Fig. 4. UV opsin immunoreactivity (-ir) in the retina, visualized with a secondary antibody attached to Alexa Fluor 488 (green) or Cy3 (red). Cell nuclei were stained with DAPI (blue). (A) Frontal section showing the distally twisted rhabdomeres with UV opsin-ir (arrow). Note the strong autofluorescence of the crystalline cones (cc). (B) Tangential section of the central part of the eye showing three types of ommatidia, those with zero (double arrowhead), one (single arrowhead) and two (arrows) UV opsin-ir rhabdomeres. An electron microscopy study of the bumblebee fused rhabdom indicates that the rhabdomeres of R1, 2 and 5,6 are physically separated by the rhabdomeres of the other photoreceptor cells (Meyer-Rochow, 1981). (C) Semi-tangential section showing the heterogeneous UV opsin-ir in the rhabdomeres of different ommatidia. Ommatidium with one (arrowhead), two (arrows) and no (double arrowhead) UV opsin-ir retinula cells. Note the UV opsin-ir vesicles (flecks) in the cell bodies adjacent to the UV opsin-ir rhabdoms. (D) Tangential section showing UV opsin mRNA expression in the cell bodies of R1 and R5 photoreceptor cells, visualized with antisense digoxigenin-labeled riboprobes and an alkaline phosphatase-conjugated anti-digoxigenin antibody. Dotted lines indicate the boundaries of individual ommatidia. Three ommatidial types are shown, those with zero, one or two UV opsin mRNA-expressing photoreceptor cells. (E) Diagram of a longitudinal view of a single bee ommatidium showing the cornea (c), crystalline cone (cc), primary pigment cells (darkly speckled), secondary pigment cells (lightly speckled) and retinular cells (white) surrounding the fused rhabdom (hatched). Tangential views of the three ommatidial types observed in the Bombus retina with respect to UV opsin-ir (purple) are shown. Numbers indicate retinula cells R1–8. The R1 and R5 cells are UV opsin-ir, following the convention of Menzel and Blakers (1976). Note that in ommatidia in which only one rhabdomere is UV opsin-ir, that cell can be either the R1 or R5 cell (only one shown). (F) Longitudinal section of the retina showing the more densely packed UV opsin-ir dorsal rim area (Dra) (arrow). La, lamina; Re, retina. Scale bars, 20 µm (A,C); 10 µm (B); 25 µm (D); 50 µm (F).