Patterned rhodopsin expression in R7 photoreceptors of mosquito retina: Implications for species-specific behavior

Abstract

Visual perception of the environment plays an important role in many mosquito behaviors. Characterization of the cellular and molecular components of mosquito vision will provide a basis for understanding these behaviors. A unique feature of the R7 photoreceptors in Aedes aegypti and Anopheles gambiae is the extreme apical projection of their rhabdomeric membrane. We show here that the compound eye of both mosquitoes is divided into specific regions based on nonoverlapping expression of specific rhodopsins in these R7 cells. The R7 cells of the upper dorsal region of both mosquitoes express a long wavelength op2 rhodopsin family member. The lower dorsal hemisphere and upper ventral hemisphere of both mosquitoes express the UV-sensitive op8 rhodopsin. At the lower boundary of this second region, the R7 cells again express the op2 family rhodopsin. In Ae. aegypti, this third region is a horizontal stripe of one to three rows of ommatidia, and op8 is expressed in a fourth region in the lower ventral hemisphere. However, in An. gambiae the op2 family member expression is expanded throughout the lower region in the ventral hemisphere. The overall conserved ommatidial organization and R7 retinal patterning show these two species retain similar visual capabilities. However, the differences within the ventral domain may facilitate species-specific visual behaviors.

Figure 1

Organization of photoreceptors in the ommatidium of Ae. aegypti and An. gambiae. A: Electron microscopy image of a single ommatidium in Ae. aegypti at the depth of the rhabdom showing the location of individual photoreceptors. B: Drawing based on A, showing the outline of the cell bodies and rhabdomeres of each of the photoreceptors. The R7 photoreceptor cell contains the nucleus (labeled N) but lacks a rhabdomere at this depth. C: Schematic illustration of the ultrastructure of an Ae. aegypti ommatidial unit. The diagram is based on the electron microscopic analyses of Brammer (1970) and of this report. D: Electron microscopy image of an An. gambiae ommatidial unit. The view is similar to that shown for Ae. aegypti in A. E: A distal retinal section of An. gambiae showing the extension of the R7 rhabdomere, marked with “*”, over the fused rhabdom. R1-R8, R1-R8 photoreceptor cells; N, nucleus; CC, cone cell. Scale bars in A, D, and E are approximately 10 µm.

Figure 2

Expression of op8 rhodopsin in Ae. aegypti and An. gambiae R7 photoreceptor cells. A: Multiple sequence alignment of the C-terminal regions of Ae. aegypti Aaop8 and Aaop2 with An. gambiae Agop8 and D. melanogaster Rh3 and Rh4 rhodopsins. Amino acids identical to the Aaop8 sequence are shaded gray. B, C, D: Ae. aegypti ommatidia labeled with Aaop8 antibody (green, B), phalloidin (magenta, C), and the resulting merged image (D). Phalloidin detects actin and heavily stains the rhabdom (labeled). E, F, G: An. gambiae ommatidia labeled with Aaop8 antibody (green, E), phalloidin (magenta, F), and the merged image (G). Scale bars in D and G are approximately 15 µm.

Figure 3

Expression of op2 rhodopsin in Ae. aegypti and An. gambiae R7 photoreceptor cells. A: Multiple sequence alignment of the C-terminal regions of Ae. aegypti Aaop2, Aaop1, Aaop3, Aaop4, Aaop5 with An. gambiae Agop1, Agop3, Agop6 rhodopsins. Amino acids identical to the Aaop2 sequence are shaded gray. B, C, D: Ae. aegypti ommatidia labeled with Aaop2 antibody (green, B), phalloidin (magenta, C), and the merged image (D). E, F, G: An. gambiae ommatidia labeled with Aaop2 antibody (green, E), phalloidin (magenta, F), and the merged image (G). Scale bars in D and G are approximately 15 µm.

Figure 4

op8 and op2 rhodopsins are expressed in nonoverlapping sets of R7 photoreceptor cells in Ae. aegypti and An. gambiae. A: Whole mount retina of Ae. aegypti stained with Aaop8 antibody (green) and co-labeled with phalloidin (magenta). The two regions not stained by Aaop8 antibody are the dorsal region (large white arrowhead) and the ventral stripe (small white arrow). The equator is marked by white lines at the margins of the eye. B: whole mount retina of Ae. aegypti stained with Aaop2 antibody (green) and co-labeled with phalloidin (magenta). C: Sectioned Ae. aegypti retina co-stained with Aaop8 antibody (green) and Aaop2 antibody (magenta). D: Whole mount retina of An. gambiae stained with Aaop8 antibody (green) and co-labeled with phalloidin (magenta). E: Whole mount retina of An. gambiae stained with Aaop2 antibody (green) and co-labeled with phalloidin (magenta). F: Sectioned An. gambiae retina co-stained with Aaop8 antibody (green) and Aaop2 antibody (magenta). Scale bars are approximately 50 µm.