Tracing evolutionary and developmental implications of mitochondrial stoichiometric shifting in the common bean

The recombination and copy number shifting activities of the plant mitochon drial genome are widely documented across plant genera, but these genome pr ocesses have not been as well examined with regard to their roles in plant evolution. Because of the extensive plant collections of Phaseolus spp and the degree to which cytoplasmic male sterility (cms) has been characterized in the common bean, this system would be valuable for investigating mitoch ondrial genome dynamics in natural populations. We have used the ems-associ ated sequence pvs-orf239 as a mitochondrial genetic marker for these studie s and have demonstrated its universal presence throughout a diversity of un domesticated Phaseolus lines. Within these populations, the pvs-orf239 sequ ence is present in high copy number in similar to 10% of the lines, but sub stoichiometric in all others. This mitochondrial sequence, derived apparent ly by at least two recombination events, is well conserved with two point m utations identified that are both apparently silent with regard to the ster ility phenotype. A putative progenitor sequence was identified in Phaseolus glabelus in substoichiometric levels, suggesting that the present-day pvs- orf239 sequence was likely introduced substoichiometrically. Copy number sh ifting within the mitochondrial genome results in a 1000- to 2000-fold chan ge, so that substoichiometric forms are estimated at less than one copy per every 100 cells. On the basis of PCR analysis of root tips, we postulate t hat a mitochondrial "transmitted form" resides within the meristem to assur e transmission of a complete genetic complement to progeny.