FIGURES FOR : TEMPERATURE-DEPENDENT BEHAVIOURS ARE GENETICALLY VARIABLE IN THE NEMATODE CAENORABDITIS BRIGGSAE

INTRODUCTION Temperature-dependent behaviours in the model organism Caenorhabditis elegans represent one of the best-understood behavioural neural circuits in biology, with much known about the essential neuronal and molecular components (Mori and Ohshima, 1995; Cassata et al., 2000; Samuel et al., 2003; Colosimo et al., 2004; Satterlee et al., 2004; Inada et al., 2006; Yoshinori Tanizawa et al., 2006; Ohnishi et al., 2011). These behaviours are complex and ecologically important and, when combined with comparative studies within and between species, will help determine the selective pressures and changes that act on ecologically relevant traits over the course of evolution (Bendesky and Bargmann, 2011). Variability in temperature is a universal environmental factor in biology that affects organisms through a wide-range of temperature-dependent mechanisms, from defining biomolecular reaction rates to developmental timing and geographic range distributions. For ectotherms, temperature-dependent behaviours provide an essential means of regulating body temperature within an ideal range for growth and reproduction, especially for smaller animals with little thermal inertia, such as rhabditid nematodes. These behaviours are complex in that they integrate many neurologic components including temperature sensation, locomotion, memory, foraging and decision-making. Caenorhabditis elegans has been used extensively to study the biological circuitry involved in temperature-dependent behaviours, starting with the classic work by Hedgecock and Russell (Hedgecock and Russell, 1975). However, most thermotaxis research in Caenorhabditis has focused on only a single species, C. elegans, and primarily a single genetic background within that species, the laboratory-adapted N2 strain. Here, we expand the scope of this system by explicitly quantifying temperature-dependent behaviours in two genetic backgrounds of C. elegans and 15 wild genetic backgrounds of Caenorhabditis briggsae. When removed from their food source and placed on a thermal gradient, C. elegans forage using two key behavioural patterns that depend on their prior temperature history: thermotaxis and isothermal tracking (Hedgecock and Russell, 1975). On a thermal gradient, C. elegans move towards their cultivation temperature via a biased random-walk strategy, a behaviour known as thermotaxis (Hedgecock and Russell, 1975; Mori and Ohshima, 1995; Ryu and Samuel, 2002; Zariwala et al., 2003). Caenorhabditis elegans worms are amenable to cryophilic thermotaxis under most experimental conditions, whereas thermophilic thermotaxis depends on the steepness of the gradient and the relative starting position of worms on the gradient (Ramot et al., 2008; Jurado et al., 2010). When close to the preferred temperature on a thermal gradient, individual C. elegans animals are capable of long runs with minor heading adjustments to follow isotherms, a behaviour known as isothermal tracking (Mori and Ohshima, 1995; Ryu and Samuel, 2002; Luo et al., 2006). Temperature plays a crucial role in the ecology and evolution of C. briggsae. The fitness of different wild genetic backgrounds of C. briggsae is affected differentially by rearing temperature in accord SUMMARY Temperature-dependent behaviours in Caenorhabditis elegans, such as thermotaxis and isothermal tracking, are complex behavioural responses that integrate sensation, foraging and learning, and have driven investigations to discover many essential genetic and neural pathways. The ease of manipulation of the Caenorhabditis model system also has encouraged its application to comparative analyses of phenotypic evolution, particularly contrasts of the classic model C. elegans with C. briggsae. And yet few studies have investigated natural genetic variation in behaviour in any nematode. Here we measure thermotaxis and isothermal tracking behaviour in genetically distinct strains of C. briggsae, further motivated by the latitudinal differentiation in C. briggsae that is associated with temperature-dependent fitness differences in this species. We demonstrate that C. briggsae performs thermotaxis and isothermal tracking largely similar to that of C. elegans, with a tendency to prefer its rearing temperature. Comparisons of these behaviours among strains reveal substantial heritable natural variation within each species that corresponds to three general patterns of behavioural response. However, intraspecific genetic differences in thermal behaviour often exceed interspecific differences. These patterns of temperature-dependent behaviour motivate further development of C. briggsae as a model system for dissecting the genetic underpinnings of complex behavioural traits.