Fig. 6. The Gr21a-expressing neuron mediates CO₂ detection in larvae. (A) Confocal image of the anterior of a third instar larva expressing membrane-bound UAS-mCD8::GFP (green) driven by a Gr21a-Gal4 construct. Dotted line indicates the position of the dome of the dorsal organ in the transmission image; arrows point to the terminal organs. (B,C) Distributions of larvae in the four-field olfactometer as in Fig. 5. (B) Responses to 1% CO₂ by larvae lacking the Gr21a-expressing neurons due to Gr21a-driven expression of the apoptotic gene reaper (rpr) (Gr21a-rpr, N=22), compared to their genetic controls carrying only the driver construct (Gr21a, N=20) or the reaper construct (rpr, N=17). Significant avoidance is only seen in the controls. (C) Response to apple cider vinegar of larvae lacking the Gr21a-expressing neuron (Gr21a-rpr, N=18) is normal when compared to wild-type larvae (CS, same data as in Fig. 5C at 10:00 min; ns, no significant difference, Mann-Whitney U-test). Abbreviations and statistics are as in Fig. 1. The broken line at 25% indicates an equal distribution in all fields. In contrast to Figs 1 and 2 the test field is not significantly different from all control fields (Friedman-ANOVA, P<0.05 for CO₂, P<0.001 for vinegar; Wilcoxon-Wilcox: P<0.05); ns, no significant difference. Values are means ± s.e.m.