Give and take: plant parasites dole out genes while stealing nutrients.

Parasites are the ultimate moochers, earning a living by stealing hard-earned nutrients from their hosts. Now, a new study in plants suggests that parasites sometimes give something back: foreign genes.

Gene swapping between species, a process known as horizontal gene transfer, is relatively common and well studied in bacteria. However, scientists have only recently found evidence of horizontal gene transfer in plants. Last summer, two groups of researchers--one led by Jeffrey Palmer of Indiana University in Bloomington and the other by Charles Davis of the University of Michigan in Ann Arbor--reported independently that some host plants pass their genes to the parasitic plant species that feed off them.

To determine whether gene transfer takes place in the opposite direction--from parasite to host--Palmer's team compared several genes from different species of the common weed genus Plantago with those in parasitic flower species of the Bartsia and Cuscuta genera.

Generally, the more closely related the species, the more similar their gene sequences. However, Palmer and his colleagues found that several Plantago species carry versions of a gene called atp1 that looks more like the comparable gene in either Bartsia or Cuscuta than like the one found in other Plantago species.

These results, published in the Nov. 11 Nature, suggest that Bartsia and Cuscuta each passed an atp1 gene to its Plantago host sometime during the past few million years.

Although the researchers aren't sure how the gene transfer took place, they speculate that it required tight physical contact between the parasites and their host plants. Both Bartsia and Cuscuta tap into their hosts by jutting tiny tendrils through the host's protective outer layer, perhaps making it possible for foreign DNA to sidestep the host's natural defenses.

Although the foreign atp1 gene that the researchers found in some Plantago species doesn't appear to function, Palmer suggests that horizontal transfer may have played a significant role in introducing genes for important traits in plants. "Parasitic plants could potentially be loaded with genes stolen from other plants, and they might serve as a reservoir to pass on these different genes," he says.

Davis, an ecologist and evolutionary biologist, calls Palmer's findings "really exciting stuff. We speculated that transfer of DNA between hosts and parasites was a two-way street, but they actually documented that fact."

Botanist Susanne Renner of the University of Hamburg in Germany notes that such gene swapping could make it difficult to figure out how species are related. Research that traces an organism's family tree usually presumes that the kind of DNA used in this study, mitochondrial DNA, doesn't travel between species.

"We need to be superwary about trusting mitochondrial genes when it comes to reconstructing an organism's family tree," she says.