. /Science Daily/
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