The evolutionary implications of promiscuous DNA are far reaching and it is speculated that chloroplasts and mitochondria, before invading eukaryotic cell were free living primitive prokaryotes. The endosymboint hypothesis suggests that in course of time these entrapped organisms lost their independence by losing genetic information to nuclear genome. Thus the promiscuity of DNA gave further support to endosymboint hypothesis, but exchange of DNA between mitochondria and chloroplasts is surprising, since they are independent of each other. This may be partly explained on the basis of great variation and bigger size of mtDNA, relative to cpDNA which is tightly constrained in size and sequence even between species. While mtDNA can tolerate foreign DNA and other disturbances, chloroplasts are sensitive to any such variation or modification. This is confirmed by evidence produced at Carnegie Institute of Washington, Stanford, U.S.A., where it was shown that chloroplast sequences from a variety of plants are scattered throughout mitochondrial genomes, and some of these transfers are believed to be rather recent.
The mechanism of this transfer of so called promiscuous DNA is not known but two possibilities are considered, (i) Some kind of a vector or transposon or transducing phage may be involved, (ii) The two organelles might have undergone fusion at some stage. This second mechanism involving fusion is preferred on the basis of electron microscopic observations. However, more than one mechanisms may be involved in such DNA transfers.
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