The Unlikely Relationship Between a Brittle Star and a Sea Pansy

The presence of similar light-emitting enzymes in the distantly related organisms lends new insight into bioluminescence evolution.

By | July 17, 2017

SEA STAR LUMINESCENCE: High sensitivity macrophotography captures a brittle star arm emitting light.JÉRÔME MALLEFET, JÉRÔME DELROISSE

EDITOR'S CHOICE IN EVOLUTIONARY BIOLOGY

The paper
J. Delroisse et al., “A puzzling homology: A brittle star using a putative cnidarian-type luciferase for bioluminescence,” Open Biology, 7:160300, 2017.

Degrees of separation
Although the long-tentacled brittle star (Amphiura filiformis) differs from the stout sea pansy (Renilla) in both appearance and phylogeny, researchers have now demonstrated that they share a similar luciferase—an enzyme that catalyzes the light-producing reaction that results in the invertebrates’ bioluminescence.

Unexpected homology
When an international group of researchers searched the brittle star’s genome and transcriptome for known luciferase sequences, they detected sequences in the echinoderm that were homologous to those of the luciferase of the sea pansy—a cnidarian. The sequences were so similar, in fact, that antibodies specific to the sea pansy luciferase could also detect the brittle star luciferase.  

Defying convention
Conventional dogma states that every taxonomic group has its own distinct luciferase, explains lead author Jérôme Delroisse of the University of Mons; but previous work has found similar homologies in distantly related species. How such different species acquired similar luciferases remains unclear, however.

An evolving hypothesis
The sea pansy’s luciferase has known homology to a nonbioluminescent bacterial enzyme, and the authors uncovered similar proteins in other nonluminous organisms. The data suggest that both brittle star and sea pansy luciferases evolved “from a common ancestral protein originally not involved in light emission,” says Delroisse, and that the gene for this protein horizontally transferred from bacteria to a common ancestor. It’s becoming clearer that “not all independently evolved bioluminescent enzymes have to be structurally different,” says Miriam Sharpe of the University of Otago who was not involved in the study.

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