EVOLUTION OF THE NUCLEUS. “One important event in early eukaryotic evolution was the development of a symbiotic relationship between the α-proteobacteria progenitor of mitochondria and an archaebacteria-like host. As with most endosymbiotically derived organelles, DNA was transferred from mitochondria to the host.”
http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1004351