Mitochondrial pAL2-1 plasmid homologs are senescence factors in Podospora anserina independent of intrinsic senescence

Since the first description of a linear mitochondrial plasmid in Podospora anserina, pAL2-1, and homologous plasmids have gone from being considered beneficial longevity plasmids, via neutral genetic elements, toward mutator plasmids causing senescence. The plasmid has an invertron structure, with terminal inverted repeats and encodes a DNA and a RNA polymerase. Here we test whether pAL2-1 homologs cause rapid aging independent of intrinsic and external conditions. We first analyzed a natural population of P. anserina and in 40% of the 112 isolates we detected pAL2-1 homologous plasmids. Though the lifespan varied considerably among the strains, plasmid-infected wild-type strains are on average shorter lived than plasmid-free strains and typically show a reduced lifespan extending effect of calorie restriction (CR). However, interesting exceptions were found, inviting further study. To further investigate the effect of pAL2-1 homologs under various conditions, we constructed and analyzed isogenic lines with and without the plasmid. We found that the presence of pAL2-1 homologs did not significantly affect growth rate as suggested by the population analysis, but reduced lifespan under all conditions. This effect was particularly clear for the lifespan extending conditions tested (CR, low temperature, antibiotics) supporting the idea that pAL2-1 homologs are additional senescence factors independent of the intrinsic senescence determinants