[
Worm Breeder's Gazette,
1995]
An NGM plate full of worms and bacteria forms a complete two-species microcosm that can be useful for studies of ecological dynamics. I wasled to this project while trying to develop a suitable discrete generationprotocol for artificial selection studies. Cleaning plates with asodiumhypochlorite solution every four days (20C) produces sequentialpopulations without having to worry about overlapping generations or dauerformation. While working out this method, however, I noticed that in somegenerations there would be tens of thousands of worms on a plate whereasother generations would yield only a few thousand worms. The functionalexplanation for this turned out to be density dependent populationregulation. When there are a few worms on a plate, there is plenty offood and many eggs are produced for the next generation (the populationincreases). When the number of worms becomes large, however, worms begindeveloping more slowly, and may not be producing eggs by day four. This can lead to a crash in population size. Ordinarily, we would expect asteady state to be reached at the carrying capacity of the plate. C.elegans produces an extremely large number of offspring, however, and itsnet reproductive rates is large enough to theoretically generate complexpatterns of population dynamics, including chaos. To test this hypothesis, I raised four replicate lineages of N2worms for 100 generations using the serial transfer technique describedabove (10cm plates). The the volume of OP50 inoculum was controlled,and worm densities were estimated using serial dilutions of the wormsrinsed off the plates. Although the population dynamics during this timewere indeed complex (see figure--record high 78,900; carrying capacityprobably around 15,000), the fact that replicates occasionally tend tocycle in concert suggests that environmental factors as well as populationregulation effects are influencing population dynamics. Possibleenvironmental influences are small variations in the time betweencleanings, 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 suchas C. elegans plates, then doing it in natural populations might well beimpossible.