Dinosaurs: Comparative Cytogenomics of Their Reptile Cousins and Avian Descendants

SIMPLE SUMMARY: Dinosaurs have been in scientific and popular culture since early fossil discoveries, but increased interest, particularly in their genomes, is expanding. Birds are reptiles, specifically theropod dinosaurs, meaning that if we compare the genomes of related reptile relations, we can get an idea of what the extinct dinosaur genomes looked like. In all animals/plants/fungi, we think of genome organization in terms of chromosomes. Genes sit on chromosomes and each cell of each individual of each species has its own unique organization. Every gene is in exactly the same spot on each chromosome, organized like continents and islands, with the genes as the cities/towns/villages. All reptiles apart from crocodilians have both big and small chromosomes in their genomes but birds particularly so, like the Philippines or Polynesia. Birds have ~80 chromosomes (far more than most organisms) and this is very consistent in most species. Recent studies suggest that this pattern was probably established ~255 million years ago as it is also mostly present in some turtles. In other words, most dinosaurs probably had chromosomes (genome organization) like chickens or emus. In this paper, we present ideas of how this may have contributed to dinosaurs being so diverse in appearance and function. ABSTRACT: Reptiles known as dinosaurs pervade scientific and popular culture, while interest in their genomics has increased since the 1990s. Birds (part of the crown group Reptilia) are living theropod dinosaurs. Chromosome-level genome assemblies cannot be made from long-extinct biological material, but dinosaur genome organization can be inferred through comparative genomics of related extant species. Most reptiles apart from crocodilians have both macro- and microchromosomes; comparative genomics involving molecular cytogenetics and bioinformatics has established chromosomal relationships between many species. The capacity of dinosaurs to survive multiple extinction events is now well established, and birds now have more species in comparison with any other terrestrial vertebrate. This may be due, in part, to their karyotypic features, including a distinctive karyotype of around n = 40 (~10 macro and 30 microchromosomes). Similarity in genome organization in distantly related species suggests that the common avian ancestor had a similar karyotype to e.g., the chicken/emu/zebra finch. The close karyotypic similarity to the soft-shelled turtle (n = 33) suggests that this basic pattern was mostly established before the Testudine–Archosaur divergence, ~255 MYA. That is, dinosaurs most likely had similar karyotypes and their extensive phenotypic variation may have been mediated by increased random chromosome segregation and genetic recombination, which is inherently higher in karyotypes with more and smaller chromosomes.