Worm Breeder's Gazette 13(5): 13 (February 1, 1995)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Biology Department, University of Texas, Arlington, TX 76019 An NGM plate full of worms and bacteria forms a complete two-species microcosm that can be useful for studies of ecological dynamics. I was led to this project while trying to develop a suitable discrete generation protocol for artificial selection studies. Cleaning plates with a sodium±hypochlorite solution every four days (20C) produces sequential populations without having to worry about overlapping generations or dauer formation. While working out this method, however, I noticed that in some generations there would be tens of thousands of worms on a plate whereas other generations would yield only a few thousand worms. The functional explanation for this turned out to be density dependent population regulation. When there are a few worms on a plate, there is plenty of food and many eggs are produced for the next generation (the population increases). When the number of worms becomes large, however, worms begin developing more slowly, and may not be producing eggs by day four. This can lead to a crash in population size. Ordinarily, we would expect a steady state to be reached at the carrying capacity of the plate. C. elegans produces an extremely large number of offspring, however, and its net reproductive rates is large enough to theoretically generate complex patterns of population dynamics, including chaos. To test this hypothesis, I raised four replicate lineages of N2 worms for 100 generations using the serial transfer technique described above (10cm plates). The the volume of OP50 inoculum was controlled, and worm densities were estimated using serial dilutions of the worms rinsed off the plates. Although the population dynamics during this time were indeed complex (see figure--record high 78,900; carrying capacity probably around 15,000), the fact that replicates occasionally tend to cycle in concert suggests that environmental factors as well as population regulation effects are influencing population dynamics. Possible environmental influences are small variations in the time between cleanings, plate age and quality, and E. coli culture age and quality. The data still need to be analyzed using more sophisticated methods before can I determine whether a chaotic signature emerges from the environmental noise, but one message I take from this is that if it is difficult to separate these features in a tightly controlled system such as C. elegans plates, then doing it in natural populations might well be impossible.