October
19, 2005
UCSD Study Shows 'Junk' DNA Has
Evolutionary Importance
By Kim McDonald
Genetic material
derisively called “junk” DNA because it does not
contain the instructions for protein-coding genes and appears
to have little or no function is actually critically important
to an organism’s evolutionary survival, according to a
study conducted by a biologist at UCSD.
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Fruit fly Drosophila
melanogaster
Credit:
UCSD
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In the October 20 issue
of Nature, Peter Andolfatto, an assistant professor
of biology at UCSD, shows that these non-coding regions play
an important role in maintaining an organism’s genetic
integrity. In his study of the genes from the fruit fly Drosophila
melanogaster, he discovered that these regions are strongly
affected by natural selection, the evolutionary process that
preferentially leads to the survival of organisms and genes
best adapted to the environment.
Andolfatto’s
findings are important because the similarity of genome sequences
in fruit flies, worms and humans suggest that similar processes
are probably responsible for the differences between humans
and their close evolutionary relatives.
“Sequencing
of the complete genome in humans, fruit flies, nematodes and
plants has revealed that the number of protein-coding genes
is much more similar among these species than expected,”
he says. “Curiously, the largest differences between major
species groups appear to be the amount of ‘junk’
DNA rather than the number of genes.”
Using a recently developed
population genetic approach, Andolfatto showed in his study
that these expansive regions of “junk” DNA—which
in Drosophila accounts for about 80 percent of the
fly’s total genome—are evolving more slowly than
expected due to natural selection pressures on the non-protein-coding
DNA to remain the same over time.
“This pattern
most likely reflects resistance to the incorporation of new
mutations,” he says. “In fact, 40 to 70 percent
of new mutations that arise in non-coding DNA fail to be incorporated
by this species, which suggests that these non-protein-coding
regions are not ‘junk,’ but are somehow functionally
important to the organism.”
Andolfatto also found
that “junk” regions exhibit an unusually large amount
of functional genetic divergence between different species of
Drosophila, further evidence that these regions are
evolutionarily important to organisms. This implies that, like
evolutionary changes to proteins, changes to these “junk”
parts of the genome also play an important role in the evolution
of new species.
“Protein evolution
has traditionally been emphasized as a key facet of genome evolution
and the evolution of new species,” says Andolfatto. “The
degree of protein sequence similarity between humans and chimpanzees,
and other closely-related but morphologically distinct taxa,
has prompted several researchers to speculate that most adaptive
differences between taxa are due to changes in gene regulation
and not protein evolution. My results lend support to this view
by demonstrating that regulatory changes have been of great
importance in the evolution of new Drosophila species.”
Comment: Peter
Andolfatto (858) 334-8039
Media Contact: Kim
McDonald (858) 534-7572
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