Dorsal views of chimeric larvae containing Biotin-Dextran- (BD) or GFP-marked cells transplanted from a donor embryo of noted genotype into a wildtype (wt) host; throughout the figure the neurons derived from the donor embryo are displayed in magenta, while those from the host are in blue. Synapses (stained with anti-human-Cx36, yellow) associated with a transplanted neuron (cyan circles and boxes) can be directly compared to wildtype host synapses (red circles and boxes). Hindbrain, lateral dendrite, and spinal cord images are maximum intensity projections of ~30,~5, and ~10 uM, respectively. Anterior is to the left. Scale bar = 10 uM. (A–C) At the Aud/M synapses presynaptic auditory afferent neurons (in A, stained with BD, magenta) synapse onto the postsynaptic Mauthner lateral dendrite (stained with anti-GFP, blue in A, magenta in B,C). (D,E) At the M/CoLo synapses the presynaptic Mauthner axon (stained with anti-GFP, magenta in D) synapses with the postsynaptic CoLo (stained with anti-GFP, magenta in E). (F–J) Presynaptic removal of gjd1a/cx34.1 function has no effect on Aud/M and M/CoLo synapses (cyan circles in F,I). By contrast, removing gjd1a/cx34.1 postsynaptically causes a loss of electrical synapse staining (cyan boxes and circles in G,H,J; note residual Cx36 staining at Aud/M synapses when gjd1a/cx34.1 is removed from only the postsynaptic neuron, (H). (K–O) Conversely, gjd2a/cx35.5 function is required exclusively presynaptically (K,N) and is dispensable postsynaptically (L,M,O). (P,Q) Quantitation of the ratio of Cx36 at donor-associated synapses to wildtype host synapses in chimaeric embryos. Each circle represents the average ratio of 1–8 donor-associated to 8–12 host-associated synapses within an animal, varying depending on the synapse and chimaera. Associated quantitation of Cx36 at chimeric synapses can be found in Figure 6—source data 1 for Figure 6 and Figure 6—source data 2 for Figure 6.