Reciprocal chromosome painting shows that the great difference in diploid number between human and African green monkey is mostly due to non-Robertsonian fissions

We used reciprocal chromosome painting with both African green monkey (C. a ethiops) and human chromosome specific DNA probes to delineate homologous r egions in the two species. Probes were derived by fluorescence-activated ch romosome flow sorting and then were reciprocally hybridized to metaphase sp reads of each species. Segments in the size range of a single chromosome ba nd were identified, demonstrating the sensitivity of the approach when comp aring species that diverged more than 20 million years ago. Outgroup analys is shows that the great difference in diploid numbers between the African g reen monkey (2n = 60) and humans (2n = 46) is mainly owing to fissions, and the direction of change is towards increasing diploid numbers. However, mo st break points apparently lie outside of the centromere regions, suggestin g that the changes were not solely Robertsonian as has been previously assu med. No reciprocal translocations have occurred in the phylogenetic lines l eading to humans or African green monkeys. The primate paints established h ere are a valuable tool to establish interspecies homology, to define rearr angements, and to determine the mechanisms of chromosomal evolution in prim ate species.