This paper investigates senescence, a programmed growth limitation in the fungus Podospora anserina. The authors demonstrate that mycelia exist in two distinct physiological states: a 'non-senescent' state with constant growth potential and a 'senescent' state with declining growth potential. The transition occurs via a single random event—appearance of a cytoplasmic 'senescence' factor—after which a race-specific incubation distance (I.D.) must be grown before senescent morphology manifests. Using subcultured hyphal samples at different positions along growing mycelia, the researchers confirm that race A mycelia follow the same two-state model previously described for race S. They provide evidence that the senescence factor is particulate and increases exponentially during mycelial growth. The transformation rate (probability of senescence factor appearance per unit growth) is directly proportional to the number of hyphal tips per culture, supporting a random event model. Finally, brief exposure to cycloheximide or cyanide extends median growth length, suggesting mitochondrial function influences senescence development.

Key findings

• Mycelia of Podospora anserina exist in two physiological states ('non-senescent' and 'senescent') defined by distinct growth characteristics, with transition occurring via a single random cytoplasmic event.
• The senescence factor is particulate and increases exponentially in concentration during mycelial growth, from one factor per mycelium to approximately 1-100 factors per cell.
• The transformation rate from non-senescent to senescent state is directly proportional to the number of hyphal tips per culture, consistent with random event probability.
• Short-term growth in media containing cycloheximide or cyanide substantially extends median growth length, suggesting mitochondrial function modulates senescence development.