A staple of warm summer nights, fireflies have charmed generations with their magical evening glow. Children setting out to capture them in jars can tell you the trick is to catch sight of each flash of light as the bugs fly around. What we might not realize as children is that fireflies emit their greenish glow as a signal to potential mates while they navigate their habitats. Male fireflies use specific colors and patterns of light flashes like a sexy Morse code to signal their interest to nearby females. In return, females signal back to high-quality males by emitting their own light.
The pattern of light flashes and color of light varies from species to species. Because mating with the wrong species can lead to weak or dead on arrival offspring, it is important for each firefly to detect differences in signals among species to ensure it mates with fireflies of the same species. In order to differentiate between species, fireflies must detect subtle color changes on a spectrum between green and yellow. Previous studies, however, have not investigated whether fireflies are better at perceiving colors associated with their species compared to colors that are associated with other species.
In order to detect different colors of light, all animals rely on specialized proteins called opsins. There are several different types of opsins, each of which has its own spectral sensitivity to light. For example, one opsin might be sensitive to blue light whereas another might be responsive to red light. Insects usually have three types of opsins: one to detect blue, one to detect red (long wavelength), and one to detect ultraviolet light. While it is not uncommon for insects to lose or gain opsins as speciation events occur or environmental demands change, previous studies have shown that, in general, fireflies can respond to all of these colors. But does their ability to see color depend on each species’ specific mating behaviors? For instance, are fireflies that emit green light better at seeing green than fireflies that emit yellow light?
To answer these questions, University of Georgia scientists Sarah E. Sander and David W. Hall tested whether fireflies that use different colors to find mates have evolved different opsin proteins to detect color differences. They compared levels of gene expression for genes that encode opsins in the eyes of 38 different firefly species. If fireflies evolved different opsin proteins to detect color differences, then they would expect to see differences in gene expression in opsin genes across species. For example, if a firefly species was better at detecting green light than yellow light, they would expect to see increased gene expression for the blue light opsin gene compared to species who do not use green light. If there were no differences in gene expression between fireflies with different color emissions, then the authors could conclude that ability to detect color was not due to differences in opsin gene expression.
Across all 38 species, the scientists found evidence for only two opsin proteins: ultraviolet and red light. Ultraviolet opsins help fireflies detect sunlight and decide when to start signaling, but because fireflies do not emit ultraviolet light, this opsin does not play a role in signaling to mates. Instead, this finding indicates that any differences in the fly’s ability to detect green or yellow light must derive from differences in the red opsin. This is a surprising finding because green and yellow are closer to blue on the color spectrum than to long, red wavelengths.
Because all of the fireflies tested had the same two opsins, the researchers suggested that mate-finding behavior does not change the types of opsin present across species. However, small mutations in the genes encoding each opsin could arise between species and alter the sensitivity of the opsin to green and yellow light. To test this, they looked at specific changes to the amino acid sequences of each opsin protein using genomic sequencing for mutations that might affect the color sensitivity of each opsin. Mutations in the long wavelength opsin indicate changes in signal detection, whereas mutations in the ultraviolet opsin indicate changes to background light detection since fireflies do not emit ultraviolet light. Here, Sander and Hall found that there were in fact mutations in the amino acid sequences of the long wavelength detecting opsin, but only in four of the 38 species. These four species had recently transitioned to using aerosolized chemical signals called pheromones to find mates during the day rather than emitting flashes of colored light. The ultraviolet light opsin did not show any signs of mutations.
These results demonstrate that despite using different shades of green and yellow light to signal for mates, all of the firefly species tested were capable of detecting each color. This may indicate that light signals for mating are not reflected in changes to opsin proteins or the divergence between species has occurred too recently to drive opsin changes. Without sensitivity to specific colors fireflies are still able to discriminate between colors, likely because the spectral overlap between red and ultraviolet opsins can be used to distinguish green and yellow. It is only when fireflies stop using light to signal to each other that their ability to detect light is changed. It’s unclear why this change in mating behavior, flirting with pheromones, would lead to evolutionary changes in the detection of an entirely different signal. The researchers did not find fireflies using light signals with the same mutations, indicating that in this case, the fireflies adopted this new mating behavior before opsin evolution began. One possibility is that mutations in the red, long wavelength opsins were inherited when the ability to distinguish green and yellow light no longer prevented individuals from finding a mate.
Fireflies are both a beloved member of many backyard ecosystems and a great model for studying the evolution of signal detection. In this study, fireflies’ ability to detect light as a mating signal isn’t specific to the color of light they use and only changes after they evolve a new mating strategy. While this study does not address whether this is advantageous to the fireflies or not, it offers us a glimpse into the paired, or in this case unpaired, evolution of signals and signal detection. The ability to detect all light signals, rather than just that of one species, indicates that fireflies maintain a certain amount of flexibility in signal detection. If they no longer rely on light to find mates this perhaps allows individuals with mutations for detecting pheromones to pass on their new opsins to the next generation. All creatures rely on signals to remain healthy and successful in their environments, but with a rapidly changing world, many animals who find themselves in habitats modified by humans will have to adapt and evolve to maximize their signal detection. Flexibility, like what these fireflies have demonstrated with their ability to see color, will be necessary for staying safe and healthy as the environment changes. By studying how mating behaviors of fireflies alter signal detection, as well as signaling changes in other species, we can begin to understand how genetic flexibility can help animals adapt to new conditions and perhaps predict how our own actions could be impacting their evolution.
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