The role of opsins in search image tuning in the damselfly Ischnura elegans

The damselfly Ischnura elegans is characterized by female-limited colour polymorphism, with one female morph (andromorph) that mimics the blue colouration of males, and a second morph (gynomorph) that exhibits female-specific green or orange colouration. The learned mate recognition (LMR) hypothesis suggests that the presence of multiple female morphs reduces males’ ability to recognize any one morph as a suitable mate. Males, in turn, are predicted to improve their ability to detect high-frequency females by forming a search image. Hence, behavioral tests have shown that mature males consistently bias courtship towards the most common female morphs, supporting that they develop LMR with experience. However, whether changes at the peripheral visual system affect male colour image search formation, and the underlying molecular mechanisms remain unknown. In this study, we quantify opsin gene expression changes between eyes of immature and mature male I. elegans from southern Sweden populations varying in female colour morph frequencies. We test whether male opsin expression changes during adult development and in an environment-dependent context. We then assay the spectral absorbance of short and long-wavelength opsin proteins found in the ventral eye, by in vitro heterologous expression and spectroscopy. Finally, we examine how the male visual system matches female body reflectance and corroborates with varying morph abundance across populations. Overall, our results examine the extent to which peripheral visual adaptations can act as mechanism of improving image search for female detection and contributing to the persistence of within-population polymorphism in this well-studied insect group