SLOWING OF AGE-SPECIFIC MORTALITY-RATES IN DROSOPHILA-MELANOGASTER

We have studied age-dependent mortality in large cohorts of male and f emale D. melanogaster from four inbred lines. Average longevity varies substantially between genotypes (broad-sense heritability = 22%). Con trary to the predictions of the Gompertz model, mortality rates tend t o decelerate at the most advanced ages. Fitting Gompertz, Weibull, Log istic, and Two-stage Gompertz mortality models to the data, we find th at the best fit is obtained with the two-stage model, with exponential ly increasing mortality at early ages, and zero or nearly zero increas e at older ages. There is little microenvironmental effect from cage t o cage. There is a sex-dependent mortality crossover: males and female s differ in initial mortality rate and degree of acceleration of morta lity rate, but the ordering of the sexes according to mortality parame ters depends on genotype. Model fitting can be affected by gaps betwee n deaths in the tail of the survivorship distribution. The observation s are inconsistent with the limited life-span paradigm, which predicts sudden and well-defined drops in survivorship and corresponding sharp increases in mortality at advanced ages for large cohorts of genetica lly identical individuals.