基因组测序解释长颈鹿为何长脖子

For the first time, the genomes of the giraffe and its closest living relative, the reclusive okapi of the African rainforest, have been sequenced -- revealing the first clues about the genetic changes that led to the evolution of the giraffe's exceptionally long neck and its record-holding ranking as the world's tallest land species. The research will be published in the scientific journal Nature Communications on May 17, 2016. "The giraffe's stature, dominated by its long neck and legs and an overall height that can reach 19 feet (~ 6 m), is an extraordinary feat of evolution that has inspired awe and wonder for at least 8,000 years -- as far back as the famous rock carvings at Dabous in the Republic of Niger," said Douglas Cavener of Penn State University, who led the research team with Morris Agaba of the Nelson Mandela African Institute for Science and Technology in Tanzania.

"The evolutionary changes required to build the giraffe's imposing structure and to equip it with the necessary modifications for its high-speed sprinting and powerful cardiovascular functions have remained a source of scientific mystery since the 1800s, when Charles Darwin first puzzled over the giraffe's evolutionary origins," said Cavener, a professor of biology and the Verne M. Willaman Dean of the Eberly College of Science at Penn State. The giraffe's heart, for example, must pump blood two meters straight up in order to provide an ample blood supply to its brain. This feat is possible because the giraffe's heart has evolved to have an unusually large left ventricle, and the species also has blood pressure that is twice as high as other mammals.

To identify genetic changes likely to be responsible for the giraffe's unique characteristics, including sprints that can reach 37 miles per hour (60 km/h), Cavener and Agaba compared the gene-coding sequences of the giraffe and the okapi to more than forty other mammals including the cow, sheep, goat, camel, and human. "Okapi's gene sequences are very similar to the giraffe's because the okapi and giraffe diverged from a common ancestor only 11-to-12 million years ago - relatively recently on an evolution timescale," Cavener said. "In spite of this close evolutionary relationship, the okapi looks more like a zebra and it lacks the giraffe's imposing height and impressive cardiovascular capabilities. For these two reasons, Okapi's genome sequence provides a powerful screen that we have used to identify some of the giraffe's unique genetic changes."

Using a battery of comparative tests to study the genome sequences of the giraffe and the okapi, the scientists discovered 70 genes that showed multiple signs of adaptations. "These adaptations include unique amino-acid-sequence substitutions that are predicted to alter protein function, protein-sequence divergence, and positive natural selection," Cavener said. Over half of the 70 genes code for proteins that are known to regulate development and physiology of the skeletal, cardiovascular, and nervous system - just the type of genes predicted to be necessary for driving the development of the giraffe's unique characteristics.