Plasticity of opsin gene expression in the adult red shiner (Cyprinella lutrensis) in response to turbid habitats

Parallel shifts of visual sensitivity and body coloration in replicate populations of extremophile fish

Visual sensitivity and body pigmentation are often shaped by both natural selection from the environment and sexual selection from mate choice. One way of quantifying the impact of the environment is by measuring how traits have changed after colonization of a novel habitat. To do this, we studied Poecilia mexicana populations that have repeatedly adapted to extreme sulphidic (H₂ S-containing) environments. We measured visual sensitivity using opsin gene expression, as well as body pigmentation, for populations in four independent drainages. Both visual sensitivity and body pigmentation showed significant parallel shifts towards greater medium-wavelength sensitivity and reflectance in sulphidic populations. Altogether we found that sulphidic habitats select for differences in visual sensitivity and pigmentation. Shifts between habitats may be due to both differences in the water's spectral properties and correlated ecological changes.

Evolutionary ecology of the visual opsin gene sequence and its expression in turbot (Scophthalmus maximus)

Background

As flatfish, turbot undergo metamorphosis as part of their life cycle. In the larval stage, turbot live at the ocean surface, but after metamorphosis they move to deeper water and turn to benthic life. Thus, the light environment differs greatly between life stages. The visual system plays a great role in organic evolution, but reports of the relationship between the visual system and benthic life are rare. In this study, we reported the molecular and evolutionary analysis of opsin genes in turbot, and the heterochronic shifts in opsin expression during development.

Results

Our gene synteny analysis showed that subtype RH2C was not on the same gene cluster as the other four green-sensitive opsin genes (RH2) in turbot. It was translocated to chromosome 8 from chromosome 6. Based on branch-site test and spectral tuning sites analyses, E122Q and M207L substitutions in RH2C, which were found to be under positive selection, are closely related to the blue shift of optimum light sensitivities. And real-time PCR results indicated the dominant opsin gene shifted from red-sensitive (LWS) to RH2B1 during turbot development, which may lead to spectral sensitivity shifts to shorter wavelengths.

Conclusions

This is the first report that RH2C may be an important subtype of green opsin gene that was retained by turbot and possibly other flatfish species during evolution. Moreover, E122Q and M207L substitutions in RH2C may contribute to the survival of turbot in the bluish colored ocean. And heterochronic shifts in opsin expression may be an important strategy for turbot to adapt to benthic life.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01837-2.

The sensory impacts of climate change: bathymetric shifts and visually mediated interactions in aquatic species

Visual perception is, in part, a function of the ambient illumination spectrum. In aquatic environments, illumination depends upon the water's optical properties and depth, both of which can change due to anthropogenic impacts: turbidity is increasing in many aquatic habitats, and many species have shifted deeper in response to warming surface waters (known as bathymetric shifts). Although increasing turbidity and bathymetric shifts can result in similarly large changes to a species' optical environment, no studies have yet examined the impact of the latter on visually mediated interactions. Here, we examine a potential link between climate change and visual perception, with a focus on colour. We discuss (i) what is known about bathymetric shifts; (ii) how the impacts of bathymetric shifts on visual interactions may be distributed across species; (iii) which interactions might be affected; and (iv) the ways that animals have to respond to these changes. As warming continues and temperature fluctuations grow more extreme, many species may move into even deeper waters. There is thus a need for studies that examine how such shifts can affect an organism's visual world, interfere with behaviour, and impact fitness, population dynamics, and community structure.

Phylogenetic analysis and ontogenetic changes in the cone opsins of the western mosquitofish (Gambusia affinis)

To convert external light into internal neural signal, vertebrates rely on a special group of proteins, the visual opsins. Four of the five types of visual opsins—short-wavelength sensitive 1 (Sws1), short-wavelength sensitive 2 (Sws2), medium-wavelength sensitive (Rh2), and long-wavelength sensitive (Lws)—are expressed in cone cells for scotopic vision, with the fifth, rhodopsin (Rh1), being expressed in rod cells for photopic vision. Fish often display differing ontogenetic cone opsin expression profiles, which may be related to dietary and/or habitat ontogenetic shift. The western mosquitofish (Gambusia affinis) is an aggressive invader that has successfully colonized every continent except Antarctica. The strong invasiveness of this species may be linked to its visual acuity since it can inhabit turbid waters better than other fishes. By genome screening and transcriptome analysis, we identify seven cone opsin genes in the western mosquitofish, including one sws1, two sws2, one rh2, and three lws. The predicted maximal absorbance wavelength (λ_max) values of the respective proteins are 353 nm for Sws1, 449 nm for Sws2a, 408 nm for Sws2b, 516 nm for Rh2-1, 571 nm for Lws-1, and 519 nm for Lws-3. Retention of an intron in the lws-r transcript likely renders this visual opsin gene non-functional. Our real-time quantitative PCR demonstrates that adult male and female western mosquitofish do not differ in their cone opsin expression profiles, but we do reveal an ontogenetic shift in cone opsin expression. Compared to adults, larvae express proportionally more sws1 and less lws-1, suggesting that the western mosquitofish is more sensitive to shorter wavelengths in the larval stage, but becomes more sensitive to longer wavelengths in adulthood.

Reversibility of Multimodal Shift: Zebrafish Shift to Olfactory Cues When the Visual Environment Changes

Animals can shift their reliance on different sensory modalities in response to environmental conditions, and knowing the degree to which traits are reversible may help us to predict their chances of survival in a changing environment. Here, using adult zebrafish (Danio rerio), we found that 6 weeks in different light environments alone were sufficient to shift whether fish approached visual or chemical cues first, and that a subsequent reversal of lighting conditions also reversed their sensory preferences. In addition, we measured simple behavioral responses to sensory stimuli presented alone, and found that zebrafish housed in dim light for 6 weeks responded weakly to an optomotor assay, but strongly to an olfactory cue, whereas fish experiencing bright light for 6 weeks responded strongly to the visual optomotor stimulus and weakly in an olfactory assay. Visual and olfactory responses were equally reversible, and shifted to the opposite pattern when we reversed lighting conditions for 6 weeks. In contrast, we did not find a change in activity level, suggesting that changes in multiple sensory modalities can buffer animals from changes in more complex forms of behavior. This reversal of sensory response provides insight into how animals may use sensory shifts to keep up with environmental change.