Non-excitable fluorescent protein orthologs found in ctenophores

Fluorescent proteins are optically active proteins found across many clades in metazoans. A fluorescent protein was recently identified in a ctenophore, but this has been suggested to derive from a cnidarian, raising again the question of origins of this group of proteins.

Through analysis of transcriptome data from 30 ctenophores, we identified a member of an orthologous group of proteins similar to fluorescent proteins in each of them, as well as in the genome of Mnemiopsis leidyi. These orthologs lack canonical residues involved in chromophore formation, suggesting another function.

The phylogenetic position of the ctenophore protein family among fluorescent proteins suggests that this gene was present in the common ancestor of all ctenophores and that the fluorescent protein previously found in a ctenophore actually derives from a siphonophore.