Bloomington, Indiana
July 9, 2003
from
Indiana University via
Agnet July
9/03-II
Scientists have long thought gene exchange between individuals
of unrelated species to be an extremely rare event among
eukaryotes -- the massive group of organisms that counts among
its members humans, oak trees, kelp and mushrooms -- throughout
the group's 2 billion year history.
But a new Indiana
University Bloomington study in this week's Nature suggests that
such genetic events, called horizontal gene transfers, have
happened more often than previously thought during the evolution
of flowering plants. The finding hints other eukaryotes have had
significant genetic influence from completely unrelated species.
"It appears horizontal
gene transfer occurs for just about any gene in the plant
mitochondrial genome," said biologist and Class of 1955 Endowed
Professor Jeffrey Palmer, who led the research. "There is no
reason to believe that this finding would apply only to plants.
We already know from past studies that other eukaryotes
experience the same mechanisms of horizontal transfer for
certain special pieces of DNA called transposable elements. Our
results now extend this phenomenon to the thousands of ordinary
genes in a genome."
It has been common
knowledge for years that horizontal gene transfer among bacteria
is extremely common. Some scientists believe that as much as 25
percent of certain bacterial species' chromosomal DNA has been
acquired by way of horizontal transfer.
In eukaryotes, the rule
remains that individuals get their genes from parents
intergenerationally through the more familiar process called
vertical transmission. But Palmer said scientists have probably
underestimated the rate at which non-traditional gene transfer
happens in eukaryotes.
"While our data set was
small and real rates of eukaryotic horizontal gene transfer were
therefore hard to predict, we can infer that even
conservatively, horizontal gene transfer must have happened in
flowering plants thousands of times," Palmer said.
One of the assumptions
scientists make when comparing DNA from different species is
that the DNA has followed basic lines of heredity connected in
the past by a common ancestor. If DNA used in these gene studies
does not descend vertically, from parent to offspring, but
horizontally, by jumping from another lineage, analyses might
turn up confusing or misleading evolutionary relationships
between species. But Palmer isn't worried about that.
"We don't believe
horizontal gene transfer happens often enough to throw a monkey
wrench into molecular genealogical studies," he said.
While the mechanisms
of horizontal gene transfer are still unknown, various
explanations suggest that viruses, bacteria and fungi pack
errant genetic material, or that accidental cross-species mating
may play a role. However it happens, Palmer said there is no
question it doeshappen. Many scientists have reported
unexpectedly finding one species' gene in another species with
no reasonable explanation except horizontal gene transfer.
After encountering unique
gene sequences along circular mitochondrial DNA chromosomes in
three flowering plant species, Palmer and his team sought to
determine the source of the anomalous genetic material. Part of
the mystery was that closely related flowering plants did not
possess the same gene sequences. Most of the genes the
researchers examined encode parts of ribosomes, tiny assembly
plants that make proteins by connecting amino acids.
Palmer's team amassed
mitochondrial gene sequence data from about 100 angiosperm
species and looked for sequence similarities between them. In
creating a tree of relatedness between the sequences, the
scientists found that the mitochondrial genes from five
flowering species -- kiwi fruit, honeysuckle, birch, bloodroot
and Amborella (the most primitive flowering plant in existence)
-- appeared far more related to unrelated species than to
species more closely related to them, strongly suggesting that
the four species had acquired these particular genes by way of
horizontal transfer.
In the case of the
bloodroot, the researchers were astonished to find a hybrid,
"chimeric" mitochondrial gene. Half of this gene was captured by
horizontal transfer from an unrelated plant over 100 million
years distant in time, while the other half had been transmitted
faithfully from parent to offspring in the lineage leading to
bloodroot. "This result was so surprising, our first thought was
that we'd made a mistake," Palmer said. "Once it was confirmed
we had not made an error, we understood that what we'd found was
very exciting." |