Worm Breeder's Gazette 15(4): 29 (October 1, 1998)
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.
| 1 | Laboratory of Molecular Neurobiology, Division of Biological Science, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan |
| 2 | Laboratory of Molecular Neurobiology, Division of Biological Science, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan, PRESTO, JST |
Thermotaxis is an important behavior that C. elegans can use to
ensure its own survival and reproduction in the natural environment. In
the laboratory, thermotaxis is analyzed by observing the animal movement
on a temperature gradient. After cultivation with food for several
hours, C. elegans first migrates toward its cultivation temperature and
then moves isothermally on a temperature gradient. Hedgecock and Russell
(1975) reported that this "positive" thermotaxis turns into "negative"
thermotaxis after cultivation under conditions that involve starvation
or overcrowding (1). In order to understand how C. elegans associates
memorized cultivation temperatures with environmental conditions, we
began to investigate what triggers this switch from positive to negative
thermotaxis.
Negative thermotaxis could be induced by starvation,
overcrowding or both. We demonstrated that starvation alone can induce
negative thermotaxis. For example, adult animals that had been grown
with food at 25-degree for 1-3 days (fed animals) were starved at
25-degree for 4 hours on agar plate under uncrowded condition (starved
animals). Then, both fed and starved animals were individually assayed
for thermotaxis on 9cm plate with a radial temperature gradient, and the
fraction of animals that migrated to and stayed around 25-degree were
scored. Although 60% of fed animals migrated to 25-degree (0.60 +/-
0.06, N=228), only 25% of starved animals migrated to 25-degree (0.25
+/- 0.07, N=174) and the rest (75%) of starved animals showed several
migration patterns indicative of negative thermotaxis. Studies on
pumping and egg laying behaviors showed that exogenous serotonin and
octopamine mimic well-fed and starved state, respectively (2). Our
preliminary results suggest that regulation of serotonin and octopamine
levels is also important for the reversal of thermotactic responses.
When the animals had been cultivated at 25-degree for 4 hours on
unseeded plates containing serotonin and were individually assayed for
thermotaxis, 60-70% of animals, depending on the experiments, migrated
to 25-degree. By contrast, when the animals had been cultivated at
25-degree for 4 hours on seeded plates containing octopamine, 10-40% of
animals migrated to 25-degree.
We are attempting to isolate mutants that are only defective in
response to starvation. We so far screened about 40,000 mutagenized
genomes for animals that are normal in positive thermotaxis and also
migrate to cultivation temperatures regardless of starvation experience.
Several candidates were isolated, although they need more
characterization behaviorally and pharmacologically at this stage.
Through the analysis of these mutants, we are hoping to identify genes
and cells required for associating temperature memory with internal
food-deprived state. With this study, we also hope to dissect
neuroendocrine system at molecular and cellular levels.
We are grateful to Yasumi Ohshima of Kyushu University, where
this work at its initial stage was done.
(1) Hedgecock and Russell, 1975, PNAS 72, 4061-4065.
(2) Horvitz et al., 1982, Science 216, 1-12-1014.