It is evidence for increased frequency of gene transfer
Parasite found in 70 per cent of world’s invertebrates
Large-scale heritable gene transfers may allow species to acquire new genes and functions extremely quickly. If such genes provide new abilities in species that cause or transmit disease, they could provide new targets for fighting these diseases.
Scientists at the University of Rochester 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 discovery has dramatic implications for evolution, pest, and disease control.
The research, reported in Science, 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.
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, according to a University of Rochester press release.
It is stunning evidence for increased frequency of gene transfer. “It didn’t seem possible at first,” says Jack Werren, professor of biology at the University of Rochester and a world-leading authority on the parasite, called Wolbachia.
“This parasite has implanted itself inside the cells of 70 per cent of the world’s invertebrates, coevolving 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 Wolbachia 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.
Since Wolbachia typically live within the reproductive organs of their hosts, Werren reasoned that gene exchanges between the two would frequently pass on to subsequent generations.
The researchers found evidence that some of the Wolbachia genes seemed to be fused to the genes of the fruitfly, Drosophila ananassae, as if they were part of the same genome. Michael Clark, a research associate at Rochester then brought a colony of ananassae into Werren’s lab to look into the mystery.
To isolate the fly’s genome from the parasite’s, Clark fed the flies a simple antibiotic, killing the Wolbachia.
To confirm the ananassae flies were indeed cured of the wolbachia, Clark tested a few samples of DNA for the presence of several Wolbachia genes.
To his dismay, he found them. “For several months, I thought I was just failing,” says Clark.
“I kept administering antibiotics, but every single Wolbachia gene I tested for was still there. I started thinking maybe the strain had grown antibiotic resistance. After months of this I finally went back and looked at the tissue again, and there was no Wolbachia there at all.”
Since Wolbachia typically live within the reproductive organs of their hosts, Werren reasoned that gene exchanges between the two would frequently pass on to subsequent generations. Clark had cured the fly of the parasite, but a copy of the parasite’s genome was still present in the fly’s genome.
Clark was able to see that Wolbachia genes were present on the second chromosome of the insect.
Clark confirmed that the Wolbachia genes are inherited like “normal” insect genes in the chromosomes, and some of the genes are “transcribed” in uninfected flies, meaning that copies of the gene sequence are made in cells that could be used to make Wolbachia proteins.
Werren doesn’t believe that the Wolbachia “intentionally” insert their genes into the hosts. Rather, it is a consequence of cells routinely repairing their damaged DNA.
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. — Our Bureau