Comparative study of human and chimpanzee chromosomes by fluorescence in situ hybridization

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Human chromosome 2 is the only obvious exception from the chromosomal homologies between the human karyotype and that of the great apes (Wienberg et al.1990,1992). Human chromosome 2 has been described that it arose out of the fusion of two ancestral ape chromosomes on the basis of comparisons of chromosome banding patterns between human and great apes. This has been confirmed by molecular data ; human 2q12-q14 region is homologous with chimpanzee (Pan troglodytes, PTR) 12p and 13p by comparative chromosome mapping with DNA microlibraries. Furthermore, the telomere-to-telomere fusion point has been conjectured to be localized in chromosomal band 2q13. Recently a high resolution cytogenetic map of human chromosome 2 with 434 cosmid makers was constructed by Takahashi et al. (1995). The present report describes the physical order of cosmid makers in the region 2q12-14 using FISH.The cosmids were labeled with either biotin-14-dATP or digoxigenin-11-dUTP by nick-translation. Two-color FISH for ordering was performed on metaphase chromosomes decondensed by actinomycin D and on DNA fibers. Preparation of DNA fibers was our modifications of the standard protocol (Suto et al., 1996). Totally 36 cosmids were localized on human and chimpanzee chromosomes. Seventeen human chromosome 2-originated markers were located to PTR 12 short arm, and nineteen to PTR 13 short arm. The order of clones on chimpanzee chromosomes was identical to that of human chromosome 2. The results suggest that human chromosome 2 arose out of tandem fusion between two ancestral ape-type chromosomes, and that the organization of the ancestral genome has been considerably conserved. Comparative mapping of this region among great apes may contribute to the dating of chromosomal reorganization in evolution.