Scientists reconstruct genome of common ancestor of crocodiles, birds, dinosaurs

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."

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Scientists reconstruct genome of common ancestor of crocodiles, birds, dinosaurs