The adaptational and evolutionary history of spectral sensitivity in ecdysozoan r-opsins

Visual r-opsins originated more than 600Ma in the common ancestor of all protostomes, and underwent multiple duplications within the evolutionary history of Ecdysozoa, leading to a remarkable number of visual pigments in extant insect and crustacean species. Combining phylogenetic, computational and molecular approaches we explore the evolution of visual pigment functions across major duplications throughout Arthropoda (insects, crustaceans, chelicerates, myriapods). We capture key evolutionary transitions, including the terrestrialisation of insects, tardigrades and chelicerates, giving a functional view of opsin adaptation to new visual environments.
All 4 opsin families shared across Arthropoda (LWS-MWS-Rh7 and UV/SWS) originated prior to terrestrialisation of arthropods but also of vascular plants. For shallow water and coastal organisms, the water would likely have been red-shifted, owing to sediment influx without roots to bind the soil. The LWS/MWS to LWS+MWS duplication (second opsin duplication) might have resulted in a bathochromatic LWS opsin adapted to Cambrian coastal environments. Similarly, UV-light has poor depth penetration in saltwater, yet the modern UV/SWS/Rh7 clade originated in the first opsin duplication. As the ancestral ecdysozoan opsin is likely blue-green sensitive, the evolution of LWS and UV/SWS pigments should tell us about the early ecologies of Cambrian shallow marine environments.
A comprehensive phylogeny was built combining large-scale opsin datasets of modern UV-SW-MWS-LWS-Rh7 paralog lineages across arthropods and non-arthropod ecdysozoans. Ancestral states were robustly calculated using Lazarus with additional Gap Correction, alongside a novel comparative approach to better approximate flanking regions, providing 8 ancestral sequences (around 370-aa long) at key internal nodes representing duplication events preceeding modern arthropod opsin lineages.
Our first ancestral sequence is placed at the origin of all modern opsin lineages, incorporating arthropod-specific families Rh7, UV, SWS, LWS and MWS and non-arthropod ecdysozoans (ancestral 1). Our second (ancestral 2) represents the common ancestor of all arthropod-specific visual r-opsin families, which upon duplication around 599Ma, gave rise to LWS/MWS opsins (ancestral 3) in Chelicerata-Pancrustacea-Myriapoda, and Rh7/UV/SWS opsins in Chelicerata-Pancrustacea (ancestral 4). The LWS/MWS (ancestral 3) lineage evolved into MWS+LWS opsins 590Ma before the split between Chelicerata-Pancrustacea-Myriapoda, with an internal division between Chelicerate and Mandibulate LWS opsins (ancestral 5). Duplications of the RH7/UV/SWS (ancestral 4) lineage resulted in Rh7 and a combined UV/SWS (ancestral 6) 570Ma, followed by evolution of the Chelicerate UV family (ancestral 7,), then the UV/SWS split into modern UV+SWS opsins (ancestral 8).
We explore functionally these resurrected opsins by heterologous expression to assess the spectral range of early light-sensitive receptors.