Where’d you get them eyes?

Molecular Evolution of Malacostracan Short Wavelength Sensitive Opsins
Mantis shrimp (Stomatopods) are believed to have the most complex visual system in kingdom Animalia. Whereas apes get by with just three opsins, the mantis shrimp may have as many as 33 distinct opsin sequences. This extreme opsin repertoire reflects a long history of profligate gene duplication. These arthropods can detect light from red to near UV, and detect polarized light as well.

There is speculation that the many photoreceptor types in the compound eye serves the same purpose as the highly-developed color-specific neurons in the human brain. Both provide an efficient system for color identification, but shrimp eyes pre-process color, and so do not require a larger brain to process the information. The advantage of their highly developed visual capabilities is incompletely understood, but may lie in improved prey or predator recognition, accurate depth perception, and/or in mating rituals where female mantis shrimps actively fluoresce.

The mantis shrimps’ broad spectrum of light sensitivity is achieved with a single photosensitive cofactor, retinol. When the diverse opsin proteins bind retinol, their interaction with its electronic structure determines the absorption spectrum. Depending on the opsin sequence, the absorption maxima range from 700 to 300 nm.

Palecanda and co-workers studied the relationship between opsin sequence and absorption spectrum for the UV-detecting mantis shrimp. Their goal was to understand how the local protein environment tunes the absorption spectrum of the bound cofactor.

Prior studies have identified three basic pancrustacean opsin protein groups originating from ancestral duplication events. These are termed Long Wavelength (LWS), Medium Wavelength (MWS), and Short Wavelength (SWS/UV) based on their absorption maxima. The authors reviewed public RNAseq data from a set of pancrustaceans, then used CIPRES to create a maximum likelihood tree using RAxML. New sequences were determined from eight additional Stomatopod species, providing a total of 24 SWS opsin sequences from 13 Stomatopod species.

The authors then looked for evidence of key residues near the bound retinol molecule that might “tune” the absorption spectrum.

Key Findings:

  • Most mantis shrimp possess 2 or 3 SWS opsins, depending on the species, whereas decapods more typically have just 1.
  • The SWS opsins of Stomatopods fall into two clades, cUV1 (which has two sub clades) and cUV2. There was minimal support for previously described SWa and SWb clades.
  • While UV vision in invertebrates has been ascribed to a single Glu -> Lys substitution based on results from Drosophila opsins, this correlation does not hold for Stomatopod opsins.

Bottom Line: The work helps clarify the evolutionary history of the Stomatopod SWS proteins. It provides a great starting point for detailed experimental evaluation of the influence of opsin sequence on the absorption spectrum of covalently bound retinal. Together with recent work on the Jewel beetle it is clear the relationship between protein sequence and absorption spectrum is not always simple, obvious, or easy to predict.

hummingbird in flight

Get 1000 Hours free

On the UCSD Supercomputer

Start Your Trial