Along rivers and streams around the world, mayflies are a rite of spring. The mosquito-size insects lead double lives, with the young thriving in water and the adults emerging by the millions around June for just a few hours to mate and quickly die. There can be so many that they clog traffic, make roads slick, and even create a smelly mess.
Now, by sequencing the genome of one remarkable mayfly species—whose males have a second set of skyward-pointing eyes—researchers have learned how aquatic young transform into airborne adults. They’ve also discovered new clues about how all insects evolved to fly in the first place.
The amount of information gleaned from the study is impressive, says Craig Macadam, an entomologist at the U.K.-based nonprofit insect conservation organization Buglife. “It really shows that once we know the genetic makeup of a species, we can start to work out a huge amount about [it].”
Because of their sheer numbers, mayflies are important food for birds, fish, and mammals. They spend most of their lives underwater eating dead bits of leaves and other material. When the temperature warms up in late spring, the muscular young, which look like miniature six-legged shrimp, crawl out of the water and molt into the delicate, two- or four-winged adults. Fly fishermen rush out to streams when the adults emerge because they know fish will be biting.
Evolutionary biologists are intrigued by mayflies because they and their cousin, the dragonfly, belong to an early evolving branch of the insect family tree; indeed, they still look a lot like the earliest airborne insects.
Getting a high-quality genome from a tiny insect is tough, however. Scientists need to combine the DNA from several individuals, and each individual insect has a slightly different genome. So, before they could sequence the mayfly Cloeon dipterum, Isabel Almudi and Fernando Casares, integrative biologists at the Andalusian Center for Developmental Biology and colleagues had to figure out how to breed this species in the lab. That enabled them to sequence the genome and then measure gene activity in all life stages and in a variety of tissues, a first for mayflies.
There were several surprises. For one, the males have two extra light sensing proteins called opsins that females lack. These opsins sense blue and ultraviolent light and are active in the second set of eyes, perhaps helping the male see females above him, the team speculates.
The biggest news, however, came from the genetic activity of the juvenile mayfly’s gills. Once thought to be used only to take in oxygen, mayfly gills also seem to be a nose of sorts. Almudi and her team discovered unusually high numbers of proteins involved in smell and taste in this tissue, they report in Nature Communications.
The finding “may help us understand more about how mayflies sense their surroundings,” Macadam says. These extra proteins may also help mayflies coordinate their mass transformation to adults, suggests Yoshi Tomoyasu, an evolutionary biologist at Miami University.
While they were teasing out genes active in development, the researchers discovered several in juvenile mayfly gills that also play a role in the development of adult wings.
Insect wings are a major evolutionary innovation, and their origin has been hotly debated. “The discovery of common genetic programming between gills and wings is another piece towards understanding the puzzling origins of insect wings—and flight,” says Luke Jacobus, an entomologist at Purdue University who runs a mayfly website.
Some biologists think wings first evolved as an extension of the body wall, whereas others think they evolved from ancient legs. Mayfly gills are thought to arise from the upper legs. So, the finding of similar gene activity in both wings and gills “provides the first genomic support” for the idea that wings and gills evolved at least in part from legs, Tomoyasu says. But, Macadam says, “Whether this means that wings are derived from gills is still debatable.”
Jacobus sees the mayfly genome and these initial findings as just a first step. “I hope that this study stimulates more work on mayflies,” he says. They “have untapped potential,” he says, for understanding how all insects evolved.
sciencemag.org, 2 June 2020