The gene map of the Norway rat (Rattus norvegicus) and comparative mapping with mouse and man.

The current status of the rat gene map is presented. Mapping information is now available for a total of 214 loci and the number of mapped genes is increasing steadily. The corresponding number of loci quoted at HGM10 was 128. Genes have been assigned to 20 of the 22 chromosomes in the rat. Some aspects of comparative mapping with mouse and man are also discussed. It was found that there is a good correlation between the morphological homologies detectable in rat and mouse chromosomes, on the one hand, and homology at the gene level on the other. For 10 rat synteny groups all the genes so far mapped are syntenic also in the mouse. For the remaining rat synteny groups it appears that the majority of the genes will be syntenic on specific (homologous) mouse chromosomes, with only a few genes dispersed to other members of the mouse karyotype. Furthermore, the data indicate that mouse chromosome 1 genetically corresponds to two rat chromosomes, viz., 9 and 13, equalizing the difference in chromosome number between the two species. Further mappings will show whether the genetic homology will prove to be as extensive as these preliminary results indicate. As might be expected from evolutionary considerations, rat synteny groups are much more dispersed in the human genome. It is clear, however, that many groups of genes have remained syntenic during the period since man and rat shared a common ancestor. One further point was noted. In two cases groups of genes were syntenic in the mouse but dispersed to two chromosomes in rat and man, whereas in a third case a group of genes was syntenic in the rat but dispersed to two chromosomes in mouse and man. This finding argues in favor of the notion that the original gene groups were on separate ancestral chromosomes, which have fused in one rodent species but remained separate in the other and in man.