A genome analysis overturns a leading hypothesis about the origin of fireflies. It is hypothesized that the bright light emitted by many species of the beetle family Lampyridae - better known as fireflies - originally evolved as a warning signal to predators, indicating the fireflies' toxicity, and was then repurposed as a mating signal. This explanation could explain why eggs, larvae, and pupae also glow.
YingZhen and his colleagues put the conventional wisdom to the test by compiling a family tree of fireflies and tracing the evolution of lucibufagins, the compounds that make fireflies toxic. The team collected fresh samples of 16 species of Lampyridae, as well as two related species, from different locations in China, which they analyzed alongside previous collections and genetic data. In total, the authors compiled genome-level data for 41 species. For each species, the authors also used liquid chromatography-mass spectrometry to look for lucibufagins.
The team was able to show that lucibufagins are only found in one subfamily of fireflies, whereas bioluminescence is widespread throughout the family, strongly suggesting that this toxin evolved after the development of bioluminescence.
So why do fireflies start glowing? Luciferin, the substrate for firefly bioluminescence, has previously been shown to have antioxidant properties. YingZhen and his colleagues found that firefly ancestors evolved and diversified during a historical period in which atmospheric oxygen levels continued to rise from historic lows following the ToarcianOceanic anoxic event.
The authors also note that glowing millipedes are thought to have originally evolved bioluminescence in response to oxidative stress in hot, dry environments, suggesting that fireflies may have followed a similar path.