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Abstract Opsin proteins are central to photoreception and have diversified extensively to support both visual and non-visual functions in animals. While the molecular conservation is striking, the evolutionary pathways that shaped opsin diversity, particularly in relation to visual and extraocular roles, remain poorly understood. To address this gap, we examined opsin expression in Pteriomorphia, a diverse group of mostly sessile marine bivalves with multiple photoreceptor systems and ecologies. We generated transcriptomes for 12 species, including eyed and eyeless adults, and analysed published RNA-seq data from five additional species covering all taxonomic orders and eye types. We identified and classified opsins expressed in the photosensitive mantle margin and eyes using phylogenetically informed annotation. We tested whether opsin diversity is associated with eye presence or mobility and determined which opsins are widely used. Our results suggest that bivalve eyes generally express more opsins than extraocular tissues. Additionally, convergent expansions and reductions of opsin diversity were estimated, likely derived from an ancestral repertoire in the mantle margin. Curiously, differences in mobility among species do not influence opsin repertoire. Our findings further support that the rhodopsin-retinochrome system is pervasive across bivalves and identify visual and non-visual opsins used in both eyes and dermal photoreception.
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Opsin expression varies across larval development and taxa in pteriomorphian bivalves
Many marine organisms have a biphasic life cycle that transitions between a swimming larva with a more sedentary adult form. At the end of the first phase, larvae must identify suitable sites to settle and undergo a dramatic morphological change. Environmental factors, including photic and chemical cues, appear to influence settlement, but the sensory receptors involved are largely unknown. We targeted the protein receptor, opsin, which belongs to large superfamily of transmembrane receptors that detects environmental stimuli, hormones, and neurotransmitters. While opsins are well-known for light-sensing, including vision, a growing number of studies have demonstrated light-independent functions. We therefore examined opsin expression in the Pteriomorphia, a large, diverse clade of marine bivalves, that includes commercially important species, such as oysters, mussels, and scallops. Methods Genomic annotations combined with phylogenetic analysis show great variation of opsin abundance among pteriomorphian bivalves, including surprisingly high genomic abundance in many species that are eyeless as adults, such as mussels. Therefore, we investigated the diversity of opsin expression from the perspective of larval development. We collected opsin gene expression in four families of Pteriomorphia, across three distinct larval stages, i.e., trochophore, veliger, and pediveliger, and compared those to adult tissues.Results We found larvae express all opsin types in these bivalves, but opsin expression patterns are largely species-specific across development. Few opsins are expressed in the adult mantle, but many are highly expressed in adult eyes. Intriguingly, opsin genes such as retinochrome, xenopsins, and Go-opsins have higher levels of expression in the later larval stages when substrates for settlement are being tested, such as the pediveliger. Conclusion Investigating opsin gene expression during larval development provides crucial insights into their intricate interactions with the surroundings, which may shed light on how opsin receptors of these organisms respond to various environmental cues that play a pivotal role in their settlement process.
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- Award ID(s):
- 2148203
- PAR ID:
- 10530600
- Publisher / Repository:
- Frontiersin.org
- Date Published:
- Journal Name:
- Frontiers in Neuroscience
- Volume:
- 18
- ISSN:
- 1662-453X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.3389/fnins.2024.1357873
