Why the Galapagos cormorant lost its ability to fly
A new study points to a number of genes that may underlie the loss of flight in the Galapagos cormorant. Intriguingly, the genes are analogous to those associated with some bone disorders in humans. The evolution of loss of flight is one the most recurrent limb modifications encountered in nature, and a phenomenon that Darwin cited as an argument in favor of natural selection. He proposed that loss of flight could evolve as a result of positive selection for larger bodies or as a result of relaxed selection due to the absence of predators. To gain a better understanding of the genetics underlying the loss of flight, Alejandro Burga et al. analyzed the genomes of the Galapagos cormorant (Phalacrocorax harrisi) – which, exhibiting stunted wings, is the only flightless cormorant – and three other species of cormorant. In the Galapagos cormorant but not the other species, they found enrichment for genes that are associated with human developmental disorders, including those affecting limb development, such as polydactyly, syndactyly, and duplication of limb bones. Many of these disorders are related to abnormal production of cilia, the antennae of cells. The researchers then studied two of the genes, Ift122 and cux1, by editing them into mice and roundworms, respectively. In the mice, expression of cux1 disrupted the differentiation of chondrocytes, cells essential to bone growth, while in the roundworms, Ift122 expression impaired cilia function. The authors propose that these genes resulting in bone and cilia deficits may have been positively selected in the Galapagos cormorant, as shorter wings could be an advantage during diving by decreasing the buoyancy of the bird. A Perspective by Kimberly L. Cooper discusses these finding in greater detail.
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