Stress resistance declines with age: analysis of data from a survival experiment with Drosophila melanogaster.

An approach towards analyzing survivorship data is proposed for the study of changes in stress resistance with age in the population of Drosophila melanogaster. This is based on the model of heterogeneous mortality (frailty model). Results of the data analysis show that observed populations of flies are heterogeneous and the accelerated selection, debilitative effect and changes in individual frailties are the aftermath of stress. These results also reveal that debilitative effect and accelerated selection are much better pronounced in survivals of flies that are stressed at an older age. Mild stress, when applied at both ages, produced a reduction in frailty variance. Stress of greater magnitude produced higher frailty variance in the young-treated flies. Among the old-treated insects, stress of longer duration led to a reduction of both the mean and the variance of frailty distribution. Population of young-treated flies became more heterogeneous, population of old-treated flies became less heterogeneous, and both populations became more robust in average after stress.