Evolutionary conservatism in arrangement of genetic material

The diploid number of the Rhesus macaque, Macaca mulatta, is 42. All chromosomes are biarmed and all constitutive heterochromatins are centromeric. The diploid number of the African Green monkey, Cercopithecus aethiops, is 60. Again all chromosomes are biarmed, but seven pairs possess very short second arms which are heterochromatic. The heterochromatins of remaining chromosomes are centromeric. Using G-banding and deleting the heterochromatic short arms, the chromosomes of the African Green monkey can be artificially fused to reconstruct a karyotype of the Rhesus with only one pair of unmatched small metacentrics. In addition to the Robertsonian type of translocations, several sets of centromere-telomere translocations were found. The latter type of translocation reduced three arms into two. Thus the fundamental number can be changed by two mechanisms: growing extra heterochromatic arms and the centromere-telomere fusions.