Contribution of various classes of defective mitochondrial DNA molecules to senescence in Podospora anserina

The unavoidable arrest of vegetative growth in Podospora anserina (senescence process) is always correlated with rearrangements of the mitochondrial chromosome, mainly consisting in the amplification of particular regions as tandemly repeated circular molecules (senDNAs). One sequence systematically amplified in senescent cultures corresponds precisely to the first intron (intron alpha) of the cox1 gene; nevertheless, other regions (called beta and gamma) are also frequently amplified. The experiments presented in this paper show that cellular death is in some cases associated with the sole presence of large amounts of senDNA beta. In addition, we provide evidence that senDNA beta and senDNA alpha accumulate by different mechanisms, as previously proposed. This suggests that beta senDNAs have a lethal effect on the mycelium on their own and most likely have replicative properties independent of the presence of sequence alpha. These data do not fit well with the current opinion that gives an essential role to intron alpha in the senescence of P. anserina.