How a Galapagos bird got tiny wings: cormorant's flightlessness tied to several genetic changes.
Galapagos cormorants (Phalacrocorax harrisi) are the only cormorant species with wings too small to lift the birds off the ground. Broken primary cilia--antennas that cells need to receive developmental messages--left the birds with stunted wings, evolutionary biologist Alejandro Burga suggested May 12.
Burga, of UCLA, and colleagues compared DNA of flightless Galapagos cormorants with that of their close relatives, which have large wings and can fly. The researchers found more than 23,000 differences in more than 12,000 genes. Those changes have occurred within the last 2 million years, a short time by evolutionary standards.
The researchers narrowed down which genes might have had the biggest effect on cormorant evolution using a computer program that predicts whether a change in a gene will affect function. Of the genes predicted to have altered function, the researchers selected the 3.3 percent that have changed most drastically.
To determine what these genes do, Burga examined whether any of the human versions of these genes cause problems when they are mutated in people. Eight of the banged-up genes were associated with limb defects caused by faulty primary cilia, hairlike structures that grow from cells. The cilia receive signals important for cell development and functioning (SN: 11/3/12, p. 16). In people, genetic mutations that damage primary cilia lead to a variety of diseases, including developmental defects.
Normal versions of those eight genes are necessary for primary cilia to pick up signals sent by an important protein called hedgehog. Three other genes that are mutated in the flightless cormorants affect other aspects of the primary cilia.
It wasn't clear whether the cilia defects were the primary cause of the birds' flightlessness. So Burga focused on 10 of the altered Galapagos cormorant genes predicted by the computer program to give the biggest functional and evolutionary consequences. Those genes would be the most important wing shrinkers, Burga and colleagues reasoned.
One of those genes is called CUX1. The protein it produces helps turn on other genes. Most vertebrates have nearly identical versions of the gene. But in flightless cormorants, four amino acids have been lost from the protein, suggesting that it can no longer do its job or does it poorly. In chickens, a defective form of CUX1 shrinks wings. Galapagos cormorant's altered form of CUX1 might also make wings smaller because it fails to turn on limb growth genes.
Many researchers would have left the story at that, says evolutionary biologist Ludovic Orlando of the University of Copenhagen. But "they made an effort to validate their findings."
Burga and colleagues wondered whether CUX1 and the primary cilia changes were related. The team injected cells used to mimic skeletal development in lab dishes with the normal vertebrate version of CUX1. Activity levels of two cilia genes rose by about 50 percent. That is evidence that CUX1 normally helps to regulate activity of primary cilia genes.
But the Galapagos cormorant version of CUX1 barely budged activity of the cilia genes. It also was not as good at stimulating growth and development of bone cells as the normal version, the researchers found. Those findings strengthen the case that CUX1 and primary cilia together were involved in shrinking the flightless fowl's wings.
Caption: Galapagos cormorants' wings have dwindled so much over the last 2 million years that the birds can no longer fly. New genetic data implicate faulty cellular antennas, called primary cilia, in shrinking the wings.
Please note: Illustration(s) are not available due to copyright restrictions.
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|Title Annotation:||GENES & CELLS|
|Author:||Saey, Tina Hesman|
|Date:||Jun 11, 2016|
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