Cross-species chromosome painting corroborates microchromosome fusion during karyotype evolution of birds

The stone curlew, also known as thick-knee (Burhinus oedicnemus, BOE), represents a phylogenetically young species of the shorebirds (Charadriiformes) that exhibits one of the most atypical genome organizations known within the class of Aves, due to an extremely low diploid number (2n = 42) and only 6 pairs of microchromosomes in its complement. This distinct deviation from the 'typical' avian karyotype is attributed to repeated fusions of ancestral microchromosomes. In order to compare different species with this atypical avian karyotype and to investigate the chromosome rearrangement patterns, chromosome-specific painting probes representing the whole genome of the stone curlew were used to delineate chromosome homology between BOE and 5 species belonging to 5 different avian orders: herring gull (Charadriiformes), cockatiel (Psittaciformes), rock pigeon (Columbiformes), great gray owl (Strigiformes) and Eurasian coot (Gruiformes). Paints derived from the 20 BOE autosomes delimited 28 to 33 evolutionarily conserved segments in the karyotypes of the 5 species, similar to the number recognized by BOE paints in such a basal lineage as the chicken (28 conserved segments). This suggests a high degree of conservation in genome organization in birds. BOE paints also revealed some species-specific rearrangements. In particular, chromosomes BOE1-4 and 14, as well as to a large extent BOE5 and 6, showed conserved synteny with macrochromosomes, whereas homologous regions for BOE7-13 are found to be largely distributed on microchromosomes in the species investigated. Interestingly, the 6 pairs of BOE microchromosomes 15-20 appear to have undergone very few rearrangements in the 5 lineages investigated. Although the arrangements of BOE homologous segments on some chromosomes can be explained by complex fusions and inversions, the occurrence of homologous regions at multiple sites may point to fission of ancestral chromosomes in the karyotypes of the species investigated. However, the present results demonstrate that the ancestral microchromosomes most likely experienced fusion in the stone curlew lineage forming the medium-sized BOE chromosomes, while they have been conserved as microchromosomes in the other neoavian lineages.