Mastomys (=Praomys) natalensis is not a Rattus (Mammalia: Rodentia): karyological evidence.

Certain past studies have suggested that the taxa Mastomys (= Praomys) and Rattus were closely allied phyletically. We here present detailed (C- and G-banding and NOR's) comparisons of the karyotypes of M. natalensis (2n = 36) and R. norvegicus (2n = 42). A standardized G-banded karyotype of M. natalensis is also presented. About 45% of the G-banded karyotype of M. natalensis is homologous with that of R, norvegicus. Autosomal constitutive heterochromatin is nearly equivalent in the two species; the X and Y chromosomes of M. natalensis, however, have exceptionally large heterochromatic segments. Both species possess three pairs of NOR's, but adjacent G-banded regions are homologous for only one of these. Primarily on the basis of overall G-band homology, we regard the two taxa as only distantly related.