AGE-SPECIFIC PATTERNS OF GENETIC VARIANCE IN DROSOPHILA-MELANOGASTER .1. MORTALITY

PETER MEDAWAR proposed that senescence arises from an age-related decl ine in tile force of selection, which allows late-acting deleterious m utations to accumulate. Subsequent workers have suggested that mutatio n accumulation could produce an age-related increase in additive genet ic variance (V-A) for fitness traits, as recently found in Drosophila melanogaster. Here we report results from a genetic analysis of mortal ity in 65,134 D. melanogaster. Additive genetic variance for female mo rtality rates increases from 0.007 in the first week of life to 0.325 by the third week, and then declines to 0.002 by the seventh week. Mal es show a similar pattern, though total variance is lower than in fema les. In contrast to a predicted divergence in mortality curves, mortal ity curves of different genotypes are roughly parallel. Using a three- parameter model, we find significant V-A for the slope and constant te rm of the curve describing age-specific mortality rates, and also for the rate at which mortality decelerates late in life. These results fa il to support a prediction derived from MEDAWAR'S ''mutation accumulat ion'' theory for the evolution of senescence. However, our results cou ld be consistent with alternative interpretations of evolutionary mode ls of aging.