London, April 22 (IANS): Sweet potatoes contain genes from the bacterium Agrobacterium and the presence of this 'foreign' DNA makes the popular food naturally "genetically modified", says a study.

Agrobacterium is specialised to transfer a part of its own DNA, the so-called T-DNA, to plants. And it is this T-DNA that has been found in sweet potato, the researchers noted.

"It demonstrates that genetic modification also happens in nature," said one of the researchers professor Godelieve Gheysen from the Ghent University in Belgium.

The researchers said sweet potatoes could be an example of "horizontal gene transfer" in which there is exchange of genes between different species -- in contrast to normal gene transfer from parents to progeny which occurs within one species.

"The natural presence of Agrobacterium T-DNA in sweet potato and its stable inheritance during evolution is a beautiful example of the possibility of DNA exchange across species barriers," Gheysen said.

The researchers discovered the foreign DNA sequences of Agrobacterium while searching the genome of sweet potato for viral diseases.

The sequences appeared to be present in each of the 291 tested sweet potato cultivars and even in some wild related species.

Different research methods confirmed the same conclusion: the specific sequences are not due to contamination, but they are part of the sweet potato genome.

One of the earliest domesticated crops, sweet potato is one of the most important food crops for human consumption, especially in Sub-Sahara Africa, parts of Asia and the Pacific islands.

The discovery was detailed in the journal PNAS - Proceedings of the National Academy of Sciences.