The bacterial invader that rewrites genes

August 31, 2007
from: http://www.timesonline.co.uk/tol/news/world/article2357994.ece

There are implications for evolution in findings on a process that may be more common than thought

Mark Henderson Science Editor


The world’s most successful bacterial parasite has effectively become a part of at least 11 different animals by writing its entire genetic code into theirs, according to research that sheds important light on evolution.

Scientists have discovered copies of the genome of the Wolbachia bacterium lurking within the genetic code of fruit flies, wasps and nematode worms. So ingrained are the bacteria’s genes that they appear to have biological roles within the host animal.

The findings have important implications for evolution, as they show that genes can transfer from bacteria to unrelated multicellular organisms and take on new functions much more readily than was supposed previously.

This suggests that large-scale gene transfers may allow larger animals and plants to acquire new genes with beneficial effects extremely quickly, and then to pass them on to future generations. Such transfers are known to occur between bacteria and to have happened in the evolutionary past. Mitochondria, structures or organelles that provide cells with energy, are relics of bacteria that were absorbed by their host cells, as are the chloroplasts that allow photosynthesis in plants.

The research, from the J. Craig Venter Institute, Maryland, and the University of Rochester, New York state, shows this process in action and indicates that it may be more common than had been thought. “In a way Wolbachia could be the next mitochondria,” said Jack Werren, Professor of Biology at the University of Rochester. “A hundred million years from now, everyone may have a Wolbachia organelle. Well not us, we’ll be long gone, but Wolbachia will still be around.”

Professor W. Ford Doolittle, of Dalhousie University, Canada, a microbial genomics specialist who was not involved in the work, said: “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. This is stunning evidence for increased frequency of gene transfer.” The study, which is published in the journal Science, also has implications for genome mapping projects. Bacterial gene sequences picked up by efforts to decode the genomes of other species have generally been dismissed as contamination, but the new results suggest that they may actually belong to the target organisms.

Wolbachia is thought to infect about 70 per cent of all invertebrates, where it invades their eggs or sperm to ensure that it is passed to the next generation, making it a good candidate for a gene transfer source.

Julie Dunning-Hotopp, of the Venter Institute, found that some Wolbachia genes seemed to be fused to those of the fruit fly, and when Michael Clark, of the University of Rochester, treated the flies with an antibiotic, killing Wolbachia, its DNA remained.

“For several months I thought I was just failing,” Dr Clark said. “I kept administering antibiotics, but every single Wolbachia gene I tested for was still there.”

The conclusion was that Wolbachia had written its genome into that of the fly. Some of these genes are even transcribed into RNA, the molecule that makes proteins, which indicates that they probably have a biological function.