Opsin gene expression plasticity and spectral sensitivity as mechanisms for search image formation in mate-searching male damselflies

Odonata (damselflies and dragonflies) are a largely diurnal, colourful, and strongly visually guided group of insects with visual systems influenced by many opsin genes that form the primary light-sensitive eye photopigments. Heritable (genetic) female-limited colour polymorphisms are also common in Odonata, with one female morph typically exhibiting male-like (androchrome) colouration and one or two morphs exhibiting female-specific colouration (gynochromes). In the Common Bluetail Damselfly ( Ischnura elegans ), androchrome females express blue body colouration similar to that of mature males while gynochrome females express either green or red-brown colouration. This and other colour polymorphisms in damselflies are thought to be maintained by male mating harassment and frequency-dependent sexual conflict, in which males form search images for certain morphs which suffer disproportionally from mating harassment. Here, we investigate the role of opsin sensitivity and expression plasticity in visual mate detection in I. elegans by quantifying relative opsin mRNA expression over adult maturation in populations with different female morph frequencies in southern Sweden. We find evidence for opsin-specific plasticity in relative and proportion opsin expression, suggesting changes in opsin regulation and visual sensitivity over adult maturation. Furthermore, the relative expression of the long-wavelength sensitive opsin LWF2 changed in response to female morph frequencies. The highest relative expression levels were found in populations with either a high or low proportion of androchrome females. In vitro results indicate that long-wavelength sensitive opsins in I. elegans provide a good visual match to the colouration of green gynomorph females and could in principle confer male colour discrimination between female morphs. We discuss these results in relation to frequency dependent selection, male sensory adaptations, plastic search images and mate searching costs. We suggest that opsin gene expression could play an important role in male search image formation of suitable mates. ### Competing Interest Statement The authors have declared no competing interest.