First published online August 17, 2007
Journal of Experimental Biology 210, 3107-3116 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.007351
Thermal preference of Caenorhabditis elegans: a null model and empirical tests
Jennifer L. Anderson1,
Lori Albergotti1,*,
Stephen Proulx2,
Colin Peden1,
Raymond B. Huey3 and
Patrick C. Phillips1,
1 Center for Ecology and Evolutionary Biology, University of Oregon, Eugene,
OR 97402, USA
2 Department of Ecology, Evolution and Organismal Biology, Iowa State
University, Ames, IA 50011, USA
3 Department of Biology, University of Washington, Seattle, WA 98195,
USA
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Fig. 1. Rate of dispersal/diffusion at fixed temperatures for CB4856 measured as
the variance in individual straight-line distance travelled after 60 min (see
Fig. 2 for the actual
distributions). Increase in the rate of diffusion (D) with
temperature (T) is fairly linear over this range
(R2=0.92), as fit by the equation
D=–0.41+0.046T.
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Fig. 2. Dispersal at fixed temperatures for the CB4856 natural isolate. The filled
bars show the actual distributions of individual nematodes allowed to freely
disperse for 10 or 60 min at a given temperature (mean number of individuals
assayed per temperature=452). Individuals did not disperse after 10 min at
10°C. Open bars show the predicted distribution of individuals derived
from the diffusion simulation based on the temperature-dependent linear change
in diffusion coefficient estimated in Fig.
1. The diffusion process closely approximates the actual dispersal
distribution at both time scales.
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Fig. 3. Expected distribution of individuals from the CB4856 natural isolate across
a thermal gradient starting at 24°C, assuming that locomotion is
determined only by the temperature-dependent rate of diffusion. Results are
from the diffusion simulation parameterized by the estimated diffusion
coefficients at fixed temperatures (Fig.
1).
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Fig. 4. Comparison of the actual distribution of individuals of CB4856 on a thermal
gradient at different time points (filled bars), with the null
temperature-dependent-only diffusion prediction from the diffusion simulation
(open bars). Individuals starting at 24°C clearly move towards the colder
region of the thermal gradient at a much quicker rate than would be expected
under the null model. Sample sizes for the empirical distributions: 15 min,
N=495; 30 min, N=556; 60 min, N=1077; 8 h,
N=886.
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Fig. 5. Comparison of the actual distribution of individuals of N2 on a thermal
gradient at different time points (filled bars), with the null
temperature-dependent-only diffusion prediction from the diffusion simulation
(open bars). Individuals starting at 24°C avoid high temperatures, but
otherwise do not move. Sample sizes for the empirical distributions: 60 min,
N=432; 8 h, N=649.
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Fig. 6. Effect of start temperature on thermal preference in N2 (grey bars) and in
CB4856 (black bars) cultivated at 20°C. CB4856 individuals consistently
preferred cool temperatures, whereas N2 tended to disperse around their start
temperature. Mean thermal preferences are based on a mean of four replicates,
with 168 individuals per replicate (4038 total individuals). Values are
least-square means ± 2 s.e.m. Within each strain, treatments not
connected by the same letter are significantly different (P<0.05;
Tukey HSD).
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Fig. 7. Thermal preference in N2 (grey bars) and CB4856 (black bars) is not
influenced by food quality. Control (C): thermal gels seeded with E.
coli strain OP50 were incubated at room temperature overnight.
Ultra-violet (UV): prepared as for control then subjected to UV radiation to
kill the bacteria. Heat (H): bacteria grown in liquid culture overnight,
killed via exposure to high temperature, concentrated, and applied to
thermal gels to approximate a live lawn. Mean thermal preferences are based on
3–8 replicates with an average of 144 individuals per replicate for a
total of 3446 observations. Values are least square means ± 2 s.e.m.
Treatments not sharing the same letter are significantly different
(P<0.05; Tukey HSD).
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Fig. 8. Neither N2 (grey lines) nor CB4856 (black lines) migrates to their
cultivation temperature regardless of food availability. Because starved worms
rapidly migrate off unseeded thermal preference gels, the duration of these
assays was reduced to 1 h, resulting in the observation of slightly higher
estimates of mean thermal preference (see text). Worms raised for two
generations at 14°C, 20°C or 24°C were assayed for thermal
preference with OP50 present on the thermal gel (solid line) or without food
(broken line). Mean thermal preferences are based on 3–5 replicates with
a mean of 181 individuals per replicate for a total of 7059 individuals.
Values are least square means ± 2 s.e.m.
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© The Company of Biologists Ltd 2007
