Jurassic Park: What Did the Genomes of Dinosaurs Look Like?

Abstract

Recent palaeontological evidence is clear that birds are extant dinosaurs. Evolving along the lineage Diapsida—Archelosauria—Archosauria—Avemetatarsalia—Dinosauria—Ornithoscelida—Theropoda—Maniraptora—Avialae, birds are the latest example of dinosaurs emerging from catastrophic extinction events as speciose and diverse. Indeed, rather than being wiped out by the Cretaceous-Paleogene meteor strike, they are the survivors of at least three extinction events. Dinosaurs capture the public imagination through art, literature, television and film, most recently through the Jurassic Park/World franchise. Claims in the scientific literature of isolating dinosaur DNA (from amber-preserved insects or elsewhere) have largely been debunked. Nonetheless, the overall structure of dinosaur genomes along the above lineage can be determined by inference from chromosome-level genome assemblies. Our work focused first on determining the likely karyotype of the avian ancestor (probably a small, bipedal, feathered, terrestrial Jurassic dinosaur) finding great similarity to the chicken. We then progressed to determining the likely karyotype of the diapsid ancestor and the changes that have occurred to form extant animals. A combination of bioinformatics and molecular cytogenetics indicates considerable interchromosomal rearrangement from a “lizard-like” karyotype of 2n = 36−46 to one similar to that of certain turtles from 275 to 255 million years ago (mya). Remarkable karyotypic similarities between some turtles and chicken suggest identity by descent, in other words that, aside from ~7 fissions, there were few interchromosomal changes from the archelosaur (bird-turtle) ancestor to the Avemetatarsalia (dinosaurs and pterosaurs), through the theropods to modern birds. Indeed, a similar rate of change beyond 255 mya would have meant that the avian-like karyotype was in place about 240 mya when the first dinosaurs and pterosaurs emerged. We mapped 49 intrachromosomal changes in the intervening period, finding significant gene ontology enrichment in homologous synteny block and evolutionary breakpoint regions. The avian-like karyotype with its many chromosomes provides the substrate for variation (the driver of natural selection) through increased random segregation and recombination. It thus may impact on the ability of dinosaurs to survive and thrive, despite multiple extinction events.