How the clumsy Galapagos cormorant lost its flight

By Ryan CrossJun. 1, 2017 , 3:15 PM

 

Fernandina, the westernmost island in the Galapagos archipelago, is a pristine spot. It is also a place regularly inundated by lava flows that set its waters boiling. Yet that hasn’t stopped one odd bird from calling Fernandina home: the world’s only flightless cormorant. Now, a new study proposes an explanation for how the stumpy-winged seabird lost its ability to fly—through more than a dozen genetic anomalies that it shares with humans suffering from a variety of rare skeletal disorders.
For most birds, flightlessness would be a severe problem. But, as Charles Darwin concluded on his famous voyage to the Galapagos, isolation can allow species with such seeming disadvantages to thrive. The big question for modern scientists is how animals like the flightless cormorant got to be this way in the first place. Unlike penguins, ostriches, kiwis, and emus—which evolved into their flightless forms more than 50 million years ago—the Galapagos cormorant (Phalacrocorax harrisi) diverged from its soaring relatives a mere 2 million years ago. That more recent split suggests a relatively small number of genetic changes differentiate high-flying cormorants from their land-lubber cousins.

University of California, Los Angeles, geneticist Leonid Kruglyak began looking into the evolution of flightless cormorants after visiting the islands. Since he could find no conclusive studies on the large-bodied bird, he set out to sequence its DNA, using samples from the lab of Patricia Parker, an ecologist at the University of Missouri in St. Louis and the St. Louis Zoo. Parker and her team have spent years in the islands, sleeping outdoors and working from converted fishing boats to collect more than 20,000 blood samples from Galapagos animals. Kruglyak’s team then compared the Galapagos cormorant DNA to that of three other related birds—the double-crested cormorant, the neotropical cormorant, and the pelagic cormorant.

Since many developmental genes shoulder multiple roles, Kruglyak’s team reasoned that a genetic factor for flightlessness would not be found in a protein mutation, which could lead to a fatal outcome. Instead, they began searching for irregularities in the vast segments of DNA between genes called the noncoding regions, hoping to find clues about how the same genes might be regulated differently.

But that comparison yielded no results, so they turned back to the coding regions—the genes that produce proteins—to search for mutations that would change a protein’s ability to function normally. They discovered about a dozen mutated genes in the Galapagos cormorants known to trigger rare skeletal disorders in humans called ciliopathies, often characterized by misshapen skulls, short limbs, and small ribcages. Since Galapagos cormorants have short wings and an unusually small sternum, the researchers suspected this link was significant, they write today in Science.

Ciliopathies in humans arise from gene mutations affecting cilia—the microscopic hairlike extensions used to convey chemical messages between cells that control vertebrate development. When those signals go off-kilter, the body can grow in a visibly abnormal way. Sensenbrenner syndrome is one example, a rare condition reported in only a few dozen people characterized by an elongated skull, short limbs and fingers, a narrow chest, and respiratory problems. One of the genes linked to Sensenbrenner, called Ift122, was similarly mutated in the Galapagos cormorant. Another gene responsible for cilia production, Cux1, seemed to play a role in the cormorant’s stubby wings.
Next, the researchers put Ift122 and Cux1 to the test. They inserted the mutated Ift122 gene into soil roundworms, which use cilia to detect their surroundings. Compared to their regular counterparts, the mutated worms clumped together instead of dispersing across their petri dish environment, thanks to improperly functioning cilia. When they inserted the cormorant’s Cux1 gene into cartilage-producing mouse cells growing in a dish, the cells showed stunted development.

But the connection between these genes and flightnessness is still a hypothesis, Kruglyak notes. “The ideal experiment would make a Galapagos cormorant fly or another cormorant not fly,” he says, which one day could be done with a tool like CRISPR gene editing. “As technologies improve, we can imagine testing these gene mutations in birds and watching the wings develop.”
“This study is important and exciting for adding a mechanism for how flightlessness might evolve,” says Natalie Wright, a biologist at the University of Montana in Missoula who studies the evolution of flightlessness on islands. She adds that most researchers suspect flight is lost thanks to changes that cause birds to retain juvenile characteristics into adulthood. The Galapagos cormorant—whose stubby wings make it resemble an overgrown baby bird—is a perfect example.

But researchers caution that this isn’t the end of the story. “The biggest caveat to this study is that the authors did only a relatively basic screen for changes in noncoding regions,” says Tim Sackton, who studies the genomics of flightless birds at Harvard University. No single mutation alone caused the cormorants to lose their ability to fly. So even though it is more straightforward to study the effects of mutations in protein-coding genes, there are likely more, undiscovered mutations that affect flightlessness in the noncoding regions, Sackton suggests.

Do the Galapagos cormorants gain anything by their ungainliness? Parker thinks not. “In fact, it might be possible that the Galapagos cormorant is a little worse at catching fish, since they don’t have to muster up the energy for flight,” she says. Granted, they might just be freeloading off their largely predator- and pathogen-free island abode. “That may be one reason why those bizarre clunky animals are able to trundle along and do just fine,” Parker says.