Drosophila yakuba, one of several fruit fly species whose DNA appears to harbor genetic material from a virus, according to a University at Buffalo study. Photo courtesy of D.J. Obbard, University of Edinburgh. |
In
a curious evolutionary twist, several species of a commonly studied
fruit fly appear to have incorporated genetic material from a virus into
their genomes, according to new research by University at Buffalo
biologists.
The
study found that several types of fruit fly—scientific name
Drosophila—harbored genes similar to those that code for the sigma
virus, a fly virus in the same family as rabies. The authors believe the
genetic information was acquired during past viral infections and
passed on from fruit fly parent to offspring through many generations.
The
discovery could open the door for research on why flies and other
organisms selectively retain viral genes—dubbed “fossil” genes—through
evolution, said lead author Matthew Ballinger, a PhD candidate in UB’s
Department of Biological Sciences.
One
hypothesis is that viral genes provide an anti-viral defense, but
scientists have had trouble testing this theory because viral genes
found in animals are often millions of years old—ancient enough that the
genes’ genetic sequence differs significantly from that of modern-day
viruses.
The
new study, in contrast, uncovered a viral gene that appears to be
relatively young, with genetic material closely mirroring that of a
modern sigma virus.
“We
don’t know that these genes have an anti-viral function, but it’s
something we’d like to test,” Ballinger said. “It’s tempting to think
that these genes are retained and express RNA because there’s some kind
of advantage to the host.”
He
and his co-authors—Professor Jeremy Bruenn and Associate Professor
Derek Taylor in UB’s Department of Biological Sciences—reported their
results online on June 26 in the journal Molecular Phylogenetics and
Evolution. The research, supported in part by UB’s Center for Advanced
Molecular Biology and Immunology, will also appear in a forthcoming
print edition of the journal.
“Our
findings establish that sigma virus-like (genes) are present in
Drosophila species and that these infection scars represent a rich
evolutionary history between virus and host,” the researchers wrote in
their paper.
Another
important contribution the study makes is advancing our understanding
of how flies and other organisms acquire copies of virus-like genes in
the first place.
The
sigma virus belongs to a class of RNA viruses that lack an important
enzyme, reverse transcriptase, that enables other viruses to convert
their genetic material into DNA for integration into host genomes.
Given this limitation, how did sigma virus genes get into fly genomes?
The
new study supplies one possible answer, suggesting that viruses may use
reverse transcriptase present in host cells to facilitate incorporation
of viral genes into host DNA.
In
the genome of one fly, the researchers found a sigma fossil gene right
in the middle of a retrotransposon, a genetic sequence that produces
reverse transcriptase for the purpose of making new copies of itself to
paste into the genome.
The
position and context of the viral gene suggests that the
retrotransposon made a copying error and copied and pasted virus genes
into the fly genome. This is the clearest evidence yet that
non-retroviral RNA virus genes naturally enter host genomes by the
action of enzymes already present in the cell, Ballinger said.
The study builds on prior research by Taylor and Bruenn, who previously co-authored a paper
showing that bats, rodents and wallabies harbor fossil copies of genes
that code for filoviruses, which cause deadly Ebola and Marburg
hemorrhagic fevers in humans.
The
next step in the research is to continue exploring how and why flies
and other organisms acquire copies of virus genes. To find out whether
sigma virus-like genes have an anti-viral function in fruit flies,
scientists could splice the genes into flies that can contract modern
sigma viruses, or introduce modern sigma viruses into flies that already
harbor the genes.
Source: University of Buffalo