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Rafael L. Rodríguez, Thomas K. Wood, Frank W. Stearns, Robert L. Snyder, Kelley J. Tilmon, Michael S. Cast, Randy E. Hunt, and Reginald B. Cocroft : Read the article
How do mating signals diverge early in speciation? Experimental host shifts with Enchenopa treehoppers resulted in subtle but non-trivial signal divergence in a few generations. This was fueled by standing genetic variation and plasticity, and unrelated to host specialization
One of my favorite things to experience at a music concert is hearing an artist change up their song mid-performance by tweaking a lyric or hitting a new note. These changes are often satisfying to the ear, and they almost seem like a love language between that artist and that crowd. After all, the artist went out of their way to make your concert feel unique by crafting a location-specific lyric or singing a new note just for you to enjoy.
For us humans, modifying music like this is a form of entertainment. For many species of animals, though, even minor adjustments to calls, songs, and chirps can be the difference between finding a mate and being single for the rest of the breeding season. Over the course of many generations, these minor adjustments can become fixtures that begin to separate different groups from one another.
For one species to split into two, a lot of changes—ranging from adapting to new locales to only mating within one’s subgroup of the population—need to occur. In their new article, Rodríguez et al. set out to understand the early stages of one species of treehopper bug splitting into multiple and the roles that genetics and the environment play in facilitating that divide.
Male treehoppers produce vibrations to court females, forming a duet if successful. These vibrations are strikingly complex, with an initial elongated whine followed by a rhythm of staccato pulses. Females are particular with which signals they respond to, and even slight deviations in sound characteristics like pitch can take a male from being the talk of the plant to being the lone wolf at the end of the branch.
During a multi-year, collaborative experimental evolution project, Rodríguez et al. raised populations of treehoppers on three different host plants for five generations. Then, they collected eggs and newly hatched nymphs from these treehopper lineages and placed them on either their original host plant species or their new one. Once they reached adulthood, they recorded the treehoppers’ mating signals and analyzed what they heard.
The authors found that when treehoppers were evolving in enclosures with both their original and new plant hosts, any observed changes in the males’ signals when raised on either plant were unique to the identity of the plant on which they evolved. For one host shift, the signals stayed the same between plants. However, in a different shift, signals were different due to 1) changes in the treehoppers’ genetics, 2) changes in which plant the eggs and males were reared on, and 3) changes in genetics that affect how signals vary by which plant they were reared on.
These signal changes were observable after only five generations of being raised on novel host plants. Together with past information about how well treehoppers adapt to new host plants, these authors conclude that sexual divergence (i.e., the changing acoustic signals) occurs very early in the process of one treehopper species diverging into multiple—a process commonly known as speciation— while adaptation to a new host occurs separately and likely later in evolutionary time. This means that as populations start on separate evolutionary trajectories, the first fork in the road may originate in deciding with whom to mate.
So with all this in mind, the next time you’re at a show and the artist sings a note at a different key, you can think to yourself: is this the artist simply being flexible with their range, or could this be the start of an entirely new love language?
Derek Wu is a Ph.D. student in the Population Biology, Ecology, and Evolution program at Emory University in Atlanta, Georgia. Working in Nicole Gerardo’s insect-microbe ecology and evolution lab, he researches the mutualism between squash bugs and their bacterial gut symbiont. He is interested in the environmental, host, and microbial factors that mediate bacterial coinfections in the squash bugs. Outside of lab, he enjoys watching shows and listening to music with his cat, Archie.
