Thermotolerance evolution of poikilothermic organisms consists in adaptation to temperature conditions of habitats of such physiological features as the lower and upper limits of temperature for reproduction, development and behavior and organism's resistance to short-term temperature extremes.
Previous researches of Caenorhabditis revealed both interspecific and intraspecific significant differences for the upper limits of temperature for reproduction and development. These differences are in accordance with temperature conditions of habitats (Fatt and Dougherty, 1963; Fodor et al., 1983). Nevertheless interspecific differences in tolerance to temperature extremes are still uninvestigated.
Therefore we compared resistance to high temperature extremes of C. elegans (strain N2) and C. briggsae (strain AF16). Experiments were carried out with young adults grown in Petri dishes with NGM and E.coli OP50 at 23°C. Resistance to constant temperature 36°C or 37°C was measured for worms incubated individually in 1 ml of liquid medium (NGM without agar, peptone, cholesterol).
Results of our experiments show that organism's resistance to extreme high temperature is significantly higher in C. briggsae than in C. elegans:
- Mean time of reversible disturbances of swimming induced by intensive mechanic stimulus (shaking of test tube) was similar at 36°C in C. elegans and at 37°C in C. briggsae.
- Mean time of heat death was rather more in C. briggsae at 37°C than in C. elegans at 36°C.
Rapid adaptations of worms to temperature elevation by 2 hour incubation at 30°C or by heat hardening (1 hour at 33°C followed by 1 hour at 23°C) increased behavior resistance to short-term heat stress (36°C for C. elegans and 37°C for C. briggsae). Efficiency of both adaptations was similar in C. elegans and C. briggsae.
Therefore we conclude that adaptation of C. elegans and C. briggsae to different temperature conditions of habitats changed only base resistance to extreme high temperature.
It is known that the upper temperature limit for reproduction of C. elegans N2 is 26.5°C (Fatt and Dougherty, 1963), while C. briggsae AF16 can grow at 27.5°C (Gupta et al., 2007). Therefore our data indicate that evolution of Caenorhabditis thermotolerance involves unidirectional adaptive changes of the upper limits of temperature for reproduction and resistance to short-term heat stress. This pattern reveals in the evolution of most poikilotherms.
Thus soil nematodes Caenorhabditis may be used as convenient model organisms for study thermotolerance evolution.
References
Fatt HV, Dougherty EC. (1963). Genetic control of differential heat tolerance in two strains of the nematode, Caenorhabditis elegans. Science. 141, 266-267. 
Fodor A, Riddle DL, Nelson FK, Golden JW. (1983). Comparison of a new wild-type Caenorhabditis briggsae with laboratory strains of C. briggsae and C. elegans. Nematologica 29, 203–217. Abstract
Gupta BP, Johnsen R, Chen N. (2007). Genomics and biology of the nematode Caenorhabditis briggsae, Wormbook, ed. The C. elegans Research Community, Wormbook, doi/10.1895/wormbook.1.136.1, http//www.wormbook.org.
