In order to represent complex stimuli, principle neurons of associative learning regions receive combinatorial sensory inputs. Density of combinatorial innervation is theorized to determine the number of distinct stimuli that can be represented and distinguished from one another, with sparse innervation thought to optimize the complexity of representations in networks of limited size. How the convergence of combinatorial inputs to principle neurons of associative brain regions is established during development is unknown. Here, we explore the developmental patterning of sparse olfactory inputs to Kenyon cells of the Drosophila melanogaster mushroom body. By manipulating the ratio between pre- and post-synaptic cells, we find that postsynaptic Kenyon cells set convergence ratio: Kenyon cells produce fixed distributions of dendritic claws while presynaptic processes are plastic. Moreover, we show that sparse odor responses are preserved in mushroom bodies with reduced cellular repertoires, suggesting that developmental specification of convergence ratio allows functional robustness.
In order to represent complex stimuli, principle neurons of associative learning re-16 gions receive combinatorial sensory inputs. Developmental wiring programs can produce inner-17 vation densities that vary by several orders of magnitude, such that cortical pyramidal cells re-18 ceive on the order of 1000 inputs, while principle neurons of cerebellum-like structures receive 19 <10 inputs. These innervation patterns are theorized to determine the number of distinct stimuli 20 that can be represented and distinguished from one another in sensory processing regions, with 21 sparse, combinatorial (or "distributed") innervation thought to optimize the complexity of repre-22 sentations in networks of limited size. How the convergence of combinatorial inputs to principle 23 neurons of associative brain regions is established during development is unknown. Here, we 24 explore the developmental patterning of sensory inputs to principle neurons of the Drosophila 25 melanogaster mushroom body, a cerebellum-like structure required for olfactory learning. Indi-26 vidual mushroom body Kenyon cells receive only 3-10 of the 50 available olfactory projection 27 neuron inputs through large, claw-like dendritic structures. By manipulating the ratio between 28 pre-and post-synaptic cells, we find that convergence ratio is set by postsynaptic Kenyon cells: 29Kenyon cells produce largely fixed distributions of dendritic claws while presynaptic projection 30 neurons exhibit extensive plasticity in their repertoire of presynaptic processes. Moreover, we 31show that sparse odor responses are preserved in mushroom bodies with severe reductions in 32 cellular repertoires, suggesting that developmental specification of convergence ratio allows 33 functional robustness.
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