Category Archives: Transhuman News

PUBLIC RELEASE DATE:

11-Dec-2014

Contact: Tim Stephens stephens@ucsc.edu 831-459-4352 University of California - Santa Cruz @ucsc

Crocodiles are the closest living relatives of the birds, sharing a common ancestor that lived around 240 million years ago and also gave rise to the dinosaurs. A new study of crocodilian genomes led by scientists at UC Santa Cruz reveals an exceptionally slow rate of genome evolution in the crocodilians (a group that includes crocodiles, caimans, alligators, and gharials).

The UC Santa Cruz team used the crocodilian genomes, combined with newly published bird genomes, to reconstruct a partial genome of the common ancestor of crocodiles, birds, and dinosaurs. The study, part of an ambitious international collaboration to analyze the genomes of modern birds and gain insights into their evolution, is one of eight papers from the Avian Phylogenomics Consortium being published in a December 12 special issue of Science.

Richard E. (Ed) Green, lead author of the crocodilian genome paper and an assistant professor of biomolecular engineering at UC Santa Cruz, said the slow evolutionary rate in the crocodilian lineage was helpful in reconstructing the genome of the common ancestor.

"The ticking of the molecular clock in the crocodilians is much slower than in other lineages we're used to looking at, like mammals, which means we can see back into their past more cleanly," Green said.

The reconstructed genome of the common ancestor will be a valuable tool for investigating the evolution of the "archosaurs," the group that includes all dinosaurs, pterosaurs, birds, and crocodilians. (Crocodilians are actually more closely related to birds and dinosaurs than they are to other reptiles, i.e., lizards, snakes, and turtles.) Green said the genome reconstruction effort, led by UC Santa Cruz research specialist Benedict Paten, yielded about half of the genome sequence of the common ancestor with an accuracy of about 91 percent, and he expects that to improve as more data on bird and crocodile genomes become available.

The team sequenced the genomes of three crocodilian species: the American alligator, the saltwater crocodile, and the Indian gharial. Their analysis indicates that the ancestor of all archosaurs probably had an extremely slow rate of molecular evolution, and that the rate of change sped up in the bird lineage. The rate of molecular evolution of crocodilians is an order of magnitude slower than that of mammals. The most likely reason for this relates to the relatively long time between generations in crocodilians, Green said.

"When it takes longer to get from one generation to the next, you expect the evolutionary rate to be slower, and big animals tend to have long generation times," he said. "We know from fossils that the body plan of crocs has remained largely unchanged for millions of years. Mammals, however, if you go back 50 or 60 million years there were no big mammals, so we see a faster rate of evolutionary change."

Read more from the original source:
Scientists reconstruct genome of common ancestor of crocodiles, birds, dinosaurs

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A Texas Tech University biologist led a team of more than 50 scientists who mapped the genomes of three crocodilians.

By mapping these genomes, scientists may better understand the evolution of birds, which are the toothy predators' closest living relatives, said David Ray, an associate professor of biology. The team completed genomes of a crocodile, an alligator and a true gharial to complete the genomic family portrait.

"One of the major finds in our case was that crocodilian genomes change very slowly when compared to birds," Ray said. "We compared both birds and crocodilians to turtles, which are the closest living relatives of the group that includes both birds and crocodilians. We found that they evolved slowly also. The best explanation for this is that the common ancestor of all three was a 'slow evolver,' which in turn suggests that rapid evolution is something that evolved independently in birds."

Research began in 2009 as an attempt to map only 1 percent of crocodilian DNA. However, shortly after starting, the price for mapping a million bases dropped from $1,000 eventually down to $1.

"We had proposed to sequence about 2.4 million bases from the three crocodilians in the original proposal," Ray said. "By the time we got the funds, it became clear that we could easily accomplish a thousand times that much and could afford to sequence an entire genome of 3 billion bases."

Ray said that when biologists look at a group of organisms, they look for what makes that group unique as well as what all members of one group of organisms share that other groups do not. The best way to do that is to examine their closest relatives.

"Technically, birds' closest relatives are the dinosaurs," he said. "So we can only look at their fossils and this can provide only limited information on their biology when compared to examining organisms that are alive today. We get insight into differences in behavior, structures that don't fossilize, and in our case, the makeup of the genome."

Ray said he and other scientists were surprised to see how genetically uniform the alligators that the group sequenced were. Initially, the group suspected severe hunting during most of the 20th century may be to blame.

"Because alligators underwent a severe population decline, we first thought that might be what happened," he said. "However, we see the same pattern in all three species and the likelihood that all three were subject to the same genetic bottlenecks is small. We suggested instead that change just occurs slowly in crocodilians. In other words, it wasn't that the genetic differences were reduced because of overhunting. Rather, the amount of variation in crocodilians is low because change simply occurs slowly in these genomes."

The DNA in alligators, crocodiles and gharials is about 93 percent identical across the genome. By comparison, a human shares about 93 percent of his or her DNA with a macaque.

Read more:
Biologists map crocodilian genomes