De novo adult transcriptomes of two European brittle stars: spotlight on opsin-based photoreception

Next generation sequencing (NGS) technology allows to obtain a deeper andmore complete
view of transcriptomes. For non-model or emergingmodelmarine organisms, NGS technologies
offer a great opportunity for rapid access to genetic information. In this study, paired-end
Illumina HiSeqTM technology has been employed to analyse transcriptomes from the arm tissues
of two European brittle star species, Amphiura filiformis and Ophiopsila aranea. About 48
million Illumina reads were generated and 136,387 total unigenes were predicted from A. filiformis
armtissues. For O. aranea armtissues, about 47 million reads were generated and
123,324 total unigenes were obtained. Twenty-four percent of the total unigenes fromA. filiformis
show significant matches with sequences present in reference online databases,
whereas, for O. aranea, this percentage amounts to 23%. In both species, around 50% of the
predicted annotated unigenes were significantly similar to transcripts from the purple sea
urchin, the closest species to date that has undergone complete genome sequencing and
annotation. GO, COG and KEGG analyses were performed on predicted brittle star unigenes.
We focused our analyses on the phototransduction actors involved in light perception. Firstly,
two new echinodermopsins were identified in O. aranea: one rhabdomeric opsin (homologous
to vertebrate melanopsin) and one RGR opsin. The RGR-opsin is supposed to be involved in
retinal regeneration while the r-opsin is suspected to play a role in visual-like behaviour. Secondly,
potential phototransduction actors were identified in both transcriptomes using the fly
(rhabdomeric) and mammal (ciliary) classical phototransduction pathways as references.
Finally, the sensitivity of O.aranea tomonochromatic light was investigated to complement
data available for A. filiformis. The presence of microlens-like structures at the surface of dorsal
arm plate of O. aranea could potentially explain phototactic behaviour differences between
the two species. The results confirm (i) the ability of these brittle stars to perceive light using
opsin-based photoreception, (ii) suggest the co-occurrence of both rhabdomeric and ciliary
photoreceptors, and (iii) emphasise the complexity of light perception in this echinodermclass.