Are C. elegans and D. melanogaster
valid animal models for studies on aging?
Independent researcher, Naples, Italy
C. elegans and D. melanogaster are common animal models for studies on aging, but there are strong arguments against the validity of these models for this type of studies:
I) Many bird and mammal species - our species included - show an increment of mortality with increasing chronological age in natural conditions. This phenomenon ("A" phenomenon) is well documented and, being existent in the wild, is influenced by natural selection. On the contrary, animals as C. elegans and D. melanogaster show in natural conditions a constant mortality rate but, in artificial protected conditions, they display an age-related mortality increment starting from ages not existing in the wild. In fact, in natural conditions: 1) the longevity of C. elegans is reduced up to 10 fold compared with standard laboratory culture conditions and few individuals of this species remain fertile in the wild after 10 days; 2) D. melanogaster has a reported adult life span in the wild of 10-12 days. Therefore, the mortality increment for these two species ("B" phenomenon), being a laboratory artefact, cannot be influenced by selection. "A" and "B" phenomena are radically different in their possible evolutionary determinants and so the results of experiments on "B" phenomenon are not automatically applicable to "A" phenomenon.
II) C. elegans and D. melanogaster (and in general the adult insects) are composed by cells with no turnover, while birds and mammals have cells and tissues with turnover. If, as it seems likely, the slowdown and later the stopping of cell turnover, and the correlated cell senescence, are pivotal elements in the age-related fitness decline of birds and mammals, it is rather doubtful to use experiments on animals with no cell turnover to explain the fitness decline in animals with cell turnover.
III) Animals as C. elegans and D. melanogaster have life cycles thoroughly different from those of bird and mammal species. Studies on aging that use these animal models implicitly assume that their adult stages are equivalent to the postnatal stages of birds and mammals for the extension of their results to these species. But this assumption is not proved and seems quite doubtful.
The appropriateness of C. elegans and D. melanogaster as animal models for aging is a problem that cannot be neglected in aging studies. Unfortunately, in renowned texts and very influential journals, the issue is not considered and it is frequent that experiments modifying – in laboratory conditions and at ages non-existent in the wild - the modifications of C. elegans and D. melanogaster life tables are presented as meaningful advances in the understanding of human aging!
Homo Sapiens Liberatus Workshop, Moscow State University, May 2010