Abstract
BackgroundOpsins are key proteins in animal photoreception. Together with a light-sensitive group, the chromophore, they form visual pigments which initiate the visual transduction cascade when photoactivated. The spectral absorption properties of visual pigments are mainly determined by their opsins, and thus opsins are crucial for understanding the adaptations of animal eyes. Studies on the phylogeny and expression pattern of opsins have received considerable attention, but our knowledge about insect visual opsins is still limited. Up to now, researchers have focused on holometabolous insects, while general conclusions require sampling from a broader range of taxa. We have therefore investigated visual opsins in the ocelli and compound eyes of the two-spotted cricket Gryllus bimaculatus, a hemimetabolous insect.ResultsPhylogenetic analyses place all identified cricket sequences within the three main visual opsin clades of insects. We assign three of these opsins to visual pigments found in the compound eyes with peak absorbances in the green (515 nm), blue (445 nm) and UV (332 nm) spectral range. Their expression pattern divides the retina into distinct regions: (1) the polarization-sensitive dorsal rim area with blue- and UV-opsin, (2) a newly-discovered ventral band of ommatidia with blue- and green-opsin and (3) the remainder of the compound eye with UV- and green-opsin. In addition, we provide evidence for two ocellar photopigments with peak absorbances in the green (511 nm) and UV (350 nm) spectral range, and with opsins that differ from those expressed in the compound eyes.ConclusionsOur data show that cricket eyes are spectrally more specialized than has previously been assumed, suggesting that similar adaptations in other insect species might have been overlooked. The arrangement of spectral receptor types within some ommatidia of the cricket compound eyes differs from the generally accepted pattern found in holometabolous insect taxa and awaits a functional explanation. From the opsin phylogeny, we conclude that gene duplications, which permitted differential opsin expression in insect ocelli and compound eyes, occurred independently in several insect lineages and are recent compared to the origin of the eyes themselves.
Highlights
Opsins are key proteins in animal photoreception
Cricket opsin paralogs cluster in the three visual opsin clades of insects To add to the understanding of insect opsin expression and evolution, we have cloned visual opsins in a hemimetabolous insect, the two-spotted cricket Gryllus bimaculatus (Gb)
In order to clarify their evolutionary origin, we have reconstructed a molecular phylogenetic tree of arthropod visual opsins based on the Maximum likelihood algorithm (Figure 2; see Additional File 4)
Summary
Opsins are key proteins in animal photoreception. Studies on the phylogeny and expression pattern of opsins have received considerable attention, but our knowledge about insect visual opsins is still limited. Researchers have focused on holometabolous insects, while general conclusions require sampling from a broader range of taxa. Visual opsins are key proteins in animal photoreception. They belong to the superfamily of G-protein coupled transmembrane receptors and form visual pigments together with a light-sensitive prosthetic group, the chromophore. Even though questions concerning the evolutionary history of opsins have lately received considerable attention (for a recent review see [1]), we still have very limited knowledge of insect visual opsins. Comprehensive data on earlierbranching insect lineages are missing, which makes generalized conclusions questionable
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