Rather, overexpression of the potassium channel changes the resting membrane potential and the pH of the cytoplasm, which makes the cell more active and responsive. “It’s not like a receptor-ligand interaction where things are binding to the receptor on the inside of the cell, and they travel down and trigger transcription of something,” Harris said. How the potassium channel mutation causes the extra growth in the fin is still a mystery. “Nature has targeted the same specific gene in a couple of different contexts,” said Sarah McMenamin, a fish evolutionary developmental biologist at Boston College. The leucine transporter mutations in the different lineages are not identical, but they cause the same amino acid change - a clue that the lineages independently hit on the same genetic trick to evolve this shape. The flying fish body plan evolved independently several times in various lineages, and it always relied on the same types of mutations in the leucine transporter and the potassium channel. “I don’t know of many other systems where there’s that level of simplicity in terms of major scale changes to an organ’s size like that.” “A single point mutation in some cases can give you really, really big fins,” said Daane, who recently launched his own lab at the University of Houston.
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. Either of those mutations by itself produces a clumsy fish. 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. 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. 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. This approach was unusual because genetic studies of zebra fish typically focus on the embryonic development of the animals. They searched those that survived to adulthood for interesting adult traits or phenotypes. His team used chemicals and gamma rays to create random mutations in more than 10,000 zebra fish embryos. Harris’ team accordingly turned to zebra fish ( Danio rerio), freshwater minnows widely kept as aquarium pets but also as research animals. “You have to turn to something where you can actually do that.”
“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. 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. By looking for regions of DNA that had changed unusually quickly between species, they identified genes that seemed to have evolved under selection pressure. 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. The study appeared in the November 22 issue of Current Biology. “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. 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. In a quirky triumph of evolution, creatures that were once strictly aquatic transformed into temporarily airborne ones through a few modifications in body shape. 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.
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