Genomics Sheds New Light on the Ancestral Bilaterian Opsin Repertoire and Suggests Rhabdomeric Phototransduction in Xenacoelomorpha.

Animals use opsins, light-sensing, transmembrane proteins, to detect light, which is then converted into an electrical stimulus in the photoreceptors through a process known as phototransduction. The steady generation of genomic data has facilitated the description of the opsin diversity and photoreceptor activity in different animals. However, Xenacoelomorpha still represents an important gap in our understanding of opsin evolution from early animals. Characterized by extreme morphological simplicity, some Xenacoelomorpha present ocelli, eyespots, and even eyes with lenses, but no ciliary or rhabdomeric photoreceptors have been described. Here, we have leveraged all available genomic data from this group to characterize their opsin repertoire and to annotate the genes involved in the two main phototransduction cascades, ciliary and rhabdomeric, in these worms. Only six opsin types were found in Xenacoelomorpha, with no more than three in any given species. Among them, we annotated Anthozoa I, insofar thought to be cnidarian-specific, and another one specific to xenacoelomorphs. Despite their enigmatic position in the animal tree, we extended these findings to other animals, evaluating the two main phylogenetic hypotheses-the Nephrozoa and Xenambulacraria hypotheses. We propose that the opsin repertoire of the ancestral bilaterian was composed of either seven opsins or 11, depending on the phylogenetic hypothesis, and not nine as previously suggested. We argue that the Nephrozoa hypothesis explains opsin evolution more parsimoniously than the alternative Xenambulacraria hypothesis. Furthermore, our data also suggest that despite the lack of the typical rhabdomeric or ciliary photoreceptors, xenacoelomorphs use rhabdomeric phototransduction.