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Open Access Highly Accessed Research article

Experimental evolution of aging in a bacterium

Martin Ackermann12*, Alexandra Schauerte2, Stephen C Stearns3 and Urs Jenal2

Author Affiliations

1 Institute of Integrative Biology, ETH Zürich, CH-8092 Zürich, Switzerland

2 Division of Molecular Microbiology, Biozentrum, University of Basel, CH-4056 Basel, Switzerland

3 Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06520-8106, USA

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BMC Evolutionary Biology 2007, 7:126  doi:10.1186/1471-2148-7-126

Published: 28 July 2007

Abstract

Background

Aging refers to a decline in reproduction and survival with increasing age. According to evolutionary theory, aging evolves because selection late in life is weak and mutations exist whose deleterious effects manifest only late in life. Whether the assumptions behind this theory are fulfilled in all organisms, and whether all organisms age, has not been clear. We tested the generality of this theory by experimental evolution with Caulobacter crescentus, a bacterium whose asymmetric division allows mother and daughter to be distinguished.

Results

We evolved three populations for 2000 generations in the laboratory under conditions where selection was strong early in life, but very weak later in life. All populations evolved faster growth rates, mostly by decreasing the age at first division. Evolutionary changes in aging were inconsistent. The predominant response was the unexpected evolution of slower aging, revealing the limits of theoretical predictions if mutations have unanticipated phenotypic effects. However, we also observed the spread of a mutation causing earlier aging of mothers whose negative effect was reset in the daughters.

Conclusion

Our results confirm that late-acting deleterious mutations do occur in bacteria and that they can invade populations when selection late in life is weak. They suggest that very few organisms – perhaps none- can avoid the accumulation of such mutations over evolutionary time, and thus that aging is probably a fundamental property of all cellular organisms.