The first map of all olfactory, thermosensory and hygrosensory projection neurons in the fly brain
The vinegar fly, Drosophila, is a powerful model system for genetic studies of a wide range of biological systems, including the brain. The fly brain contains approximately 100,000 neurons making an estimated 100 million connections. Producing maps of how these neurons connect with each other allows scientists to build and test theories to explain how complex behaviours can arise from electrochemical signalling across neural circuits. In two publications, Greg Jefferis’ group, in collaboration with scientists at the University of Cambridge, Janelia Research Campus, and Brandeis University, have presented the first full map of all olfactory, thermosensory, and humidity-sensing projection neurons in the fly brain.
Flies navigate the world using neurons in their antennae to sense odours, temperature, and humidity. Temperature affects all aspects of biology and avoidance of extreme temperatures is critical. Humidity detection is particularly important in insects because their large surface area to volume ratio means that they can easily become dehydrated and perish. Similarly, detection of odours can help flies find food, mates, and appropriate egg-laying sites, some smells being attractive, others repellent.
What is connectomics?
A map shows you what exists in an area and how you can get from one place to another. Connectomics is the study of brain maps identifying every neuron in an organism’s nervous system and how they connect with each other.
Mapping a brain is done similarly to manually drawing a road map from satellite images, except in three dimensions using high resolution electron microscopy images of a whole fly brain. This is a time-consuming process, with reconstruction of any individual neuron taking many hours.
Two members of Greg’s group, Alexander Bates and Philipp Schlegel, and Elizabeth Marin at the University of Cambridge led the team effort to build the most comprehensive possible maps of the neuron circuits responsible for sensing temperature, humidity, and odour. In doing so, they observed how different neurons can be wired together in different ways. For example, olfactory neurons known to respond to different food odours formed separate sub-groups and olfactory neurons interacted with other neurons thought to respond to wind-like motion, perhaps because smell and wind direction often need to be quickly co-computed.
This kind of integration across different senses – an important area of basic research in neuroscience – was even seen for the combination of odour, temperature, and humidity information. Such integration could be required because the temperature and humidity of the air affect the volatility of odours and the numbers of molecules that reach the sensory neurons, or it could be that the same neurons mediate avoidance of both repellent odours and dangerous levels of temperature or humidity.
Another interesting finding was the identification of a connection between thermosensory neurons and the part of the fly brain that controls its circadian rhythms, showing how temperature might contribute to keeping the body clock in line with the 24-hour day.
These papers represent foundational work for future studies of temperature, humidity and odour sensing, and for building models of how this circuitry functions to control behaviour. Understanding how computation works in this system will likely reveal connectivity motifs used in larger, less experimentally accessible brains, including our own.
Together, such studies showcase the very rapid progress in connectomics, which aims to map the connections of all of the neurons within a brain. Group Leaders in the LMB’s Neurobiology Division including Albert Cardona, Marta Zlatic and Greg Jefferis have major roles in collaborative efforts to obtain connectomes of larval and adult Drosophila. These represent some of the biggest steps forward in efforts to map whole brains since the first connectome, of the nematode worm C. elegans, was obtained by Sydney Brenner, John White and colleagues working at the LMB in the 1970s and 80s.
The work was funded by UKRI MRC, Wellcome Trust, ERC, Boehringer Ingelheim Fonds, a Herchel Smith Studentship, China Scholarship Council, National Institute of General Medical Sciences, National Institute of Allergy and Infectious Diseases, National Science Foundation, and a Cambridge Neuroscience-PSL collaborative grant supported by the Embassy of France in London.
Complete connectomic reconstruction of olfactory projection neurons in the fly brain. Bates, AS., Schlegel, P., Roberts, RJV., Drummond, N., Tamimi, IFM., Turnbull, R., Zhao, X., Marin, EC., Popovivi, PD., Dhawan, S., Jamasb, A., Javier, A., Serratosa Capdevila, L., Li, F., Rubin, GM., Waddell, S., Bock, DD., Costa, M., Jefferis, GSXE. Current Biology 30: 1-17
Connectomics analysis reveals first, second, and third order thermosensory and hygrosensory neurons in the adult Drosophila brain. Marin, EC., Büld, L., Theiss, M., Sarkissian, T., Roberts, RJV., Turnbull, R., Tamimi, IFM., Pleijzier, MW., Laursen, WJ., Drummond, N., Schlegel, P., Bates, AS., Li, F., Landgraf, M., Costa, M., Bock, DB., Garrity, PA., Jefferis, GSXE. Current Biology 30: 1-16
Greg’s group page
Drosophila Connectomics group page
Paul Garrity’s group page