Gross genome evolution in the Dinosauria

The Dinosaurs dominated the terrestrial environment for around 170 million years and are probably the most successful land vertebrate group to have existed. They survived several mass extinction events before finally all non-avian species were wiped out 66 million years ago in the Cretaceous-Paleogene extinction event. The neornithes (modern birds) are their living descendants. Despite the huge phenotypic diversity seen in birds, they, and some non-avian reptiles (e.g. some turtle species) display remarkably similar karyotypes with a characteristic pattern of macro and micro chromosomes, small genome size and few repetitive elements. This suggests that these were features present early in their evolution.

The availability whole genome sequences and the recent sequencing of around 50 avian genomes, 6 of which were assembled at sufficient read depth and coverage to permit visualization at the chromosomal level, has facilitated the reconstruction of the overall genome structure (karyotype) of both Saurian (bird-reptile) and Avian ancestors. Subsequent use of bioinformatic tools permitted the retracing of the gross evolutionary changes that have occurred along the Dinosaur (and various avian) lineages. Gene ontology analysis of homologous synteny blocks (HSBs) and evolutionary breakpoint regions (EBRs) of chromosomes has allowed us to search for enrichment for genes involved in chromosome rearrangement (consistent with the formation of the signature fragmented karyotype of birds (and probably dinosaurs)). Preliminary analyses of EBRs suggest that they appear to be enriched for genes involved in body size, consistent with the overall gross reduction in body size as dinosaurs evolved into birds. Our results also suggest a period of inter-and intra-chromosomal rearrangements up until around the divergence of turtles (approximately 210 MYA) with a relatively “fixed” pattern thereafter where intra-chromosomal rearrangement plus a few identifiable fissions predominated. It is reasonable therefore to speculate that this ‘avianstyle’ genome may be one of the key factors in the success of this extraordinarily diverse animal group, allowing rapid speciation through increased propensity for random segregation and genetic recombination.