September 4, 2007
One species’ genome discovered inside
another’s
Findings have major implications for theory of
evolution
Jack Werren, professor of biology and principle investigator of the gene transfer study
Scientists at the University and the J. Craig Venter
Institute have discovered a copy of the genome of a bacterial parasite
residing inside the genome of its host species.
The research, reported in Science, also shows that lateral
gene transfer—the movement of genes between unrelated
species—may happen much more frequently between bacteria and
multicellular organisms than scientists previously believed, posing
dramatic implications for evolution and for disease control.
Such large-scale heritable gene transfers may allow
species to acquire new genes and functions extremely quickly, says Jack
Werren, a principle investigator of the study. If such genes provide new
abilities in species that cause or transmit disease, they could provide new
targets for fighting these diseases.
The results also have serious repercussions for
genome-sequencing projects. Bacterial DNA is routinely discarded when
scientists are assembling invertebrate genomes, yet these genes may very
well be part of the organism’s genome, and might even be responsible
for functioning traits.
“This study establishes the widespread
occurrence and high frequency of a process that we would have dismissed as
science fiction until just a few years ago,” says W. Ford Doolittle,
Canada Research Chair in Comparative Microbial Genomics at Dalhousie
University, who is not connected to the study. “This is stunning
evidence for increased frequency of gene transfer.”
“It didn’t seem possible at first,”
says Werren, professor of biology and world-leading authority on the
parasite wolbachia. “This parasite has implanted itself inside the cells of 70
percent of the world’s invertebrates, co-evolving with them. And now
we’ve found at least one species where the parasite’s entire or
nearly entire genome has been absorbed and integrated into the
host’s. The host’s genes actually hold the coding information
for a completely separate species.”
Wolbachia may be
the most prolific parasite in the world—a “pandemic,” as
Werren calls it. The bacterium invades a member of a species, most often an
insect, and eventually makes its way into the host’s eggs or sperm.
Once there, the parasite is ensured passage to the next generation of its
host, and any genetic exchanges between it and the host also are much more
likely to be passed on.
Werren doesn’t believe that the parasite
“intentionally” insert their genes into the hosts. As cells go
about their regular business, they can accidentally absorb bits of DNA into
their nuclei, often sewing those foreign genes into their own DNA. But
integrating an entire genome was definitely an unexpected find.
Werren and Clark are now looking further into the huge
insert found in the fruitfly, and whether it is providing a benefit.
“The chance that a chunk of DNA of this magnitude is totally neutral,
I think, is pretty small, so the implication is that it has imparted of
some selective advantage to the host,” says Werren. “The
question is, are these foreign genes providing new functions for the host?
This is something we need to figure out.”
Before this study, geneticists knew of examples where
genes from a parasite had crossed into the host, but such an event was
considered a rare anomaly except in very simple organisms. Bacterial DNA is
very conspicuous in its structure, so if scientists sequencing a nematode
genome, for example, come across bacterial DNA, they would likely discard
it, reasonably assuming that it was merely contamination—perhaps a
bit of bacteria in the gut of the animal, or on its skin.
“Gene transfers have happened before in the
distant past,” notes Werren. “In our very own cells and those
of nearly all plants and animals are mitochondria, special structures
responsible for generating most of our cells’ supply of chemical
energy. These were once bacteria that lived inside cells, much like wolbachia does today.
Mitochondria still retain their own, albeit tiny, DNA, and most of the
genes moved into the nucleus in the very distant past. Like wolbachia, they have
passively exchanged DNA with their host cells. It’s possible wolbachia may follow
in the path of mitochondria, eventually becoming a necessary and useful
part of a cell.
“In a way, wolbachia could be the next mitochondria,” says Werren.
“A hundred million years from now, everyone may have a wolbachia organelle.”
“Well, not us,” he laughs.
“We’ll be long gone, but wolbachia will still be around.”
Jonathan Sherwood is a senior science
publicist in the University Communications Office.
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