Does nuclear integration of mitochondrial sequences occur during senescence in Podospora?

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Abstract
Observations in various organisms suggest that the transfer of mitochondrial DNA sequences to the nucleus has occurred in the course of evolution. Wright and Cummings have reported, on the basis of hybridization experiments, that there is such a transposition in the fungus Podospora anserina during senescence (arrest of vegetative growth) which is accompanied by amplification of specific mitochondrial sequences. They suggested that this transposition could explain senescence, and, as it could be regularly observed under laboratory conditions, senescence in Podospora could constitute a model system to study the molecular mechanisms of the transfer of DNA sequences from mitochondria to the nucleus. We report here experiments similar to those of Wright and Cummings but include additional critical controls. Our results do not confirm the data of Wright and Cummings; the only positive signals we observed after hybridization of mitochondrial DNA sequences on nuclear DNAs can be attributed to contaminating DNA from mitochondria or plasmids.