New Tardigrade Opsins and Differential Expression Analyses Show Ontogenic Variation in Light Perception

Opsins are light-sensitive proteins involved in many photoreceptive processes, including, but not limited to, vision and regulation of circadian rhythms. Arthropod (e.g., insects, spiders, centipedes, and crabs) opsins have been extensively researched, but the relationships and function of opsins found in lineages that are evolutionarily closely related to the arthropods remains unclear. Multiple, independent, opsin duplications are known in Tardigrada (thewater bears), evidencing that protostomeopsin duplications are not limited to theArthropoda. However, the relationships, function, and expression of these newopsins are still unknown. Here, we use two tardigrade transcriptomeswith deep coverage to greatly expand our knowledge of the diversity of tardigrade opsins. We reconstruct the phylogenetic relationships of the tardigrade opsins andinvestigate their ontogenetic expression. We foundthatwhile tardigrades havemultiple opsins that evolved from lineage-specific duplications of well-understood arthropod opsins, their expression levels change during ontogeny such that most of these opsins are not co-temporally expressed. Co-temporal expression of multiple opsins underpins color vision in Arthropoda and Vertebrata. Our results clearly show duplications of both rhabdomeric and ciliary opsins within Tardigrada, forming clades specific to both the Heterotardigrada and Eutardigrada in addition to multiple independent duplications within genera. However, lack of co-temporal, ontogenetic, expression suggests that while tardigrades possess multiple opsins, they are unlikely to be able to distinguish color.

Automated summary

Research shows that there are multiple opsins duplications within Tardigrada, however, the expression levels of these opsins change during ontogeny and most of them are not co-temporally expressed, indicating that tardigrades are unlikely to be able to distinguish color.