Avian ancestral karyotype reconstruction and differential rates of inter-and intra-chromosomal change in different lineages

In birds, genome is organised into several large chromosomes (macrochromosomes) and many smaller chromosomes (microchromosomes) that usually constitute about 25 and 75 per cent of the karyotype, respectively. Cytogenetic and molecular cytogenetic evidence suggests that avian karyotype is remarkably stable in evolution, with exception of several clades. To date, at least 21 avian genomes have been sequenced and assembled at the chromosome or scaffold level with N50 greater than 2 Mb, thereby allowing cytogenomic studies of chromosome organisation and change. To understand the comparative organisation and evolution of several avian species, we aligned chromosomes and scaffolds using an interactive genome browser (Evolution Highway), identifying homologous synteny blocks (HSBs) and evolutionary breakpoint regions (EBRs). For ancestral karyotype reconstruction, we focused on six species (chicken, turkey, duck, zebra finch, ostrich, and budgerigar; N50 > 10Mb) and reconstructed avian ancestor chromosomes using an outgroup (Anole lizard). In particular, we addressed the following biological questions: (1) whether species-specific EBRs could represent recombination hotspots, and (2) whether entire microchromosomes could be considered as blocks of conserved synteny. Our study did not reveal a significant association between EBRs and recombination. With support from molecular cytogenetic mapping, we did find that microchromosomes are characterised by a high interchromosomal conservation in almost all birds studied, except ostrich and parrots (budgerigar). By analysing HSBs in six birds and using a lizard outgroup,, we reconstructed a tentative avian ancestral genome and chromosomal rearrangements that occurred in the major avian evolutionary lineages. We identified most intrachromosomal changes (mostly inversions) in the zebra finch clade (Passeriformes) since the time when it diverged from the sister group of parrots (Psittaciformes) 54MYA. Our data also suggest the fewest number of chromosomal changes in the chicken as compared to the dinosaur-like avian ancestor.