Transforming descending input into behavior: The organization of premotor circuits in theDrosophilaMale Adult Nerve Cord connectome

Abstract

In most animals, a relatively small number of descending neurons (DNs) connect higher brain centers in the animal's head to motor neurons (MNs) in the nerve cord of the animal's body that effect movement of the limbs. To understand how brain signals generate behavior, it is critical to understand how these descending pathways are organized onto the body MNs. In the fly, Drosophila melanogaster, MNs controlling muscles in the leg, wing, and other motor systems reside in a ventral nerve cord (VNC), analogous to the mammalian spinal cord. In companion papers, we introduced a densely-reconstructed connectome of the Drosophila Male Adult Nerve Cord (MANC, Takemura et al., 2023), including cell type and developmental lineage annotation (Marin et al., 2023), which provides complete VNC connectivity at synaptic resolution. Here, we present a first look at the organization of the VNC networks connecting DNs to MNs based on this new connectome information. We proofread and curated all DNs and MNs to ensure accuracy and reliability, then systematically matched DN axon terminals and MN dendrites with light microscopy data to link their VNC morphology with their brain inputs or muscle targets. We report both broad organizational patterns of the entire network and fine-scale analysis of selected circuits of interest. We discover that direct DN-MN connections are infrequent and identify communities of intrinsic neurons linked to control of different motor systems, including putative ventral circuits for walking, dorsal circuits for flight steering and power generation, and intermediate circuits in the lower tectulum for coordinated action of wings and legs. Our analysis generates hypotheses for future functional experiments and, together with the MANC connectome, empowers others to investigate these and other circuits of the Drosophila ventral nerve cord in richer mechanistic detail.

Nested neural circuits generate distinct acoustic signals during Drosophila courtship

“…At present, connectomic analysis is also typically limited to very small sample sizes, usually just one, and so we cannot address variability both within and between species. These limitations will eventually be overcome through advances in electron (Cheong et al, 2023;Dorkenwald et al;Lesser et al, 2023;Marin et al, 2023;Phelps et al, 2021b;Scheffer et al, 2020;Schlegel et al, 2023a) and light (Lillvis et al, 2022) microscopy to provide molecular detail and to allow study of much larger sample sizes.…”

“…A male fly moves its wings in similar ways during flight and courtship song, which are produced by an overlapping set of wing muscles (O'Sullivan et al, 2018). However, the descending neurons that drive flight and song make connections to mostly different subsets of interneurons in the male VNC (Cheong et al, 2023). To identify neurons in the song circuit, we relied on previous evidence that fru + and dsx + neurons in the VNC are required to produce courtship song (von Philipsborn et al, 2011;Shirangi et al, 2016).…”

“…Having functionally characterized many of the individual song circuit neurons, we next examined their patterns of connectivity with each other and with wing motor neurons (Cheong et al, 2023) within the MANC connectome (Marin et al, 2023). This analysis revealed that the eight neuron types required to produce normal amounts of pulse and sine song (pIP10,pMP2,dPR1,dMS9,vMS12,TN1A,dMS2,and vPR9) are highly interconnected to each other and to wing motor neurons (Figures 8A-B…”

“…Split-GAL4 enhancer combinations identified in D. melanogaster have been used to target pIP10 in D. yakuba (Ding et al, 2019), suggesting that the split-GAL4 reagents we have identified may be useful for studying homologous circuits across Drosophila species. Testing anatomical hypotheses will require comparative connectomics, which should be facilitated by recent advances in EM (Dorkenwald et al;Lesser et al, 2023;Marin et al, 2023;Phelps et al, 2021a;Scheffer et al, 2020;Schlegel et al, 2023aSchlegel et al, , 2023bSheridan et al, 2022;Xu et al, 2017) and in expansion microscopy (Lillvis et al, 2022).…”

“…We assess the functions of over 40 distinct cell types by recording and analyzing a total of more than 1800 hours of songs from over 5000 male flies. We then trace the synaptic connections of the identified song neurons in the male VNC connectome (MANC) (Cheong et al, 2023;Marin et al, 2023). These anatomical and functional data define a highly interconnected core song circuit composed of eight fru and/or dsx-expressing (fru + /dsx + ) cell types organized into overlapping circuits for pulse and sine song production.…”