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.