Parasite pandemic: Cells of a fruit fly ovary (red) infected with Wolbachia (yellow).
Credit: University of Rochester
SYDNEY: Bacteria transfer genes to host organisms much more commonly than supposed, say geneticists who've discovered bacterial genes – in one case almost an entire genome – within the DNA of 11 different worms and insects.
A new study of the Wolbachia bacterium reports that lateral gene transfer — the movement of potentially functional genes between unrelated species — may happen frequently, posing dramatic implications for evolution.
It was already known that bacteria swap genes, and that some kinds of virus, such as HIV, can transfer their genes into the host organisms, but scientists believed that the multicellular organisms were largely immune to gene transfer from bacteria.
Gene switching
"It didn't seem possible at first," said study co-author Jack Werren, of the University of Rochester in New York, USA. "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 genome has been absorbed and integrated into the host's."
"The host's genes actually hold the coding information for a completely separate species," added Werren, whose team detail the findings today in the U.S. journal Science.
When the Wolbachia bacterium invades an animal host, typically an insect, it finds its way into the egg or sperm cells. It is then 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.
Led by genomicist Julie Dunning Hotopp at the J. Craig Venter Institute in Rockville, Maryland, the researchers found evidence that Wolbachia genes were interspersed within the genes of the the fruit fly, Drosophila ananassae, as if they were part of the same genome. The team went on to discover that the bacteria had inserted a range of genes into 11 different host species including flies, mosquitos and nematodes. Almost the entire Wolbachia genome was transferred to one of the fly genomes.
Acquiring new functions
"Our study demonstrates not only how widespread the transfers are but provides the first physical evidence for the transfer, namely the microscopy showing the Wolbachia gene in the insect's chromosome. We are also the first to demonstrate that the Wolbachia genes are transcribed." she said
Transcription is one of the steps required for creating functional protein from DNA and suggests the possibility that some of the transferred genes could be fully operational.
"Evolutionary biologists will be interested in how these transfers may allow species to acquire new functions," said Dunning Hotopp. "For example, can the fly inherit the genes to make certain vitamins it needs?"
Genome sequencing projects may have actually been biased against spotting bacterial genes within other genomes, commented geneticist Scott O'Neill from the University of Queensland in Brisbane, because known bacterial sequences are typically regarded as evidence of contamination in samples and discarded.
"This research… reopens the debate about the occurrence and frequency of lateral gene transfer between distantly related organisms," he said.
"This is stunning evidence for increased frequency of gene transfer," added biochemist W. Ford Doolittle of Dalhousie University in Nova Scotia, Canada. The discovery confirms the widespread occurrence of a process that would have been dismissed as pure "science fiction" until very recently, he said.
