By David Kelly | University Communications
AURORA, Colo. - Scientists from the University of Colorado School of Medicine who sequenced the genome of the Burmese python have discovered large numbers of rapidly evolved genes in snakes.
These genetic changes are linked to extreme characteristics in snakes, such as rapid increases in metabolism and organ growth after feeding, findings that open a novel window into how evolution works at the molecular level.
"The bottom line is that snakes have undergone incredible changes at all levels of their biology, from the physiological to the molecular," said David Pollock, Ph.D., Associate Professor of Biochemistry and Molecular Genetics at the CU School of Medicine. "Snakes appear to have functionally evolved much more than other species. They are a crucible of evolution."
Pollock is the principle investigator of the study to appear December 2, 2013 in the Proceedings of the National Academy of Sciences. He and fellow researchers, including lead author Todd Castoe, a former postdoctoral fellow at the CU School of Medicine now at the University of Texas at Arlington, found that snakes carry large numbers of proteins with signatures for positive selection in their ancestors.
This paper, and a companion paper in PNAS detailing the genome of the King Cobra, represent the first complete and annotated snake genomes.
"One of the fundamental questions of evolutionary biology is how vertebrates with all the same genes display such vastly different characteristics," Castoe said. "The Burmese python is a great way to study that because it is so extreme. We’d like to know how snakes uses genes we all have to do things no other vertebrate can do."
Extreme characteristics of the snake such as metabolism, spine and skull shape and cell cycle regulation are linked to positive selection in hundreds of genes. "When you have positive selection you have a lot of adaptation going on," Pollock said. "Positive selection is rare, but when it happens we are curious. What we are seeing in snakes is unprecedented."
The researcher said these multiple adaptive bursts caused evolutionary redesign of many proteins in the snake.
"We first saw these unusual molecular patterns in the snake mitochondrial DNA, and now it seems they extend throughout the nuclear genome," Pollock said.
The team also found that after Burmese pythons ate, they experienced massive changes in gene expression linked to 35 to 100 percent size increases in their heart, small intestine, liver and kidneys in just 24-48 hours.
Meanwhile, the snake’s metabolism, among the lowest of any vertebrate, ramped up significantly. Pollock compared the increase to a horse going from standing still to running a quarter mile race; but the snake isn’t moving, just digesting.
"Genes that were fully off are now full on," he said. "Snakes eat animals as big as themselves. Once they catch something that size, they need to digest it quickly before it rots in their stomach, and they have to turn a lot of genes on to do it."
The study said phenotypic novelty in snakes appears to be driven by the system-wide coordination of protein adaptation, gene expression and changes in the genome structure.
Pollock noted that the findings not only offer insights into how evolution works at the molecular level but they also have implications for humans.
"What we are seeing now can apply to people," he said. "We can link mutations to physiological effects and perhaps find a way to stop those mutations before they cause disease. There are any number of possibilities and we are only starting to unravel them."
A full list of authors is available online at www.pnas.org. The title of the paper is "The Burmese python genome reveals the molecular basis for extreme adaptation in snakes."