Did the evolution of multiple microchromosomes help save bird and other dinosaurs from extinction?

Highlights

• The avian genome consists of few macro- and many microchromosomes.

• This organisation is ancient and predates the evolution of theropod dinosaurs.

• This organisation is very stable; however, lineage-specific rearrangements occurred.

• Many micro-chromosomes allow more crossovers and thus more variability and adaptation.

Abstract

Birds currently number ∼11,000 species and, despite enormous biodiversity, have numerous class-specific characteristics including feathers, flight, nesting, brooding, longevity and a very distinctive karyotype. Central to several academic fields including developmental biology, agriculture, virology, neurobiology, evolution, ecology and conservation, many birds have a fully-sequenced genome assemblies; aligning these to the karyotype (chromosome level assembly), is key to understanding avian biology and evolution. The “signature” avian karyotype (∼2n = 80) of these Therapod dinosaurs, many of which are threatened with extinction, is defined by ∼30 microchromosomal pairs and ∼10 larger macrochromosomes. Studies of phylogenetic relationships suggest that the domestic chicken (2n = 78 - the most studied species) has close to an ancestral genome organization, from which all others are measured. Comparative genomics using whole chromosome painting and selected locus specific probes have been used to map chromosome rearrangements throughout evolution and indicate that the signature avian-karyotype first appeared before the dinosaurs emerged 240MYA. Some, if not many, extinct dinosaurs therefore probably had this unique karyotype. Why it appeared and persisted may be due to a) evolutionary advantages to retaining it, and/or b) a lack of opportunity for change. Dinosaurs survived several extinction events, mostly recently evolving as birds and, here, we review research suggesting that a contributory factor to dinosaur survival may have been this unique means of genome organization. Of note is the large number of microchromosomes, which provide the substrate for phenotypic variation, the driver of evolutionary change, through increased random segregation and genetic recombination compared to other groups or vertebrates.