The last common ancestor of most bilaterian animals possessed at least nine opsins

Abstract

The opsin gene family encodes key proteins animals use to sense light and has expanded dramatically as it originated early in animal evolution. Understanding the origins of opsin diversity can offer clues to how separate lineages of animals have repurposed different opsin paralogs for different light-detecting functions. However, the more we look for opsins outside of eyes and from additional animal phyla, themore opsinswe uncover, suggestingwe still do not knowthe true extent of opsin diversity, nor the ancestry of opsin diversity in animals. To estimate the number of opsinparalogs present in both the lastcommonancestor of theNephrozoa (bilaterians excluding Xenoacoelomorpha), and the ancestor of Cnidaria +Bilateria, we reconstructed a reconciled opsin phylogeny using sequences from14animal phyla, especially the traditionally poorly-sampled echinoderms andmolluscs.Our analysis strongly supports a repertoire of at least nine opsin paralogs in the bilaterian ancestor and at least four opsin paralogs in the last common ancestor of Cnidaria +Bilateria. Thus, the kernels of extant opsin diversity arose much earlier in animal history than previously known. Further, opsins likely duplicated and were lost many times, with different lineages of animals maintaining different repertoires of opsin paralogs. This phylogenetic information can inform hypotheses about the functions of different opsin paralogs and can be used to understand how and when opsins were incorporated into complex traits like eyes and extraocular sensors.