20Animal behaviors are robust and flexible. To elucidate how these two conflicting 21 features of behavior are encoded in the nervous system, we analyzed the neural circuits 22 generating a C. elegans thermotaxis behavior, in which animals migrate toward the past 23 cultivation temperature (T c ). We identified multiple circuits that are highly overlapping 24 but individually regulate distinct behavioral components to achieve thermotaxis. When 25 the regulation of a behavioral component is disrupted following single cell ablations, the 26 other components compensate the deficit, enabling the animals to robustly migrate 27 toward the T c . Depending on whether the environmental temperature surrounding the 28 animals is above or below the T c , different circuits regulate the same behavioral 29 components, mediating the flexible switch between migration up or down toward the T c .
30These context-dependencies within the overlapping sub-circuits reveal the 31 implementation of degeneracy in the nervous system, providing a circuit-level basis for 32 the robustness and flexibility of behavior.
105C. elegans animals are known to navigate using a series of stereotyped movements, 106 designated behavioral components (Croll, 1975; Iino and Yoshida, 2009; 107 Pierce- Shimomura et al., 1999). We first attempted to extract the behavioral components 108 during thermotaxis by employing a Multi-Worm Tracker (MWT) (Swierczek et al., 109 2011). MWT simultaneously captured the positions and postures of approximately 120 110 animals ( Figure 1A), and these data were further analyzed by a custom-built MATLAB 111 script to detect the behavioral components (see Materials and methods).
112For the thermotaxis assays, we set cultivation temperature (T c ) as 20°C and the 113 temperature of the center in the assay plate as either 17°C or 23°C ( Figure 1B). Animals 114 were placed at the center of the plate, and then we evaluated the animals' migrations by 115 calculating thermotaxis index (TTX index), according to the equation shown in Figure 116 1C. TTX index is 1 when all the animals are in the coldest fraction of the plate and 8 117 when all the animals are in the warmest fraction. Consistent with our previous report 118 (Ito et al., 2006), the animals reached their T c within approximately 30 minutes in two 119 conditions ( Figure 1D and Movie S1), plate centered at 17°C or 23°C. In this study, we 120 thus focused on the first 30 minutes from the start of the assays. To analyze the 121 behaviors during the migrations toward the T c , we analyzed the animals that were 122 distributed in the center four fractions of the assay plate; 15.5-18.5°C for the TT c condition (Figure 2A). Behaviors were 124 essentially classified into three behavioral components: turns, reversals, and curves 125 ( Figure 2B). Turns were further classified into omega turns and shallow turns (Kim et 126 al., 2011; Schild and Glauser, 2013), and reversals were further classified into reversals 127 and reve...