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A Genome within a Genome

The surprising finding suggests a new mechanism of evolution.
August 31, 2007

Wolbachia in yellow with host cells in red.
Credit: University of Rochester and J. Craig Venter Institute

In a strange genetic version of the Russian doll, scientists have discovered the genome of a bacterial parasite nestled inside the genome of its host. The findings, published today in the journal Science, suggest that organisms might quickly acquire new genes and functions through the large-scale transfer of genes.

The parasite, known as Wolbachia, invades the eggs and sperm of many different types of insects, ensuring that it is passed down to the host’s offspring. In this case, scientists discovered the bacterium’s genome within the chromosome of its fruit-fly host. While microbiologists have previously seen cases of gene swapping between microbes or between parasites and their hosts, this is the first example of such an extensive exchange.

According to a press release from the University of Rochester,

“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 [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 percent 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.”

A similar phenomenon may have happened in our own distant past.

“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.”

Fruit fly ovaries showing Wolbachia infection within.
Credit: University of Rochester and the J. Craig Venter Institute

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