Comparative painting reveals strong chromosome homology over 80 million years of bird evolution

Chickens and the great flightless emu belong to two distantly related order s of birds in the carinate and ratite subclasses that diverged at least 80 million years ago. In the first ZOO-FISH study between bird species, we hyb ridized single chromosome paints from the chicken (Gallus domesticus) onto the emu chromosomes. We found that the nine macrochromosomes show remarkabl e homology between the two species, indicating strong conservation of karyo type through evolution. One chicken macrochromosome (4) was represented by a macro- and a microchromosome in the emu, suggesting that microchromosomes and macrochromosomes are interconvertible. The chicken Z chromosome paint hybridized to the emu Z and most of the W, confirming that ratite sex chrom osomes are largely homologous; the centromeric region of the W which hybrid ized weakly may represent the location of the sex determining gene(s).