Appendix 2—figure 6. Gap-junction spread information.

Because of axonal gap-junctions between R6 and R7/R8 photoreceptors in the lamina (Shaw, 1984; Shaw et al., 1989), R1-R6s that have been genetically engineered to express UV-sensitive Rh3-rhodopsin (‘UV-flies’) can still respond to green light by different degrees (Wardill et al., 2012). This flow of extra ‘color’- information can be readily identified in intracellular responses of different R1-R6 photoreceptors in the same ‘UV-fly’ to very bright UV (385 nm) and green-yellow (505 nm) flashes. (A) First cell responded to UV but not to green. (B) Next cell (likely R6 in the same or neighboring neuro-ommatidium) responded to both UV and green. This cell cannot be R7y/p, which are less green-sensitive, or R8y/p, which are less UV-sensitive. Inset highlights a hypothetical recording path, somewhere close to the retina/lamina border (red arrow), and gap-junctions (black arrows) between photoreceptor axons. Histaminergic L1 and L2 cells receive visual information from R1-R6 photoreceptors’ output synapses in the same neuro-ommatidium. (C) Another cell responded to UV and weakly to green-yellow. Modified from (Wardill et al., 2012).