BACKGROUND: The availability of multiple avian genome sequence assemblies greatly improves our ability to define overall genome organization and reconstruct evolutionary changes. In birds, this has previously been impeded by a near intractable karyotype and relied almost exclusively on comparative molecular cytogenetics of only the largest chromosomes. Here, novel whole genome sequence information from 21 avian genome sequences (most newly assembled) made available on an interactive browser (Evolution Highway) was analyzed. RESULTS: Focusing on the six best-assembled genomes allowed us to assemble a putative karyotype of the dinosaur ancestor for each chromosome. Reconstructing evolutionary events that led to each species' genome organization, we determined that the fastest rate of change occurred in the @entity12128 and @entity5798 , consistent with rapid speciation events in the Passeriformes and Psittaciformes. Intra- and interchromosomal changes were explained most parsimoniously by a series of inversions and translocations respectively, with breakpoint reuse being commonplace. Analyzing @entity1573 and @entity12128 , we found little evidence to support the hypothesis of an association of evolutionary breakpoint regions with recombination hotspots but some evidence to support the hypothesis that microchromosomes largely represent conserved blocks of synteny in the majority of the 21 species analyzed. All but one species showed the expected number of microchromosomal rearrangements predicted by the haploid chromosome count. @entity6497 , however, appeared to retain an overall karyotype structure of 2n = 80 despite undergoing a large number (26) of hitherto un-described interchromosomal changes. CONCLUSIONS: Results suggest that mechanisms exist to preserve a static overall avian karyotype/genomic structure, including the microchromosomes, with widespread interchromosomal change occurring rarely (e.g. in @entity6497 and @entity5798 lineages). Of the species analyzed, the @entity1573 lineage appeared to have undergone the fewest changes compared to the dinosaur ancestor.

Reconstruction of gross avian genome structure, organization and evolution suggests that the XXXX lineage most closely resembles the dinosaur avian ancestor.

@entity5798 :: ('13146', 'Species') :: ['budgerigar'] @entity1573 :: ('9031', 'Species') :: ['chicken'] @entity12128 :: ('59729', 'Species') :: ['zebra finch'] @entity6497 :: ('8801', 'Species') :: ['Ostrich', 'ostrich']
@entity1573 :: (9031,Species) :: ['chicken']
