Counting mtDNA molecules in Phaseolus vulgaris: sublimons are constantly produced by recombination via short repeats and undergo rigorous selection during substoichiometric shifting.

Sublimons are substoichiometric DNA molecules which are generated by recombinations across short repeats, located in main mitochondrial genome of plants. Since short repeats are believed to recombine irreversibly and to be usually inactive, it is unknown how substoichiometric sequences are maintained. Occasionally, sublimons are amplified during substoichiometric shifting (SSS) and take the role of the main genome. Using the Phaseolus vulgaris system, we have addressed the questions concerning accumulation of sublimons, the role of recombination in their maintenance and selective amplification during SSS. We found that sublimons accompanied by parental recombination sequences were maintained by constant recombination across a short 314-bp repeat. The abundance of these sublimons was three orders of magnitude higher than accumulation of those which could not be maintained by recombination because their parental forms were absent from the main genome. As expected for active recombination, two recombination-derived sublimons were equimolar and so were their parental forms. One parental and one substoichiometric form shared the A/C polymorphism indicating their frequent inter-conversion. Only the C variant of the sublimon was amplified during substoichiometric shift implying strong selection of DNA molecules operating during SSS.