Sub-functionalization of dorsal and ventral eyes in a whirligig beetle (Coleoptera: Gyrinidae)

Abstract

ABSTRACTCompound eyes in nocturnal or fossorial insects generally express visible light opsins at higher levels than diurnal insects. In this study, we tested whether dorsal (above water) and ventral eyes (below water) of the diurnal four-eyed whirligig beetle Gyretes sericeus Laboulbène, 1853, resemble opsin expression and function of diurnal or nocturnal insect eyes respectively. By immunocytochemistry, we compared expression of green LW-opsin in dorsal and ventral rhabdoms of whirligig beetle ommatidia. Basal rhabdomeres (bR) showed comparable expression levels of LW-opsin in both dorsal and ventral ommatidia. In contrast, the inner proximal (R1p) and distal (R1d) bR showed a weak and narrow expression dorsally, whereas R1p and R1d showed a higher and expanded expression ventrally. To test whether dorso-ventral specialization of ommatidia results in functional differences in light response, we studied the behavior of beetles after selective eye-occlusion experiments. During phototaxis experiments, whirligig beetles typically showed a clear preference for light. Positive phototaxis was mainly disrupted after dorsal eye occlusion suggesting dorsal dominance of light sensing. Together, differences in opsin expression, structure, and function of whirligig beetle eyes suggest dorso-ventral sub-functionalizations resembling eye adaptations of diurnal and nocturnal insects. We discuss how dorsal and ventral eye specializations may have evolved in adephagan beetles.