Fig. 7: TLHONs display privileged connectivity with descending pathways that mediate escape.

a,b Connectomic analysis centered on potential descending pathways downstream of TLHONs based on Hemibrain data. No direct TLHON > DN connections were found. Additional pathways including a single intermediate cell type were considered (threshold: 10 synapses). Cells belonging to the same cell type were treated as a unit. a Brain-VNC schematic, significant neuropils are color coded as in (b–d), leg and wing neuropils are color coded as the corresponding associated body part. Arrows = synaptic input, numbers = synapse #. b In the brain, TLHONs connect to two interneurons (LHAD1 and PVLP076) with privileged connectivity with DNp-type descending neurons. LHAD1 also receives additional recurring connections from TLHON through LHAV2b and AVLP053. Together, LHAD1, PVLP076 and AVLP053 connect to 10 DNp cell types and one DNb. The strongest connectivity is observed with DNp01 (201 synapses) and with DNp06 (336 synapses from LHAD1 and 145 from PVLP076). In the VNC, the identified DNps connect preferentially with neuropils associated with locomotion, turning and escape. Neck/VNC black lines represent DN targets based on literature (see text for details). c EM reconstructions of TLHON > LH/VLP>key DNps (LH, VLP and cell types are color coded as in (a); neuropil colors are schematized in (a and c) top left, arrowheads = cell bodies). d Silencing DNp06 reduces the occurrence of “early turns” in the 25° vs 35 °C and 25° vs 40 °C conditions (green box), while (e) silencing DNp09 does not. Sideways histograms represent quantification of turning frequency within distinct bands of the thermal gradients in the 25° vs 30 °C, 25° vs 35 °C and 25° vs 40 °C conditions, for controls (Gal4/+, left) and experimental animals (Gal4/UAS-Kir2.1, right). “Early turns” (<26.5 °C) and “deep turns” (>28.5 °C or >32.5 °C) are compared within genotype to quantify the propensity for rapid escape (two-sided Chi square, * =p < 0.05, p= [9E−4, 7E−5, 3.94E−05, 1.45E−06], p= [0.353, 0.053,0.522], p = [0.112, 4.94E−16, 9.36E−10], for P6/ + , P6/Kir, P9/Kir, and 25 °C vs 30,35,40 °C, respectively; n.s.= not significant, p > 0.05. N = 20–44 flies). f Schematic of the full circuit described here. Anatomical abbreviations: LH = lateral horn, VLP = ventrolateral protocerebrum, Tct = tectulum, NTct = neck t., WTct = wing t., HTct = halthere t., IntTct = Intermediate t., LTct = lower t., ANm = abdominal neuromere, AMNP = accessory mesothoracic neuropil, IntNP(T1-3) = intermediate neuropil thoracic neuromere 1-3, MB = mushroom body, AL = antennal lobe, NC= neck connective. Source data are provided as a source data file.