Most biologists might guess that transposons, viruslike bits of DNA that jump in and out of genomes, would harm their host. But new research shows that sometimes these genetic foes can turn into friends, by boosting toxin-busting genes.
The benevolent transposons were discovered by accident, admits evolutionary geneticist Todd Schlenke of the University of California, Davis, one of the paper's authors. While studying immune system genes in a Californian population of the fruit fly Drosophila simulans, he and his colleague David Begun stumbled on something odd. Normally, genes come in slightly different versions called alleles. But all the genes along a 100,000 DNA-letters-long piece of chromosome appeared to be exactly the same from one fly to the next. "It was pure chance," Schlenke says about the unexpected finding.
Such a lack of variation usually means that a particular allele of a gene somewhere inside the uniform region has spread rapidly through the population, dragging all the alleles of neighboring genes along as genetic hitchhikers--a process known as a selective sweep. A comparison with California samples of the related Drosophila melanogaster showed that in this species, too, a region in the same part of the chromosome was remarkably similar from fly to fly, the team reports online this week in the Proceedings of the National Academy of Sciences. That was an important clue: The researchers reckoned that the area where the unchanging regions in the two species overlapped must contain the gene that natural selection had singled out.
This stretch turned out to contain an unexpected partnership: a gene called Cyp6g1, responsible for detoxifying insecticides and other toxins, flanked by a "jumping gene." Tests showed that Cyp6g1 is more active in the presence of the transposon, possibly because the jump had jammed open a genetic switch that normally keeps Cyp6g1 turned off. In the pesticide-doused orchards of California, this may well have caused the selective sweep, Schlenke speculates.
The finding is interesting because transposons normally create havoc, says evolutionary geneticist Brian Charlesworth of the University of Edinburgh, U.K. In this case they appear to have benefited their host. The results could help clarify why transposons are clustered in some parts of the genome, he says.