Genetically modified crops have specific genes transferred from one genome to another. Typically it is believed that this could not happen naturally without human assistance. However, this article reports on the evidence that the sweet potato has a gene originally found in a bacterium.

http://www.npr.org/sections/goatsandsoda/2015/05/05/404198552/natural-gmo-sweet-potato-genetically-modified-8-000-years-ago

Michaeleen Doucleff