How circuits self-assemble starting from neuronal stem cells is a fundamental question in developmental neurobiology. Here, we addressed how neurons from different stem cell lineages wire with each other to form a specific circuit motif. In Drosophila larvae, we combined developmental genetics (twin-spot mosaic analysis with a repressible cell marker, multi-color flip out, permanent labeling) with circuit analysis (calcium imaging, connectomics, network science). For many lineages, neuronal progeny are organized into subunits called temporal cohorts. Temporal cohorts are subsets of neurons born within a tight time window that have shared circuit-level function. We find sharp transitions in patterns of input connectivity at temporal cohort boundaries. In addition, we identify a feed-forward circuit that encodes the onset of vibration stimuli. This feed-forward circuit is assembled by preferential connectivity between temporal cohorts from different lineages. Connectivity does not follow the often-cited early-to-early, late-to-late model. Instead, the circuit is formed by sequential addition of temporal cohorts from different lineages, with circuit output neurons born before circuit input neurons. Further, we generate new tools for the fly community. Our data raise the possibility that sequential addition of neurons (with outputs oldest and inputs youngest) could be one fundamental strategy for assembling feed-forward circuits.
Keywords: D. melanogaster; connectomics; developmental biology; lineage; neuroblast; neuroscience; somatosensation.
The nervous system of an animal consists of complex arrangements of nerve cells or neurons. These arrangements are called neuronal circuits, and they contain both input and output neurons. Input neurons sense signals, such as external cues, and output neurons pass these signals on to the brain, for example. The nerve cells in a circuit connect to each other through so-called synapses in specific patterns. Neuronal circuits first assemble during the development of an animal. The assembly process starts when a nerve stem cell divides and gives rise to more specialized neurons, its progeny. A lot of what we know about neuronal circuit assembly comes from studying the nerve cord of fruit fly larva, which shares many features with the spinal cord of vertebrates. Previous studies had used experimental techniques to trace, or follow, the fate of the progeny of specific nerve stem cells. These approaches provided information about which nerve stem cells contribute to which neuronal circuits. However, major questions in developmental neurobiology remain about how exactly these neuronal circuits assemble. For example, it was not clear in what order input and output neurons build a circuit. Here Wang, Wreden et al. took a different approach by starting with a specific circuit in the fruit fly nerve cord – a circuit that detects vibrations – and looking for the stem cells contributing to that circuit. Using a number of techniques, Wang, Wreden et al. determined when particular nerve cells were ‘born’, what they looked like, and what other nerve cells they formed synapses with. Although nerve stem cells gave rise to many different neurons during development, those neurons did not change gradually over time. Instead, neurons were born in short bursts, and those in the same ‘temporal cohort’ were similar to each other, while neurons in different cohorts were different. The neuronal circuit that detects vibrations assembled itself from three temporal cohorts of neurons coming from different stem cells. The output neurons, which send information from the nerve cord to the brain, were born before the input neurons, which detect vibrations in the surroundings. All in all, these experiments offer more detailed insights into how neuronal circuits assemble during development. The study also provides experimental resources for other scientists working with fruit flies, and poses new research questions for developmental biologists studying vertebrates.
© 2022, Wang, Wreden et al.