We report on the hybridization of mouse chromosomal paints to Apodemus sylvaticus, the long-tailed field mouse. The mouse paints detected 38 conserved segments in the Apodemus karyotype. Together with the species reported here there are now six species of rodents mapped with Mus musculus painting probes. A parsimony analysis indicated that the syntenies of nine M. musculus chromosomes were most likely already formed in the muroid ancestor: 3, 4, 7, 9, 14, 18, 19, X and Y. The widespread occurrence of syntenic segment associations of mouse chromosomes 1/17, 2/13, 7/19, 10/17, 11/16, 12/17 and 13/15 suggests that these associations were ancestral syntenies for muroid rodents. The muroid ancestral karyotype probably had a diploid number of about 2n = 54. It would be desirable to have a richer phylogenetic array of species before any final conclusions are drawn about the Muridae ancestral karyotype. The ancestral karyotype presented here should be considered as a working hypothesis.

Macaca sylvanus has a diploid chromosome number of 2n = 42. With the exception of chromosome 2 all chromosomes appear to have an identical banding pattern to most other macaque species. This chromosome has a clear band, localized immediately below the centromere; therefore, the karyotype of M. sylvanus more closely resembles that of the African Papionini, which also present a large clearly staining band on chromosome 2. C-banding intensely stained this band. Fluorescent in situ hybridization (FISH) with human chromosome specific DNA probes to macaque chromosome 2 showed that human probe 7 was located on the whole q arm and on a small part of the p arm, immediately adjacent to the centromere; the remaining part of the p arm was hybridized by the DNA probe for human chromosome 21. It appears clear, from the hybridization pattern in the other primates and in the outgroups examined, that chromosome 7 was probably a syntenic region for many ancestral mammalian karyotypes and a single syntenic region in the ancestral karyotype of the primates. Human probe for chromosome 21 and 3 are found associated in New World primates and a wide range of mammalian species. Therefore, this syntenic association was probably ancestral for mammals and present in the ancestral primate karyotype. The translocation of 7 and 21 to form chromosome 2 of Macaca sylvanus and all other Papionini is probably a recent trait linking all these species.