The fractionation experiment: reducing heterogeneity to investigate age-specific mortality in Drosophila.

Age-specific mortality rates decelerate at older ages in both genetically homogenous and heterogeneous populations of Drosophila. One explanation proposed for deceleration is population heterogeneity. This hypothesis suggests that a population consists of sub-populations that differ in mortality characteristics and that the deceleration is the result of selective survival of stronger individuals. Here we describe an experiment that fractionates populations into several sub-populations without changing the physiological characteristics of the post-fractionated populations. Through a careful process of selection of Drosophila eggs, larvae, pupae and adults, we attempt to reduce as much as possible the degree of pre-adult, environmentally induced heterogeneity among individuals of a genetically identical cohort. We then ask whether such cohorts, when compared to non-fractionated populations, exhibit a lesser degree of mortality deceleration at advanced ages. From a total of 106 fractionated and control populations, consisting of 51331 individuals, 101 populations (93% of the fractionated populations and 100% of the control populations) exhibit a significant amount of mortality deceleration late in life. These observations suggest that environmental heterogeneity accrued during larval development is not a major factor contributing to mortality deceleration at older ages.