DECISION MAKING IN DROSOPHILA LARVAE: AN ANALOGOUS SYSTEM TO VERTEBRATES
Last August, researchers from the Systems Biology programme of the Centre for Genomic Regulation (CRG) have published a study on the processes used by Drosophila melanogaster larvae to track odours and spatially orient themselves in response to olfactory signals. The findings show that larvae of the vinegar fly, unlike other organisms with simpler nervous systems, use an odour tracking strategy similar to vertebrates. This discovery represents a crucial advance as Drosophila is one of the most widely-used genetic model organisms in neuroscience.
The study was conducted by researchers Alex Gomez-Marin (CRG), Greg J. Stephens (Princeton University) and directed by Matthieu Louis, leader of the Sensory Systems and Behaviour research group at the CRG. The Louis lab studies how odorant stimuli are encoded and processed by the olfactory system. This work represents a starting point from which to decipher the neural computation underlying the processing of complex sensory signals. The long-term goal of the Louis lab is to identify the circuits integrating olfactory information and directing orientation decisions, which allow larvae to navigate towards the source of food.
Chemotaxis is a process by which cells and microorganisms direct their movements according to "chemical trails" present in the environment. This is essential for survival (for example, to locate nutrient sources or escape from danger). Chemotaxis also plays an important role during the early and late stages of development (for example, in the migration of axons or the motility of sperm).
Drosophila larvae navigate by alternating straight runs with turns biased towards the direction of higher stimulus concentrations. Researchers have discovered that larval chemotaxis relies on a form of active sampling similar to that in vertebrates. To orient each of their turns, larvae perform side-to-side head movements - head casts - to perceive differences in the odour concentrations in their surroundings. Each of these head casts provides a snapshot of the olfactory world, similar to a “sniff” in dogs and other mammals. This finding may be a new source of inspiration in the design of bio-inspired robots searching for explosives or chemical pollutants.
Simpler organisms, such as the bacterium E. coli, also widely-used as a model system, chemotax based on a biased random walk where the orientation of individual turns is chosen haphazardly. Animals with more complex nervous systems, like honey bees, rats and humans, make use of comparisons between the inputs of bilaterally symmetric olfactory organs (stereo-olfaction): they can identify differences in concentration gradients between their left and right noses. They are thus capable of directing their course on the spot without having to sequentially sample their olfactory environment. "The case of Drosophila larva can be seen as an intermediate navigational strategy, since active sampling through head casts is analogous to sniffing in vertebrates," says Alex Gomez-Marín. Whether larvae utilize stereo-olfaction or not remains unanswered.
About Systems Biology
This line of research focuses on the function and interaction of different components of a biological system (e.g., nervous systems, cells and bacteria). The computational principles extracted from the relatively simple nervous system of the larva are then replicated using computer models to make behavioural predictions for more complex systems.
Reference work: Gomez-Marin, A. et al. “Active sampling and decision making in Drosophila chemotaxis.” Nat. Commun, 2:441 doi:10.1038/ncomms1455 (2011). Link
For more information: Juan Manuel Sarasua, Press Office, Communication and Public Relations Dept., Centre for Genomic Regulation. Tel 93 316 02 37. E-mail: juan.sarasua@crg.eu