Evidence for a phototransduction cascade in an early brachiopod embryo

Characterization of eyes, photoreceptors, and opsins in developmental stages of the arrow worm Spadella cephaloptera (Chaetognatha)

The phylogenetic position of chaetognaths, or arrow worms, has been debated for decades, however recently they have been grouped into the Gnathifera, a sister clade to all other Spiralia. Chaetognath photoreceptor cells are anatomically unique by exhibiting a highly modified cilium and are arranged differently in the eyes of the various species. Studies investigating eye development and underlying gene regulatory networks are so far missing. To gain insights into the development and the molecular toolkit of chaetognath photoreceptors and eyes a new transcriptome of the epibenthic species Spadella cephaloptera was searched for opsins. Our screen revealed two copies of xenopsin and a single copy of peropsin. Gene expression analyses demonstrated that only xenopsin1 is expressed in photoreceptor cells of the developing lateral eyes. Adults likewise exhibit two xenopsin1 + photoreceptor cells in each of their lateral eyes. Beyond that, a single cryptochrome gene was uncovered and found to be expressed in photoreceptor cells of the lateral developing eye. In addition, cryptochrome is also expressed in the cerebral ganglia in a region in which also peropsin expression was observed. This condition is reminiscent of a nonvisual photoreceptive zone in the apical nervous system of the annelid Platynereis dumerilii that performs circadian entrainment and melatonin release. Cryptochrome is also expressed in cells of the corona ciliata, an organ in the posterior dorsal head region, indicating a role in circadian entrainment. Our study highlights the importance of the Gnathifera for unraveling the evolution of photoreceptors and eyes in Spiralia and Bilateria.

Increasing complexity of opsin expression across stomatopod development

Stomatopods are well studied for their unique visual systems, which can consist of up to 16 different photoreceptor types and 33 opsin proteins expressed in the adults of some species. The light-sensing abilities of larval stomatopods are comparatively less well understood with limited information about the opsin repertoire of these early-life stages. Early work has suggested that larval stomatopods may not possess the extensive light detection abilities found in their adult counterparts. However, recent studies have shown that these larvae may have more complex photosensory systems than previously thought. To examine this idea at the molecular level, we characterized the expression of putative light-absorbing opsins across developmental stages, from embryo to adult, in the stomatopod species Pullosquilla thomassini using transcriptomic methods with a special focus on ecological and physiological transition periods. Opsin expression during the transition from the larval to the adult stage was further characterized in the species Gonodactylaceus falcatus. Opsin transcripts from short, middle, and long wavelength-sensitive clades were found in both species, and analysis of spectral tuning sites suggested differences in absorbance within these clades. This is the first study to document the changes in opsin repertoire across development in stomatopods, providing novel evidence for light detection across the visual spectrum in larvae.

The Last Common Ancestor of Most Bilaterian Animals Possessed at Least Nine Opsins

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, the more opsins we uncover, suggesting we still do not know the true extent of opsin diversity, nor the ancestry of opsin diversity in animals. To estimate the number of opsin paralogs present in both the last common ancestor of the Nephrozoa (bilaterians excluding Xenoacoelomorpha), and the ancestor of Cnidaria + Bilateria, we reconstructed a reconciled opsin phylogeny using sequences from 14 animal phyla, especially the traditionally poorly-sampled echinoderms and molluscs. 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.

Analysis of the opsin repertoire in the tardigrade Hypsibius dujardini provides insights into the evolution of opsin genes in panarthropoda

Screening of a deeply sequenced transcriptome using Illumina sequencing as well as the genome of the tardigrade Hypsibius dujardini revealed a set of five opsin genes. To clarify the phylogenetic position of these genes and to elucidate the evolutionary history of opsins in Panarthropoda (Onychophora + Tardigrada + Arthropoda), we reconstructed the phylogeny of broadly sampled metazoan opsin genes using maximum likelihood and Bayesian inference methods in conjunction with carefully selected substitution models. According to our findings, the opsin repertoire of H. dujardini comprises representatives of all three major bilaterian opsin clades, including one r-opsin, three c-opsins, and a Group 4 opsin (neuropsin/opsin-5). The identification of the tardigrade ortholog of neuropsin/opsin-5 is the first record of this opsin type in a protostome, but our screening of available metazoan genomes revealed that it is also present in other protostomes. Our opsin phylogeny further suggests that two r-opsins, including an "arthropsin," were present in the last common ancestor of Panarthropoda. Although both r-opsin lineages were retained in Onychophora and Arthropoda, the arthropsin was lost in Tardigrada. The single (most likely visual) r-opsin found in H. dujardini supports the hypothesis of monochromatic vision in the panarthropod ancestor, whereas two duplications of the ancestral panarthropod c-opsin have led to three c-opsins in tardigrades. Although the early-branching nodes are unstable within the metazoans, our findings suggest that the last common ancestor of Bilateria possessed six opsins: Two r-opsins, one c-opsin, and three Group 4 opsins, one of which (Go opsin) was lost in the ecdysozoan lineage.