Comparative karyotype of rat and mouse using bidirectional chromosome painting Marie-Noe Èlle Guilly, Pierre Fouchet, Patricia de Chamisso, Annette Schmitz & Bernard Dutrillaux CEA, DSV-DRR, 60±68 Avenue du Ge Âne Âral Leclerc, BP 6, 92265 Fontenay-aux-Roses cedex, France; Tel: 33-1-46548943; Fax: 33-1-46548886; E-mail: mnguilly@armoise.saclay.cea.fr Received 5 November 1998; received in revised form and accepted for publication by M. Schmid 29 January 1999 Key words: comparative cytogenetics, heterologous chromosome painting Abstract A comparative karyotype of rat ( Rattus norvegicus) and mouse ( Mus musculus) based on chromosome G- banding morphology, heterologous chromosome painting results and available gene mapping data is proposed. Whole chromosome painting probes from both species were generated by PARM-PCR ampli®cation of ¯ow sorted chromosomes. Bidirectional chromosome painting identi®es 36 segments of syntenic homology and allows us to propose a nearly complete comparative karyotype of mouse and rat (except for RNO 13 p and RNO 19 p12-13). Seven segments completely covered the RNO chromosomes 3, 5, 8, 11, 12, 15 and 18. Eight segments completely covered the MMU chromosomes 3, 4, 6, 7, 9, 12, 18 and 19. The RNO chromosomes 5, 8, 18 show complete homology with the MMU chromosomes 4, 9 and 18, respectively. Bidirectional hybridization results clearly assign 16 segments to subchromosomal regions in both species. Interpretation of the results allows subchromosomal assignment of all the remaining segments apart from seven distributed on chromosomes MMU 15, MMU 10 B2-D3 and MMU 17 E3-E5. The proposed comparative karyotype shows overall agreement with available comparative mapping data. The proposed repartition of syntenic homologous segments between the two species provides useful data for gene-mapping studies. In addition, these results will enable the reconstruction of the karyotype of the Cricetidae and Muridae common ancestor and the de®nition of the precise rearrangements which have occurred in both mouse and rat lineages during evolution. Introduction Mouse ( Mus musculus) and rat ( Rattus norvegicus) belong to the Muridae, a large family of rodents related to Cricetidae, another large Rodentia family. The genome of the mouse is the best known among mammals, man excepted. The rat genome has been less studied although this species, is, like the mouse, widely used as model for experimental studies (Gill et al. 1989, Jacob et al. 1995, James & Lindpainter 1997). The information accumulated on the mouse genome might be transposed to that of the rat, but, unfortunately, Muridae species underwent an intense chromosome evolution, in which many translocations and complex structural rearrangements occurred. By chromosome banding analyses, it was proposed that at least 24 rearrangements separate mouse and rat karyotypes (Viegas-Pequignot et al. 1983, 1985), which explains the dif®culty of their comparison. Among some 2700 genes known to be homologous in man and mouse, about one third are identi®ed in the rat genome but a large proportion of them is not yet assigned to a given chromosome region (Levan et al. 1991, Mouse Genome Database (MGD) 1998, Chromosome Research 7: 213±221, 1999. # 1999 Kluwer Academic Publishers. Printed in the Netherlands 213