The evolution and specialized metabolism of beetle bioluminescence

Abstract

Fireflies (Lampyridae) and certain other families of beetles including the American railroad worms (Phengodidae), Asian starworms (Rhagophthalmidae), and American click-beetles (Elateridae), produce light in a process known as bioluminescence. The bioluminescent systems of beetles, natively used for the purposes of mating communication and/or an aposematic warning signal, are now well understood and have been widely applied in biotechnology and biomedical research. There have been considerable advancements in the engineering of the luciferin substrate, and the luciferase enzyme, for beneficial characteristics such as altered emission wavelength, improved thermostability, and improved catalytic parameters, but despite this substantial effort focused on the biotechnological applications of beetle bioluminescence, major questions remain regarding its natural biochemistry and evolutionary origins.

Four major questions that were unanswered at the beginning of this PhD study were: (1) Do fireflies possess a storage form of their luciferin? (2) What is the evolutionary relationship of bioluminescence amongst the bioluminescent beetles families, and has this trait independently evolved multiple times? (3) How is firefly luciferin biosynthesized? And (4) Are there accessory genes from the bioluminescent beetles which act in bioluminescent metabolism, and might these genes be useful for biotechnological applications? Here I describe the discovery and characterization of the presumed storage form of luciferin in fireflies, sulfoluciferin, and the enzyme which produces it, luciferin-sulfotransferase.

Furthermore, I describe the sequencing, assembly, and characterization of the genome of the North American "Big Dipper" firefly Photinus pyralis, along with the Japanese "heike" firefly Aquatica lateralis genome, and the genome of the Puerto Rican bioluminescent click beetle or "cucubano" Ignelater luminosus. Genomic comparisons amongst these three species support the hypothesis that firefly and click beetle luciferase evolved independently, suggesting an independent evolutionary origin of the bioluminescent systems between these fireflies and click beetles. I also describe stable isotope tracing experiments in live fireflies, establishing that adult and larval fireflies likely do not de novo biosynthesize firefly luciferin, and may instead rely on a "recycling" pathway to re-synthesize luciferin from the luminescence product oxyluciferin. Lastly, I discuss the future directions resulting from this thesis, and the yet unanswered questions.