Reconstruction of the putative Saurian karyotype and the hypothetical chromosome rearrangements that occurred along the Dinosuar lineage

Dinosaurs hold a unique place both in the history of the earth and the imagination of many. They dominated the terrestrial environment for around 170 million years during which time they diversified into at least 1000 different species. Reptilia, within which they are placed is one of the most remarkable vertebrate groups, consisting of two structurally and physiologically distinct lineages – the birds and the non-avian reptiles, of which there are 10,000 and 7,500 extant species respectively. The dinosaurs are without doubt the most successful group of vertebrate to have existed. They survived several mass extinction events before finally non-avian dinosaurs were defeated 66 million years ago in the Cretaceous-Paleogene extinction event, leaving the neornithes (modern birds) as their living descendants. Aside from the huge phenotypic diversity seen in this group, the birds and non-avian reptiles interestingly display similar karyotypic patterns (with the exception of crocodilians); with the characteristic pattern of macro and micro chromosomes, small genome size and few repetitive elements, suggesting that these were features exhibited in their common ancestor.

In this study, the availability of multiple reptile genome sequences (including birds) on an interactive browser (Evolution Highway) allowed us to identify multi species homologous synteny blocks (msHSBs) between the putative avian ancestor (derived from six species of extant birds), the Lizard (Anolis carolensis) and the Snake (Boa constrictor). From these msHSBs we were able to produce a series of contiguous ancestral regions (CARs) representing the most likely ancestral karyotype of the Saurian (ancestor of archosaurs and lepidosaurs) that diverged from the mammalian lineage 280 mya. From this we have hypothesised the series of inter and intra-chromosomal rearrangements that have occurred along the dinosaur (archosaur) lineage to the ancestor of modern birds (100 mya) and along the lepidosaur lineage to the modern snake and lizard using the model of maximum parsimony.

Our study shows that relatively few chromosomal rearrangements took place over this period with an average of one inter or intra-chromosomal (translocations and inversions respectively) rearrangement occurring approximately every 2 million years. The majority of these rearrangements appear to be intra-chromosomal suggesting an overall karyotypic stability, which is consistent with that of that of modern birds. Our results support the hypothesis that the characteristically avian genome was present in the saurian ancestor and that it has remained remarkably stable in the 280 million years since. It is credible therefore to suggest that this ‘avian-style’ genome may be one of the key factors in the success of this extraordinarily diverse animal group.