FISH

Quanta Magazine

To escape predators beneath the waves, a flying fish can shoot out of the water and glide long distances because its paired pectoral and pelvic fins, longer and more rigid than those of other fish, act as airfoils. In a quirky triumph of evolution, creatures that were once strictly aquatic transformed into temporarily airborne ones through a few modifications in body shape.

Recently, a group of researchers led by Matthew Harris of Harvard Medical School and Boston Children’s Hospital reported the genetic basis for the evolution of those unusual fins: Through an innovative combination of techniques, they discovered that changes in just two genes were sufficient to create the distinctive body shape of flying fishes. When those mutations occurred in a species of common aquarium fish, its proportions began to shift in similar ways.

“When we started out in evo-devo, there was no thinking that we would be able to make these large jumps in form with such simple rules,” Harris said. The study appeared in the November 22 issue of Current Biology.

The findings are noteworthy in part because they hint that bioelectric signals within developing tissues, not just “morphogenetic” chemical ones, can regulate the growth and shape of developing fins and possibly other structures. This study and earlier work also clearly illustrate how small genetic changes can sometimes produce big morphological changes that have important evolutionary consequences.

Much of the unfathomable diversity of animal forms in nature arises from natural selection’s tinkering with the genetic programs that control development. Tweaks to the timing and speed of tissue growth can stretch or shrink structures, or even insert and delete bones, to create novel adaptations that open new niches for species. Evo-devo, the biological study of this process, has a long history, but only relatively recently have researchers been able to start probing for the genes responsible for specific changes.

To search for the genetic basis of the flying fish’s body shape, researchers in the Harris lab began by sequencing and comparing the genomes of 35 species of flying fish and their close relatives. By looking for regions of DNA that had changed unusually quickly between species, they identified genes that seemed to have evolved under selection pressure.

This comparative analysis allowed researchers to look for the main factors driving the formation of the new body type, explained Joost Woltering, an evolutionary biologist at the University of Konstanz in Germany who works on the evolution and development of fin and limb diversity. “But how are you going to find out if the gene really is the smoking gun that makes a difference? You cannot modify it in the flying fish,” he said. “You have to turn to something where you can actually do that.”

Harris’ team accordingly turned to zebra fish (Danio rerio), freshwater minnows widely kept as aquarium pets but also as research animals. His team used chemicals and gamma rays to create random mutations in more than 10,000 zebra fish embryos. They searched those that survived to adulthood for interesting adult traits or phenotypes. This approach was unusual because genetic studies of zebra fish typically focus on the embryonic development of the animals.

Jacob Daane, a postdoc in the Harris lab at the time, and his colleagues also screened a collection of previously known zebra fish mutants with long fins to refine their search for gene variants that might be regulating the growth of the flying fishes’ fins. They homed in on two: kcnh2a, a mutation that makes cells overexpress potassium channels on their outer membrane, and lat4a, a loss-of-function mutation that disables cells’ ability to transport the amino acid leucine.

The researchers showed that in zebra fish, loss-of-function mutations in the leucine transporter cause all fins to be short, while the overexpression mutation of the potassium channels causes all fins to be long. Either of those mutations by itself produces a clumsy fish. But when the two mutations are combined, the resulting zebra fish has long paired pectoral fins and shorter median fins, exactly the form of the flying fish.

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