The enigmatic xenopsins

Abstract

A new member of the family of light-sensitive proteins called opsins has stirred up our view of photoreceptors.

Keywords: Leptochiton asellus; evolution; evolutionary biology; eye; genomics; invertebrates; neuroscience; opsin; photoreceptor.

Figure 1.. The evolution of opsins and photoreceptor cells.

The animal evolutionary tree (light grey) shows the relationships between a number of different animal phyla; the question mark indicates the current uncertainty about when the ctenophores emerged. Different phyla have evolved their own photoreceptors (top; ‘r’ denotes rhabdomeric photoreceptors, ‘c’ denotes ciliary photoreceptors) that each use specific opsins to detect light (named in the panel of its respective photoreceptor). To investigate when these opsins and photoreceptors evolved, Vöcking et al. determined the molecular phylogeny of ten families of opsins (bottom left; colored lines correspond with the colored text in the bottom right box). Further confirmation that the bracketed opsins are closely related came from an analysis that shows that their genes share introns. Placing the opsin molecular phylogeny (colored lines in main image) on top of the animal evolutionary tree (light grey) suggests that if both are correct, certain opsins (such as the xenopsins) must have been independently lost from multiple phyla during evolution. Opsins first evolved after the divergence of the Placozoa (lightning bolt) and then went through an early period of diversification (striped box). Numbers in circles mark opsin family relationships that are supported by evidence from shared introns. Crosses indicate the loss of an opsin in a given lineage. Dashed lines indicate opsin relationships inferred from the intron comparison.