"Bar-coding" primate chromosomes: molecular cytogenetic screening for the ancestral hominoid karyotype

Two recently introduced multicolor FISH approaches, cross-species color ban ding (also termed Rx-FISH) and multiplex FISH using painting probes derived from somatic cell hybrids retaining fragments of human chromosomes, were a pplied in a comparative molecular cytogenetic study of higher primates. We analyzed these "chromosome bar code" patterns to obtain an overview of chro mosomal rearrangements that occurred during higher primate evolution. The o bjective was to reconstruct the ancestral genome organization of hominoids using the macaque as outgroup species. Approximately 160 individual and dis cernible molecular cytogenetic markers were assigned in these species. Resu lting comparative maps allowed us to identify numerous intra-chromosomal re arrangements, to discriminate them from previous contradicting chromosome b anding interpretations and to propose an ancestral karyotype for hominoids. From 25 different chromosome forms in an ancestral karyotype for all homin oids of 2N=48 we propose 21. Probes for chromosomes 2p, 4, 9 and Y were not informative in the present experiments. The orangutan karyotype was very s imilar to the proposed ancestral organization and conserved 19 of the 21 an cestral forms; thus most chromosomes were already present in early hominoid evolution, while African apes and human show various derived changes.