CONSERVATION OF HUMAN VS FELINE GENOME ORGANIZATION REVEALED BY RECIPROCAL CHROMOSOME PAINTING

We employed fluorescence in situ hybridization (FISH) with probes esta blished by flow sorting metaphase chromosomes of the domestic cat (Fel is cattus, 2n = 38) to ''paint'' homologous segments on human chromoso mes and, reciprocally, using human chromosome paints on feline metapha se preparations. The results revealed, by direct microscopic observati on, widespread conservation of genome organization between the two mam malian orders and confirmed 90% of the homologous genes mapped to both species. Fourteen of 23 human chomosomes were hybridized with single cat probes, and 9 of 19 cat chromosomes were entirely labeled by a sin gle human probe. All other chromosomes were labeled with only two or, at most, three probes of the respective species. Y-chromosome probes g ave no signals. Approximately 30 syntenic segments were identified, an d the number of translocations could be estimated to be on the order o f one new translocation per 10 million years in the phylogenetic lines leading to human and cat. Using the principle of maximum parsimony, t he primitive vs. derived human chromosome segments were identified by comparison to the feline, cattle, and pig genomes, a first step in rec onstructing the evolutionary heritage of the mammalian radiations. The results suggest that reciprocal chromosome painting will help reconst ruct the history of genomic changes by determining the polarity of chr omosomal rearrangements and establishing the ancestral karyotype for e ach principle branching point in mammalian evolution.