How do fireflies get their glow? We finally have some answers.

Each summer, fireflies famously take to the evening skies in an otherworldly display of bioluminescence, emanating electric shades of green, yellow, orange, and even blue. More than 2,000 firefly species are known to science, each with its own unique pattern of light flashes and differently shaped light organs.

But have you ever wondered how these luminous insects do what they do? Scientists sure have—and now they’ve discovered two key genes that allow it to happen.

“There are two crown jewels in firefly study,” say Xinhua Fu, a biologist at the University of Huazhong Agricultural University in China, in an email. (Fireflies are vanishing—but you can help protect them.)

The first is how firefly larvae, or glowworms, rapidly develop their light organs into a completely different physical structure as adults. The other is how adults can control these organs—called lanterns—so precisely, for instance by blinking them on and off in Morse code-like patterns when an alluring mate is nearby. (Here's how bioluminescence works in nature.)

In a new study published this week in Nature Communications, Fu and his co-author, Xinlei Zhu, decoded the genome of the aquatic firefly, Aquatica leii, found in rice paddies in China. 

In doing so, they zeroed in upon two key genes responsible for the formation, activation, and positioning of this firefly’s light organ: Alabd-B and AlUnc-4. It was a surprise, as these genes, already known to aid a firefly’s development, had never been linked to bioluminescence.

The finding, Fu says, could help scientists finally understand how these beetles evolved the ability to light up like a firework. (See fireflies magically illuminate this national park.)

It's also crucial work, Fu says, as fireflies are in a precipitous freefall across the globe. Eighteen species now face extinction in the United States, and many more globally, due to a combination of light pollution, pesticide use, loss of habitat, and other factors.

The secret of the firefly’s flash

As it turns out, the firefly central to the new study is a bit of a darling for Fu: He discovered the species and has been studying it since 2000, even setting up a captive colony in the lab that produces up to 600,000 fireflies a year. This population allowed him to intensify study of the exact 24-hour window of time right after the pupa form, which is when their adult light organs develop.

In their recent experiment, Fu and Zhu began tweaking these insects’ genome to see what effect was made by turning off or knocking out certain genes. They discovered that during pupation, the firefly activates its Alabd-B and AlUnc-4 genes, which cause the adult lantern to develop in the proper position within the abdomen. 

Jing-Ke Weng, a biochemist and director of the Institute for Plant-Human Interface at Northeastern University in Boston, called the new study “impressive” for its use of advanced genomic tools and analyses.

What’s more, Weng, who was not involved in the research, says it lays the groundwork for understanding how fireflies’ common ancestor co-opted these two genes to produce light. (Learn how these fireflies can synchronize their light shows.)

Still, many challenges lie ahead for expanding on the evolutionary research. For one, due in part to their complex courtship, rearing fireflies is much more difficult than raising mice, fruit flies, or other common study animals

What’s more, the study is only limited to males of one species, A. leii, which means this pathway remains relatively unexplored in half the population—the females. Fu plans to rectify this in a follow-up study.