A GENETIC-ANALYSIS OF SENESCENCE IN DROSOPHILA

Two attractive theories for the evolution of senescence are based on t he principle that the force of natural selection decreases with age1-5 . The theories differ in the type of age-specific gene action that the y assume. Antagonistic pleiotropy2-5 postulates that pleiotropic genes with positive effects early in life and negative effects of comparabl e magnitude late in life are favoured by selection, whereas genes with the reverse pattern of action are selected against. Mutation accumula tion1,3-5 assumes that deleterious mutant alleles with age-specific ef fects will equilibrate at a lower frequency if their effects are expre ssed early rather than late in life. Explicit models demonstrate that both mechanisms can lead to the evolution of senescent life histories under reasonable conditions3-5. Antagonistic pleiotropy has gained con siderable empirical support4-6, but the evidence in support of mutatio n accumulation is more sparse4,5,7. Here we report that the genetic va riability of mortality in male Drosophila melanogaster increases great ly at very late ages, as predicted by the mutation accumulation hypoth esis3-5. The rate of increase in mortality with age exhibits substanti al genetic and environmental variability. This result provides a possi ble explanation for recent observations of non-increasing mortality ra tes in very old flies8,9.