Sensory neuron population expansion enhances odor tracking without sensitizing projection neurons.

The evolutionary expansion of sensory neuron populations detecting important environmental cues is widespread, but functionally enigmatic. We investigated this phenomenon through comparison of homologous neural pathways of Drosophila melanogaster and its close relative Drosophila sechellia , an extreme specialist for Morinda citrifolia noni fruit. D. sechellia has evolved species-specific expansions in select, noni-detecting olfactory sensory neuron (OSN) populations, through multigenic changes. Activation and inhibition of defined proportions of neurons demonstrate that OSN population increases contribute to stronger, more persistent, noni-odor tracking behavior. These sensory neuron expansions result in increased synaptic connections with their projection neuron (PN) partners, which are conserved in number between species. Surprisingly, having more OSNs does not lead to greater odor-evoked PN sensitivity or reliability. Rather, pathways with increased sensory pooling exhibit reduced PN adaptation, likely through weakened lateral inhibition. Our work reveals an unexpected functional impact of sensory neuron expansions to explain ecologically-relevant, species-specific behavior.

Animal Behavior: A Neural Basis of Individuality.

Behavior can vary greatly even between genetically identical animals, but what determines such individuality? New work reveals that inter-individual differences in wiring of visual interneurons in Drosophila, arising from stochastic developmental events, underlie fly-specific object orientation abilities.

Circuit architecture for somatotopic action selection in invertebrates.

Invertebrate species have significantly contributed to neuroscience owing to the accessibility they provide to cellular- and molecular-level understanding of brain functions. Somatotopic action selection is one of the key features of animal behavior, and studying this process in invertebrates is potentially a sweet spot in understanding the general relationship between neuronal morphology, circuit structure, and animal behavior. In this review, we introduce circuit architectures that realize somatotopic action selection, from simple reflexes to patterned motor outputs, in different invertebrate species. We then discuss future directions towards understanding the general principles underlying the development and evolution of the circuit architecture that enables sensorimotor transformation and action selection in the animal kingdom.

Divergent Connectivity of Homologous Command-like Neurons Mediates Segment-Specific Touch Responses in Drosophila.

Animals adaptively respond to a tactile stimulus by choosing an ethologically relevant behavior depending on the location of the stimuli. Here, we investigate how somatosensory inputs on different body segments are linked to distinct motor outputs in Drosophila larvae. Larvae escape by backward locomotion when touched on the head, while they crawl forward when touched on the tail. We identify a class of segmentally repeated second-order somatosensory interneurons, that we named Wave, whose activation in anterior and posterior segments elicit backward and forward locomotion, respectively. Anterior and posterior Wave neurons extend their dendrites in opposite directions to receive somatosensory inputs from the head and tail, respectively. Downstream of anterior Wave neurons, we identify premotor circuits including the neuron A03a5, which together with Wave, is necessary for the backward locomotion touch response. Thus, Wave neurons match their receptive field to appropriate motor programs by participating in different circuits in different segments.