No. 25: Jan-Feb 1983
The cells of plants (photosynthetic eukaryotes) are genetically the most complex that biologists have discovered. Each cell has three genetic systems: its own, that of the chloroplasts; and that of the mitochondria. It is supposed that the chloroplasts and mitochondria were once free-living cells that linked up with the embryonic plant cell to form a symbiotic partnership, with the host "plant" cell being the dominant member.
Up until now, the three genetic systems were thought to be discrete, each going down its own pathway. But chloroplasts genes have now been found inside plant mitochondria, overturning conventional wisdom. To sum it all up, DNA seems promiscuous -- no respecter of privacy and breaking down all isolating genetic barriers.
This discovery at once raises a dozen questions. For example, are mitochondria genes in chloroplast cells? How far does this promiscuity go? Can the same thing happen in higher organisms; say, with humans and symbiotic microorganisms or even not-so-symbiotic disease organisms? Is there no stopping this DNA?
(Ellis, John; "Promiscuous DNA -- Chloroplast Genes inside Plant Mitochondria," Nature, 299:678, 1982.)