Defining the ancestral karyotype of all primates by multidirectional chromosome painting between tree shrews, lemurs and humans

We used multidirectional chromosome painting with probes derived by bivaria te flourescence-activated flow sol ting of chromosomes from human, black le mur (Eulemur macaco macaco) and tree shrew (Tupaia belangeri, order Scanden tia) to better define the karyological relationship of tree shrews and prim ates. An assumed close relationship between tree shrews and primates also a ssists in the reconstruction of the ancestral primate karyotype taking the tree shrew as an "out-group" species. The results indicate that T. belanger i has a highly derived karyotype. Tandem fusions or fissions of chromosomal segments seem to be the predominant mechanism in the evolution of this tre e shrew karyotype. The 22 human autosomal painting probes delineated 40 dif ferent segments, which is in the range found in most mammals analyzed by ch romosome painting up to now. There were no reciprocal translocations that w ould distinguish the karyotype of the tree shrew from an assumed primitive primate karyotype. This karyotype would have included the chromosomal forms la, Ib, 2a, 2b, 3/21, 4-11, 12a/22a, 12b/22b, 13, 14/15, 16a, 16b, 17, 18, 19a, 19b, 20 and X and Y and had a diploid chromosome number of 2n = 50. O f these forms, chromosomes la, Ib, 4, 8, 12a/22a, and 12b/22b may be common derived characters that would link the tree shrew with primates. To define the exact phylogenetic relationships of the tree shrews and the genomic re arrangements that gave rise to the primates and eventually to humans furthe r chromosome painting in Rodentia, Lagomorpha, Dermoptera and Chiroptera is needed, but many of the landmarks of genomic evolution are now known.