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COMMENTARY ON A RECENT PUBLICATION |
1 National Institute for Public Health and the Environment, Bilthoven, The Netherlands. 2 Department of Environmental Science, Radboud University, Nijmegen, The Netherlands.
Address correspondence to Dr. Christian Mulder, Laboratory for Ecological Risk Assessment, National Institute for Public Health and the Environment, P. O. Box 1, NL-3720BA Bilthoven, The Netherlands. E-mail: christian.mulder{at}rivm.nl
CHEN and colleagues (1) discussed fitness and aging relationships, considering senescence evolution in laboratory lines of nematodes. They concluded that fitness costs for Caenorhabditis elegans are consistent with antagonistic pleiotropy. Fitness and demography might be dominated by indirect genetic effects in other organisms too. We provide an additional explanation for senescence evolution by allometric mass-specific costs, as shown by long-term dynamics of reproduction, growth, and mortality of Eisenia andrei age cohorts. Empirical results collected during a 9-year experiment are provided and discussed. In this context we introduced mass-derived Gompertzian Metabolic Rates (GMR), which are benchmarks in comparative physiology.
Changes in the culture conditions affect the progeny of invertebrates, as shown by selecting populations under modeled situations in the laboratory (2), but in contrast to nematodes (3,4), concrete estimates of longevity for most invertebrates are lacking. Recently, Chen and colleagues (1) rejuvenated the discussion on fitness estimates of worm-like organisms. Our long-term study on the senescence (increase of mortality rate with age) of the earthworm Eisenia andrei supports other conclusions on mortality rates of invertebrates born and kept in captivity. The median longevity of E. andrei in the artificial OECD (Organisation for Economic Co-operation and Development, www.oecd.org) soil is 63 months, the median survivorship 240 weeks. The entire life span at 18°C of the oldest specimen was 8.73 years. E. andrei showed an individual average cocoon production of 0.8–2.0 cocoons per week. As in a study on the closely related Eisenia foetida (5), these cocoons were pale with a mucousy, jelly-like consistency at birth, although the cocoons of E. andrei took on a much more brownish hue within some hours. For large age cohorts, the mortality rate in laboratory lines from big cocoons reaches a maximum of 29.8% after 42.5 months, and to this age survived 65.0% of the Eisenia adults who survived their birth (Table 1 and Figure 1). Shortly after this event, the decaying oscillations in body mass highly increased (
2 = 101, p <.0001). Therefore, such an unexpected mortality peak can be seen as a bifurcation in our data, where the earthworm population apparently failed to meet some conditions required for their growth. Moreover, the period between weeks 170 and 173 might reflect the threshold between young and old ages previously described (9). Under standard conditions like those of our experiment, body-mass values of animals ranging in size from protists to mammals reflect their metabolism, fecundity, and life span (7). As all data were normalized to constant laboratory temperature, a metabolic rate as function of body mass depicts the maintaining energy costs for keeping the Eisenia body active, thus fit. [Our mass-specific definition of fitness differs in this aspect from that used by Chen and colleagues (1), as they have chosen an integrative stage-specific definition.] These allometric implications clearly show that oscillations in the body-mass values are primary determinants for entire populations. Such a finding is reasonable as this increasing lack of fit strictly reflects the late weeks of Eisenia's life, during which the aging process reaches a finite plateau. Only long-term experimental designs allow evaluation of possible explanations which do not involve a causative role between body and productivity.
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SUPPLEMENTARY DATASET: EISENIA ANDREI
The Supplementary Dataset is linked to the online article in the December issue at http://biomed.gerontologyjournals.org/content/vol62/issue12/.
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The authors wish it to be known that, in their opinion, the last two authors should be regarded as joint last authors.
References
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