Sensorimotor algorithm directing larval chemotaxis
(Alex Gomez-Marin and Aljoscha Schulze)
Bacteria and nematodes chemotax according to indirect orientation mechanisms, which consists in improved biased random walks. In contrast, Drosophila larvae employ a direct orientation mechanism where motion is locally aligned with the odour gradient. Previously, we have demonstrated that Drosophila larvae do not require bilateral olfactory inputs to perform chemotaxis. Having ruled out a mechanism purely based on stereo sampling (comparisons between the left and the right inputs), we are now investigating how larvae with unilateral olfactory function are capable of extracting directional information from spatially distributed odour cues.
We have dissected the sensorimotor algorithm governing larval chemotaxis. Combining high-resolution quantification of olfactory input and behavioural output, we have shown that larval chemotaxis is an active sampling process analogous to sniffing in vertebrates. Larvae orient in odour gradients through a sequential organization of behavioural modes: runs, stops, lateral head sweeps (or head casts) and directed turns. We have found that stereotypical patterns in the history of stimulus control two classes of decisions: when to turn or where to turn. Positive gradients are detected through high-amplitude head casts prior to a turn. Computer-vision analyses permitted us to generate large datasets for individual larvae engaged in chemotactic tasks.
By genetically reengineering the peripheral olfactory circuit, we have examined how orientation adapts to losses and gains of function in the olfactory circuit. Our findings suggest that larval chemotaxis represents an intermediate navigation strategy between the biased random walks of E. coli and C. elegans, and the stereo-olfaction observed in vertebrates such as rats and humans.
To test our current model about the active sampling and decision-making processes controlling navigation in odour gradients, we use optogenetics to interfere with the sensory experience of freely moving larvae. We make use of light-activated ion channels such as channelrhodopsin-2 (ChR2). Ectopic expression of ChR2 in single OSNs provides a powerful technique to control olfactory input at a single spike level. We have developed a system capable of measuring neuronal activity in single OSNs in response to controllable olfactory or light-evoked stimuli. By combining these technical advances, we have undertaken a quantitative characterization of synthetic olfactory stimuli and we are now in a position to reverse engineer naturalistic dynamic odour stimuli.