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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

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Thermal preference of Caenorhabditis elegans: a null model and empirical tests

Jennifer L. Anderson¹, Lori Albergotti^1,*, Stephen Proulx², Colin Peden¹, Raymond B. Huey³ and Patrick C. Phillips^1,

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

View larger version (3K): [in this window] [in a new window]   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.

View larger version (15K): [in this window] [in a new window]   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.

View larger version (10K): [in this window] [in a new window]   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).

View larger version (19K): [in this window] [in a new window]   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.

View larger version (7K): [in this window] [in a new window]   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.

View larger version (6K): [in this window] [in a new window]   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).

View larger version (6K): [in this window] [in a new window]   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).

View larger version (5K): [in this window] [in a new window]   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.