We present the results of selection experiments designed to distinguish between antagonistic pleiotropy and mutation accumulation, two mechanisms for the evolution of senescence. Reverse selection for early‐life fitness was applied to laboratory populations of Drosophila melanogaster that had been previously selected for late‐life fitness. These populations also exhibited reduced early‐age female fecundity and increased resistance to the stresses of starvation, desiccation, and ethanol, when compared to control populations. Reverse selection was carried out at both uncontrolled, higher larval rearing density and at controlled, lower larval density. In the uncontrolled‐density selection lines, early‐age female fecundity increased to control‐population levels in response to the reintroduction of selection for early‐age fitness. Concomitantly, resistance to starvation declined in agreement with previous observations of a negative genetic correlation between these two characters and in accordance with the antagonistic‐pleiotropy mechanism. However, resistance to stresses of desiccation and ethanol did not decline in the uncontrolled‐density lines during 22 generations of reverse selection for early‐life fitness. The latter results provide evidence that mutation accumulation has also played a role in the evolution of senescence in this set of Drosophila populations. No significant response in early‐age fecundity or starvation resistance was observed in the controlled‐density reverse‐selection lines, supporting previous observations that selection on Drosophila life‐history characters is critically sensitive to larval rearing density.
Selection has been used to create replicated outbred stocks of Drosophila melanogaster with increased longevity, increased later fecundity, and increased levels of physiological performance at later ages. The present study analyzed the quantitative transmission patterns of such stocks, employing extensive replication in numbers of stocks, individuals, and assayed characters. The populations used derived from five lines with postponed aging and five control lines, all created in 1980 from the same founding base population. The following characters were studied: early 24-hr fecundity, early ovary weight, early female starvation resistance, early male starvation resistance, female longevity and male longevity. Numerous crosses were performed to test for non-Mendelian inheritance, average dominance, maternal effects, sex-linkage and between-line heterogeneity. There was only slight evidence for any of these phenomena arising reproducibly in the characters studied. These findings suggest the value of this set of stocks for studies of the physiological basis of postponed aging. GRICLIATTI 1984). This limitation on genetic analysis arises because Drosophila spp. are subject to inbreeding depression for fitness-related characters, like longevity (CLARKE and MAYNARD SMITH 1955), making the isolation of mutant strains with postponed aging very difficult.The use of selection is feasible for two reasons. First, there is abundant quantitative genetic variability for life-historical characters in outbred stocks of D. melanogaster (ROSE and CHARLESWORTH 198 la,b). Secondly, an indirect selection procedure can be used, in which natural selection is directed to act at later ages by the use of eggs laid by older females exclusively. When applied repeatedly over a number of generations, this type of selective screen leads to the evolution of postponed aging (WATTIAUX 1968a,b; ROSE and CHARLESWORTH 198 I b; ROSE 1984; LUCK- INBILL et al. 1984). This procedure also involves pop-'
We have examined crosses between wild-type strains of Caenorhabditis elegans for heterosis effects on life span and other life history traits. Hermaphrodites of all wild strains had similar life expectancies but males of two strains had shorter life spans than hermaphrodites while males of two other strains lived longer than hermaphrodites. F1 hermaphrodite progeny showed no heterosis while some heterosis for longer life span was detected in F1 males. F1 hybrids of crosses between two widely studied wild-type strains, N2 (var. Bristol) and Berg BO (var. Bergerac), were examined for rate of development, hermaphrodite fertility, and behavior; there was no heterosis for these life history traits. Both controlled variation of temperature and uncontrolled environmental variation affected the length of life of all genotypes. Significant G x E effects on life span were observed in comparisons of N2 and Berg BO hermaphrodites, or N2 hermaphrodites and males, or N2 and a Ts mutant strain (DH26). Nevertheless, within an experiment, environmental variation was minimal and life spans were quite replicable.
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