Genetically modified crops may pass helpful traits to weeds, study
   finds
   'For the first time, researchers have shown that a gene artificially
   inserted into crop plants to fend off pests can migrate to weeds in a
   natural environment and make the weeds stronger. Scientists studied
   genetically engineered sunflowers - those modified with a gene that
   produces a chemical toxic to certain insects - to see what happened
   when these foreign genes, called transgenes, were inadvertently passed
   along to weedy relatives' ( Ohio State )
   See also this abstract, and this blog entry from last month.LINKS
   [1]http://www.hullocentral.demon.co.uk/site/anfin.[2]htm

References

   1. http://www.hullocentral.demon.co.uk/site/anfin.htm
   2. http://www.hullocentral.demon.co.uk/site/anfin.htm