Literature contains many suggestions that plant genomes are highly variable. One early indication was the discovery that maize inbreds differ in the number of rDNA copies. Until the advent of genetic engineering technology had not crossed the species barrier in terms of gene transfer between kingdoms. However, DNA from unrelated species is transferred and incorporated into plant genomes mainly through viruses The integration of viral sequences may be widespread in the plant kingdom, having occurred for a long period of time. Similarly, genes from the bacterium, Agrobacterium rhizogenes, have been found in the genome of some tobacco species while DNA from unrelated higher plants has been found to be transferred between their mitochondria, and, from there, to their nuclei.