The nematode C. elegans displays complex dynamical behaviors that are commonly used to identify relevant phenotypes. Although its maintenance is straightforward, sorting large populations of worms when looking for a behavioral phenotype is difficult, time consuming and hardly quantitative when done manually. Interestingly, when submitted to a moderate electric field, worms move steadily along straight trajectories. Here, we report an inexpensive method to measure worms crawling velocities and sort them within a few minutes by taking advantage of their electrotactic skills. This method allows to quantitatively measure the effect of mutations and aging on worm's crawling velocity. We also show that worms with different locomotory phenotypes can be spatially sorted, fast worms traveling away from slow ones. Group of nematodes with comparable locomotory fitness could then be isolated for further analysis. C. elegans is a growing model for neurodegenerative diseases and using electrotaxis for self-sorting can improve the high-throughput search of therapeutic bio-molecules.

It has been postulated that the presence of parasites causing high extrinsic mortality may trigger an inducible acceleration of the host aging. We tested this hypothesis using isogenic populations of Caenorhabditis elegans nematodes and different Escherichia coli strains. When exposed to pathogenic bacteria, nematodes showed up to fourfold higher mortality rates, reproduced earlier, produced more H(2)O(2), and accumulated more autofluorescence, than when exposed to an innocuous strain. We also observed that mortality increased at a slower rate in old animals, a phenomenon known as mortality deceleration. Mortality deceleration started earlier in populations dying faster, likely as a consequence of lifelong heterogeneity between individual tendencies to die. Taken together, our results strongly suggest that the high extrinsic mortality imposed by the pathogens results in the modulation of nematodes' life-history traits, including aging and reproduction. This could be an adaptive response aiming at the maximization of Darwinian fitness.