Table 1 Summary of assumptions and predictions of various aging models
From: Repair rather than segregation of damage is the optimal unicellular aging strategy
Assumptions | Predictions | |||
|---|---|---|---|---|
Growth & Division | Effect of damage | Removal of damage | Environment | Â |
Watve et al. (2006) [30]: Leslie matrix model with multiple cellular components of different ages | ||||
Growth rates of cellular components decline with their age. Cells divide after a fixed time without any restriction on daughter cell sizes. Cells die if they are in the oldest age class and no longer contribute to population growth | ‘Toxicity’ considered by assuming oldest and slowest growing components to be growth rate limiting | Repair converts oldest into newest components without growth rate cost | Constant | Asymmetric division increases population growth rate over the symmetric case if older components in the latter are ‘toxic’ and decline of growth rate with age is above minimal. Repair increases population growth rate since repair turns old into new components at no growth rate cost |
Ackermann et al. (2007) [31]: evolutionary model where survival depends on damage and repair | ||||
Cells do not grow, yet divide after a fixed time | Damage decreases survival probability | Repair removes damage, at cost of decreased survival probability | Constant, extrinsic mortality | Repair is only beneficial in symmetrically dividing cells. The best strategy is complete asymmetry without any repair |
Erjavec et al. (2008) [32]: metabolic model of growing cells | ||||
Growth of cells linear; cells divide once active protein reaches a threshold | Damage toxic | No repair but decay of active and damaged protein; decay without cost, no recycling of damaged into active protein | Constant | Asymmetry of damage partitioning beneficial, the stronger the asymmetry, the higher the benefit. Symmetry beneficial if offspring are smaller unless damage accumulation rate too high |
Chao (2010) [24]: damage affects time between divisions | ||||
Cells acquire active and damaged protein at linear rates; cells divide once active protein reaches a threshold | Damage toxic by linearly decreasing growth rate | Repair absent | Constant, extrinsic mortality | Complete asymmetry has highest mean fitness apart from a narrow region of intermediate damage accumulation rates where the fittest strategy is slightly below complete asymmetry |
Rashidi et al. (2012) [5]: energy budget model | ||||
Cells grow and prevent damage accumulation depending on energy allocated to growth and prevention, with a fixed total energy budget for the cell | No effect on growth or division, but can trigger instant cell death if above threshold | Damage is degraded but not repaired (recycled) | Constant | Asymmetry ensures survival of the population at high damage accumulation rates in the absence of degradation. Symmetrically dividing cells invest just enough into damage prevention to avoid instant death |
UnicellAge: metabolic model of growing and repairing cells competing for resources | ||||
Cells grow exponentially by consuming resource; cells divide once total protein reaches a threshold | Damage inert or toxic | Repair by active protein that does not contribute to growth; repair recycles material with a certain efficiency | Constant or dynamic, extrinsic mortality | Repair better than asymmetry unless damage accumulation rate high, damage toxic and efficiency of repair low |