DYNAMICS OF PLANT MITOCHONDRIAL GENOME - MODEL OF A 3-LEVEL SELECTIONPROCESS

The plant mitochondrial genome is composed of a set of molecules of va rious sizes that generate each other through recombination between rep eated sequences. Molecular observations indicate that these different molecules are present in an equilibrium state. Different compositions of molecules have been observed within species. Recombination could pr oduce deleted molecules with a high replication rate but bearing littl e useful information for the cell (such as ''petite'' mutants in yeast ). In this paper, we use a multilevel model to examine selection among rapidly replicating incomplete molecules and relatively slowly replic ating complete molecules. Our model simulates the evolution of mitocho ndrial information through a three-level selection process including i ntermolecular, intermitochondrial, and intercellular selection. The mo del demonstrates that maintenance of the mitochondrial genome can resu lt from multilevel selection, but maintenance is difficult to explain without the existence of selection at the intermitochondrial level. Th is study shows that compartmentation into mitochondria is useful for m aintenance of the mitochondrial information. Our examination of evolut ionary equilibria shows that different equilibria (with different comb inations of molecules) can be obtained when recombination rates are lo wer than a threshold value. This may be interpreted as a drift-mutatio n balance.